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115-691: ANAIS ( Annual modulation with NaI Scintillators ) is a dark matter direct detection experiment located at the Canfranc Underground Laboratory (LSC), in Spain, operated by a team of researchers of the CAPA at the University of Zaragoza. ANAIS' goal is to confirm or refute in a model independent way the DAMA/LIBRA experiment positive result: an annual modulation in the low-energy detection rate having all

230-414: A photomultiplier tube (PMT), photodiode , or silicon photomultiplier . PMTs absorb the light emitted by the scintillator and re-emit it in the form of electrons via the photoelectric effect . The subsequent multiplication of those electrons (sometimes called photo-electrons) results in an electrical pulse which can then be analyzed and yield meaningful information about the particle that originally struck

345-414: A radon -free environment. A careful low energy calibration of the region of interest (ROI), from 1 to 6 keV, is carried out by combining information from external calibrations and background. External calibrations with a Cd source are performed every two weeks, and every 1.5 months energy depositions at 3.2 and 0.87 keV from K and Na internal contaminations in one ANAIS module are selected by profiting from

460-478: A 1.2 μs window with high resolution (14 bits ). The trigger requires the coincidence of the two PMT trigger signals in a 200 ns window, while the PMT individual trigger is set at the single phe level. Another interesting feature is a Mylar window in the middle of one of the lateral faces of the detectors, which allows to calibrate simultaneously the nine modules with external x-ray / gamma sources down to 10 keV in

575-421: A 3 × 3 configuration. Among the most relevant features of ANAIS- 112 modules, it is worth highlighting its remarkable optical quality, which combined to using high quantum efficiency Hamamatsu photomultipliers (PMTs) results in a very high light collection, at the level of 15 photo electrons (phe) per keV in all the nine modules. The signals from the two PMTs coupled to each module are digitized at 2 GS/s in

690-413: A cerium-doped lanthanum bromide , LaBr 3 (Ce) . They are both very hygroscopic (i.e., damaged when exposed to moisture in the air) but offer excellent light output and energy resolution (63 photons/keV γ for LaBr 3 (Ce) versus 38 photons/keV γ for NaI(Tl) ), a fast response (16 ns for LaBr 3 (Ce) versus 230 ns for NaI(Tl) ), excellent linearity, and a very stable light output over

805-416: A certain particle (dE/dx), the "fast" and "slow" states are occupied in different proportions. The relative intensities in the light output of these states thus differs for different dE/dx. This property of scintillators allows for pulse shape discrimination: it is possible to identify which particle was detected by looking at the pulse shape. Of course, the difference in shape is visible in the trailing side of

920-462: A combination of water tanks and polyethylene bricks. An active veto made up of 16 plastic scintillators is placed between the anti- radon box and the neutron shielding, covering the top and sides of the set-up allowing to effectively tag the residual muon flux onsite along the ANAIS-112 data taking.   ANAIS-112 was commissioned during the spring of 2017 and it started the data-taking phase at

1035-412: A decay time of 2–4 nanoseconds, but perhaps the biggest advantage of plastic scintillators is their ability to be shaped, through the use of molds or other means, into almost any desired form with what is often a high degree of durability. Plastic scintillators are known to show light output saturation when the energy density is large ( Birks' Law ). The most common bases used in plastic scintillators are

1150-650: A great number of secondary electron–hole pairs are produced until the hot electrons and holes have lost sufficient energy. The large number of electrons and holes that result from this process will then undergo thermalization , i.e. dissipation of part of their energy through interaction with phonons in the material The resulting large number of energetic charge carriers will then undergo further energy dissipation called thermalization. This occurs via interaction with phonons for electrons and Auger processes for holes. The average timescale for conversion, including energy absorption and thermalization has been estimated to be in

1265-523: A product of π-orbitals . Organic materials form molecular crystals where the molecules are loosely bound by Van der Waals forces . The ground state of C is 1s 2s 2p . In valence bond theory, when carbon forms compounds, one of the 2s electrons is excited into the 2p state resulting in a configuration of 1s 2s 2p . To describe the different valencies of carbon, the four valence electron orbitals, one 2s and three 2p, are considered to be mixed or hybridized in several alternative configurations. For example, in

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1380-412: A scintillator was built in 1903 by Sir William Crookes and used a ZnS screen. The scintillations produced by the screen were visible to the naked eye if viewed by a microscope in a darkened room; the device was known as a spinthariscope . The technique led to a number of important discoveries but was obviously tedious. Scintillators gained additional attention in 1944, when Curran and Baker replaced

1495-612: A small amount of activator impurity. The most widely used is NaI(Tl) ( thallium -doped sodium iodide ); its scintillation light is blue. Other inorganic alkali halide crystals are: CsI(Tl) , CsI(Na) , CsI (pure), CsF , KI(Tl) , LiI(Eu) . Some non-alkali crystals include: BGO , BaF 2 , CaF 2 (Eu) , ZnS(Ag) , CaWO 4 , CdWO 4 , YAG(Ce) ( Y 3 Al 5 O 12 (Ce) ), GSO , LSO . (For more examples, see also phosphors ). Newly developed products include LaCl 3 (Ce) , lanthanum chloride doped with cerium, as well as

1610-404: A tetrahedral configuration the s and p orbitals combine to produce four hybrid orbitals. In another configuration, known as trigonal configuration, one of the p-orbitals (say p z ) remains unchanged and three hybrid orbitals are produced by mixing the s, p x and p y orbitals. The orbitals that are symmetrical about the bonding axes and plane of the molecule (sp ) are known as σ-electrons and

1725-409: A vibrational level. The singlet excitations immediately decay (< 10 ps) to the S state without the emission of radiation (internal degradation). The S state then decays to the ground state S 0 (typically to one of the vibrational levels above S 0 ) by emitting a scintillation photon . This is the prompt component or fluorescence . The transparency of the scintillator to the emitted photon

1840-482: A whole by impurity centers. The latter then rapidly de-excite by emitting scintillation light (fast component). The activator impurities are typically chosen so that the emitted light is in the visible range or near-UV where photomultipliers are effective. The holes associated with electrons in the conduction band are independent from the latter. Those holes and electrons are captured successively by impurity centers exciting certain metastable states not accessible to

1955-485: A wide range of temperatures. In addition LaBr 3 (Ce) offers a higher stopping power for γ rays (density of 5.08 g/cm versus 3.67 g/cm for NaI(Tl) ). LYSO ( Lu 1.8 Y 0.2 SiO 5 (Ce) ) has an even higher density (7.1 g/cm , comparable to BGO ), is non-hygroscopic, and has a higher light output than BGO (32 photons/keV γ), in addition to being rather fast (41 ns decay time versus 300 ns for BGO ). A disadvantage of some inorganic crystals, e.g., NaI,

2070-1019: Is anisotropic (which spoils energy resolution when the source is not collimated ), and they cannot be easily machined, nor can they be grown in large sizes; hence they are not very often used. Anthracene has the highest light output of all organic scintillators and is therefore chosen as a reference: the light outputs of other scintillators are sometimes expressed as a percentage of anthracene light output. These are liquid solutions of one or more organic scintillators in an organic solvent . The typical solutes are fluors such as p -terphenyl ( C 18 H 14 ), PBD ( C 20 H 14 N 2 O ), butyl PBD ( C 24 H 22 N 2 O ), PPO ( C 15 H 11 NO ), and wavelength shifter such as POPOP ( C 24 H 16 N 2 O ). The most widely used solvents are toluene , xylene , benzene , phenylcyclohexane , triethylbenzene , and decalin . Liquid scintillators are easily loaded with other additives such as wavelength shifters to match

2185-505: Is a constant that varies between 3 and 4, and E γ {\displaystyle E_{\gamma }} is the energy of the photon. At low X-ray energies, scintillator materials with atoms with high atomic numbers and densities are favored for more efficient absorption of the incident radiation. At higher energies ( E γ {\displaystyle E_{\gamma }} ≳ {\displaystyle \gtrsim } 60 keV) Compton scattering,

2300-459: Is also decreased via high density materials. This results in high segmentation of the detector and leads to better spatial resolution. Usually high density materials have heavy ions in the lattice (e.g., lead , cadmium ), significantly increasing the contribution of photoelectric effect (~Z ). The increased photo-fraction is important for some applications such as positron emission tomography . High stopping power for electromagnetic component of

2415-422: Is an area of intense research. Transitions made by the free valence electrons of the molecules are responsible for the production of scintillation light in organic crystals. These electrons are associated with the whole molecule rather than any particular atom and occupy the so-called - molecular orbitals . The ground state S 0 is a singlet state above which are the excited singlet states (S , S , ...),

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2530-422: Is an electron): ≈40 photons/keV for NaI(Tl) , ~10 photons/keV for plastic scintillators, and ~8 photons/keV for bismuth germanate ( BGO ). Scintillation detectors are generally assumed to be linear. This assumption is based on two requirements: (1) that the light output of the scintillator is proportional to the energy of the incident radiation; (2) that the electrical pulse produced by the photomultiplier tube

2645-469: Is an example of luminescence , whereby light of a characteristic spectrum is emitted following the absorption of radiation . The scintillation process can be summarized in three main stages: conversion, transport and energy transfer to the luminescence center, and luminescence. The emitted radiation is usually less energetic than the absorbed radiation, hence scintillation is generally a down-conversion process. The first stage of scintillation, conversion,

2760-697: Is due to the fact that the energy of the photon is less than that required for an S 0 → S transition (the transition is usually being to a vibrational level above S 0 ). When one of the triplet states gets excited, it immediately decays to the T 0 state with no emission of radiation (internal degradation). Since the T 0  → S 0 transition is very improbable, the T 0 state instead decays by interacting with another T 0 molecule: T 0 + T 0 → S ∗ + S 0 + photons {\displaystyle T_{0}+T_{0}\rightarrow S^{*}+S_{0}+{\text{photons}}} and leaves one of

2875-544: Is essential to prevent self reabsorption for scintillators. More recently, a new material class first reported by Professor Biwu Ma's research group, called 0D organic metal halide hybrid (OMHH), an extension of the perovskite materials. This class of materials exhibits strong exciton binding of hundreds of meV, resulting in their high photoluminescent quantum efficiency of almost unity. Their large stoke shift and reabsorption-free properties make them desirable. Their potential applications for scintillators have been reported by

2990-407: Is essentially 100% for most scintillators. But because electrons can make large angle scatterings (sometimes backscatterings ), they can exit the detector without depositing their full energy in it. The back-scattering is a rapidly increasing function of the atomic number Z of the scintillator material. Organic scintillators, having a lower Z than inorganic crystals, are therefore best suited for

3105-471: Is maintained at cryogenic temperatures because mutual repulsion drives any additional electrons into the next higher available energy level, which is in the conduction band. The spectrum of photons from this process is centered at 930 nm (1.33 eV) and there are three other emission bands centered at 860, 1070, and 1335 nm from other minor processes. Each of these emission bands has a different luminosity and decay time. The high scintillation luminosity

3220-561: Is observed and write about yields up to 200 000 ph/MeV. The quenching is attributed to the small e-h binding energy in the exciton that decreases for Cl to Br to I . Interestingly one may replace the organic MA group with Cs+ to obtain full inorganic CsPbX 3 halide perovskites. Depending on the Cl, Br, I content the triplet X-ray excited exciton emission can be tuned from 430 nm to 700 nm . One may also dilute Cs with Rb to obtain similar tuning. Above very recent developments demonstrate that

3335-451: Is performed in the same regions explored by DAMA/LIBRA collaboration, [1–6] keV and [2–6] keV, fixing the period to 1 year and the maximum of the modulation to 2 June. To evaluate the statistical significance of a possible modulation in ANAIS–112 data, the events rate of the nine detectors is calculated in 10-days bins, and it is minimized χ = Σ i (n i − μ i )/σ i , where n i is

3450-454: Is produced whenever an acceptor atom such as boron captures an ionization hole from the valence band and that hole recombines radiatively with one of the delocalized electrons. Unlike many other semiconductors, the delocalized electrons provided by the silicon are not “frozen out” at cryogenic temperatures. Above the Mott transition concentration of 8 × 10 free carriers per cm , the “metallic” state

3565-502: Is proportional to the emitted scintillation light. The linearity assumption is usually a good rough approximation, although deviations can occur (especially pronounced for particles heavier than the proton at low energies). Resistance and good behavior under high-temperature, high-vibration environments is especially important for applications such as oil exploration ( wireline logging , measurement while drilling). For most scintillators, light output and scintillation decay time depends on

ANAIS-112 - Misplaced Pages Continue

3680-535: Is slightly lower when detectors are considered independently, as expected following a priori sensitivity analysis. Therefore, this fit is chosen to quote the ANAIS-112 annual modulation final result and sensitivity for three-year exposure. The best fits are incompatible with the DAMA/LIBRA result at 3.3 and 2.6 σ in [1-6] and [2-6] keV energy regions, for a sensitivity of 2.5 (2.7)σ at [1–6] keV ([2–6] keV). ANAIS-112 results for 1.5, 2 and 3 years of data-taking fully confirm

3795-407: Is strong, the overall decay time constant varies with the type of incident particle. Such scintillators enable pulse shape discrimination, i.e., particle identification based on the decay characteristics of the PMT electric pulse. For instance, when BaF 2 is used, γ rays typically excite the fast component, while α particles excite the slow component: it is thus possible to identify them based on

3910-412: Is surprising because (1) with a refractive index of about 3.5, escape is inhibited by total internal reflection and (2) experiments at 90K report narrow-beam infrared absorption coefficients of several per cm. Recent Monte Carlo and Feynman path integral calculations have shown that the high luminosity could be explained if most of the narrow beam absorption is actually a novel optical scattering from

4025-425: Is the rest mass of the electron and c {\displaystyle c} is the speed of light . Hence, at high γ-ray energies, the energy absorption depends both on the density and average atomic number of the scintillator. In addition, unlike for the photoelectric effect and Compton scattering, pair production becomes more probable as the energy of the incident photons increases, and pair production becomes

4140-477: Is the dominant background source, being Pb, K, Na, H contributions the most relevant ones in the region of interest. Considering altogether the nine ANAIS-112 modules, the average background in the ROI is 3.6 cpd/kg/keV after three years of data taking, while DAMA/LIBRAphase2 background is below 0.80 cpd/kg/keV in the[1–2] keV energy interval, below 0.24 cpd/kg/keV in the [2–3] keV energy interval, and below 0.12 cpd/kg/keV in

4255-528: Is the energy interval width, and ∆t the time bin width. R 0 is a free parameter, while S m is either fixed to 0 (for the null hypothesis) or left unconstrained, positive or negative (for the modulation hypothesis). The null hypothesis is well supported for the 3-years data in both energy regions, being the results for the two background models (a single exponential or a PDF based on the Monte Carlo background model) compatible. The standard deviation σ(S m )

4370-406: Is the linear attenuation coefficient, which is the sum of the attenuation coefficients of the various contributions: At lower X-ray energies ( E γ ≲ {\displaystyle E_{\gamma }\lesssim } 60 keV), the most dominant process is the photoelectric effect, where the photons are fully absorbed by bound electrons in the material, usually core electrons in

4485-419: Is the mechanism of energy absorption: energy is first absorbed by the solvent, then passed onto the scintillation solute (the details of the transfer are not clearly understood). The scintillation process in inorganic materials is due to the electronic band structure found in crystals and is not molecular in nature as is the case with organic scintillators. An incoming particle can excite an electron from

4600-570: Is the process where the energy from the incident radiation is absorbed by the scintillator and highly energetic electrons and holes are created in the material. The energy absorption mechanism by the scintillator depends on the type and energy of radiation involved. For highly energetic photons such as X-rays (0.1 keV < E γ {\displaystyle E_{\gamma }} < 100 keV) and γ-rays ( E γ {\displaystyle E_{\gamma }} > 100 keV), three types of interactions are responsible for

4715-415: Is the target in experiments to detect rare, low-energy electronic excitations from interacting dark matter. Gaseous scintillators consist of nitrogen and the noble gases helium , argon , krypton , and xenon , with helium and xenon receiving the most attention. The scintillation process is due to the de-excitation of single atoms excited by the passage of an incoming particle. This de-excitation

ANAIS-112 - Misplaced Pages Continue

4830-719: Is their hygroscopicity, a property which requires them to be housed in an airtight container to protect them from moisture. CsI(Tl) and BaF 2 are only slightly hygroscopic and do not usually need protection. CsF, NaI(Tl) , LaCl 3 (Ce) , LaBr 3 (Ce) are hygroscopic, while BGO , CaF 2 (Eu) , LYSO , and YAG(Ce) are not. Inorganic crystals can be cut to small sizes and arranged in an array configuration so as to provide position sensitivity. Such arrays are often used in medical physics or security applications to detect X-rays or γ rays: high- Z , high density materials (e.g. LYSO, BGO) are typically preferred for this type of applications. Scintillation in inorganic crystals

4945-676: Is typically slower than in organic ones, ranging typically from 1.48 ns for ZnO(Ga) to 9000 ns for CaWO 4 . Exceptions are CsF (~5 ns), fast BaF 2 (0.7 ns; the slow component is at 630 ns), as well as the newer products ( LaCl 3 (Ce) , 28 ns; LaBr 3 (Ce) , 16 ns; LYSO , 41 ns). For the imaging application, one of the advantage of inorganic crystals is very high light yield. Some high light yield scintillators above 100,000 photons/MeV at 662 keV are very recently reported for LuI 3 (Ce) , SrI 2 (Eu) , and Cs 2 HfCl 6 . Many semiconductor scintillator phosphors are known, such as ZnS(Ag) (mentioned in

5060-578: Is very rapid (~1 ns), so the detector response is quite fast. Coating the walls of the container with a wavelength shifter is generally necessary as those gases typically emit in the ultraviolet and PMTs respond better to the visible blue-green region. In nuclear physics, gaseous detectors have been used to detect fission fragments or heavy charged particles . The most common glass scintillators are cerium -activated lithium or boron silicates . Since both lithium and boron have large neutron cross-sections , glass detectors are particularly well suited to

5175-493: Is ≈10 ns, their light output is however low, typically ≈30% of that of anthracene. Scintillation properties of organic-inorganic methylamonium (MA) lead halide perovskites under proton irradiation were first reported by Shibuya et al. in 2002 and the first γ-ray pulse height spectrum, although still with poor energy resolution, was reported on ( (C 6 H 5 (CH 2 ) 2 NH 3 ) 2 PbBr 4 ) by van Eijk et al. in 2008 . Birowosuto at al. studied

5290-445: The K- or L-shell of the atom, and then ejected, leading to the ionization of the host atom. The linear attenuation coefficient contribution for the photoelectric effect is given by: where ρ {\displaystyle \rho } is the density of the scintillator, Z {\displaystyle Z} is the average atomic number, n {\displaystyle n}

5405-479: The Schrödinger wave equation are: where q is the orbital ring quantum number; the number of nodes of the wave-function. Since the electron can have spin up and spin down and can rotate about the circle in both directions all of the energy levels except the lowest are doubly degenerate. The above shows the π-electronic energy levels of an organic molecule. Absorption of radiation is followed by molecular vibration to

5520-519: The recoil proton in (n,p) reactions; materials rich in hydrogen , e.g. plastic scintillators, are therefore best suited for their detection. Slow neutrons rely on nuclear reactions such as the (n,γ) or (n,α) reactions, to produce ionization. Their mean free path is therefore quite large unless the scintillator material contains nuclides having a high cross section for these nuclear reactions such as Li or B. Materials such as LiI(Eu) or glass silicates are therefore particularly well-suited for

5635-429: The valence band to either the conduction band or the exciton band (located just below the conduction band and separated from the valence band by an energy gap ; see picture ). This leaves an associated hole behind, in the valence band. Impurities create electronic levels in the forbidden gap . The excitons are loosely bound electron-hole pairs which wander through the crystal lattice until they are captured as

5750-423: The 5σ level. Operation at Canfranc Underground Laboratory has been granted until the end of 2025. One possible systematics affecting the comparison between DAMA/LIBRA and ANAIS result is a possible different detector response to nuclear recoils, because both experiments are calibrated using x-rays/gammas. It is well known that scintillation is strongly quenched for energy deposited by nuclear recoils with respect to

5865-638: The Dark Matter Data Center: https://www.origins-cluster.de/odsl/dark-matter-data-center/available-datasets/anais Data are available upon request. ANAIS experiment operation is presently financially supported by MICIU/AEI/10.13039/501100011033 (Grants No. PID2022-138357NB-C21 and PID2019-104374GB-I00), and Unión Europea NextGenerationEU/PRTR (AstroHEP) and the Gobierno de Aragón. Funding from Grant FPA2017-83133-P, Consolider-Ingenio 2010 Programme under grants MULTIDARK CSD2009-00064 and CPAN CSD2007-00042,

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5980-474: The Gobierno de Aragón and the LSC Consortium made possible the setting-up of the detectors. The technical support from LSC and GIFNA staff as well as from Servicios de Apoyo a la Investigación de la Universidad de Zaragoza (SAIs) is warmly acknowledged. Scintillator A scintillation detector or scintillation counter is obtained when a scintillator is coupled to an electronic light sensor such as

6095-412: The PMT (typically ~30% at peak), and on the efficiency of light transmission and collection (which depends on the type of reflector material covering the scintillator and light guides, the length/shape of the light guides, any light absorption, etc.). The light output is often quantified as a number of scintillation photons produced per keV of deposited energy. Typical numbers are (when the incident particle

6210-416: The PMT window. Ruggedness and good behavior under high temperature may be desirable where resistance to vibration and high temperature is necessary (e.g., oil exploration). The practical choice of a scintillator material is usually a compromise among those properties to best fit a given application. Among the properties listed above, the light output is the most important, as it affects both the efficiency and

6325-410: The ROI, but in particular in the region from 1 to 2 keV. In this context, the application of machine learning techniques based on Boosted Decision Trees (BDTs), under development at present, could improve the rejection of these non-bulk scintillation events. Preliminary results point to a relevant sensitivity improvement. Extending the data taking for a few more years, could allow testing DAMA/LIBRA at

6440-408: The S 1 state. This is followed by a de-excitation to the S 0 state called fluorescence. The population of triplet states is also possible by other means. The triplet states decay with a much longer decay time than singlet states, which results in what is called the slow component of the decay process (the fluorescence process is called the fast component). Depending on the particular energy loss of

6555-478: The [3–4] keV energy interval. The development of filtering protocols based on the pulse shape and light sharing among the two PMTs has been crucial to fulfill the ANAIS-112 goal since the trigger rate in the ROI is dominated by non-bulk scintillation events. The determination of the corresponding efficiency is very important, and it is calculated using Cd, K and Na events. It is very close to 100% down to 2 keV, and then decreases steeply to about 15% at 1 keV, where

6670-419: The analysis threshold is set. A blind protocol for the annual modulation analysis of ANAIS-112 data has been applied: single-hit events in the ROI are kept blinded during the event selection. Up to now, three unblindings of the data have been carried out: at 1.5 years, at 2 years, and 3 years, which correspond to exposures of 157.55, 220.69, and 313.95 kg×y, respectively. ANAIS-112 annual modulation search

6785-408: The aromatic plastics, polymers with aromatic rings as pendant groups along the polymer backbone, amongst which polyvinyltoluene (PVT) and polystyrene (PS) are the most prominent. While the base does fluoresce in the presence of ionizing radiation, its low yield and negligible transparency to its own emission make the use of fluors necessary in the construction of a practical scintillator. Aside from

6900-806: The aromatic plastics, the most common base is polymethylmethacrylate (PMMA), which carries two advantages over many other bases: high ultraviolet and visible light transparency and mechanical properties and higher durability with respect to brittleness. The lack of fluorescence associated with PMMA is often compensated through the addition of an aromatic co-solvent, usually naphthalene. A plastic scintillator based on PMMA in this way boasts transparency to its own radiation, helping to ensure uniform collection of light. Other common bases include polyvinyl xylene (PVX) polymethyl, 2,4-dimethyl, 2,4,5-trimethyl styrenes, polyvinyl diphenyl, polyvinyl naphthalene, polyvinyl tetrahydronaphthalene, and copolymers of these and other bases. Also known as luminophors, these compounds absorb

7015-457: The bonds are called σ-bonds. The p z orbital is called a π-orbital. A π-bond occurs when two π-orbitals interact. This occurs when their nodal planes are coplanar. In certain organic molecules π-orbitals interact to produce a common nodal plane. These form delocalized π-electrons that can be excited by radiation. The de-excitation of the delocalized π-electrons results in luminescence. The excited states of π-electron systems can be explained by

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7130-464: The coincidence with a high energy gamma in a second module. The ANAIS-112 experiment is installed inside a shielding consisting of an inner layer of 10 cm of archaeological lead and an outer layer of 20 cm of low activity lead. This lead shielding is encased into an anti- radon box, tightly closed and kept under overpressure with radon -free nitrogen gas. The external layer of the shielding (the neutron shielding) consists of 40 cm of

7245-517: The conduction electrons with a cross section of about 5 x 10 cm that allows scintillation photons to escape total internal reflection. This cross section is about 10 times larger than Thomson scattering but comparable to the optical cross section of the conduction electrons in a metal mirror. In gases, the scintillation process is due to the de-excitation of single atoms excited by the passage of an incoming particle (a very rapid process: ≈1 ns). Scintillation counters are usually not ideal for

7360-442: The contribution from Rayleigh scattering is almost negligible and photonuclear reactions become relevant only at very high energies. After the energy of the incident radiation is absorbed and converted into so-called hot electrons and holes in the material, these energetic charge carriers will interact with other particles and quasi-particles in the scintillator (electrons, plasmons , phonons ), leading to an "avalanche event", where

7475-526: The decay time of the PMT signal. Organic scintillators are aromatic hydrocarbon compounds which contain benzene ring structures interlinked in various ways. Their luminescence typically decays within a few nanoseconds. Some organic scintillators are pure crystals. The most common types are anthracene ( C 14 H 10 , decay time ≈30 ns), stilbene ( C 14 H 12 , 4.5 ns decay time), and naphthalene ( C 10 H 8 , few ns decay time). They are very durable, but their response

7590-526: The detection of gamma rays . The three basic ways that a gamma ray interacts with matter are: the photoelectric effect , Compton scattering , and pair production . The photon is completely absorbed in photoelectric effect and pair production, while only partial energy is deposited in any given Compton scattering. The cross section for the photoelectric process is proportional to Z , that for pair production proportional to Z , whereas Compton scattering goes roughly as Z . A high- Z material therefore favors

7705-551: The detection of heavy ions for three reasons: The reduction in light output is stronger for organics than for inorganic crystals. Therefore, where needed, inorganic crystals, e.g. CsI(Tl) , ZnS(Ag) (typically used in thin sheets as α-particle monitors), CaF 2 (Eu) , should be preferred to organic materials. Typical applications are α- survey instruments , dosimetry instruments, and heavy ion dE / dx detectors. Gaseous scintillators have also been used in nuclear physics experiments. The detection efficiency for electrons

7820-424: The detection of thermal (slow) neutrons . Lithium is more widely used than boron since it has a greater energy release on capturing a neutron and therefore greater light output. Glass scintillators are however sensitive to electrons and γ rays as well (pulse height discrimination can be used for particle identification). Being very robust, they are also well-suited to harsh environmental conditions. Their response time

7935-426: The detection of low-energy (< 10 MeV) beta particles . The situation is different for high energy electrons: since they mostly lose their energy by bremsstrahlung at the higher energies, a higher- Z material is better suited for the detection of the bremsstrahlung photon and the production of the electromagnetic shower which it can induce. High- Z materials, e.g. inorganic crystals, are best suited for

8050-615: The detection of slow (thermal) neutrons. The following is a list of commonly used inorganic crystals: Scintillation (physics) In condensed matter physics , scintillation ( / ˈ s ɪ n t ɪ l eɪ ʃ ən / SIN -til-ay-shun ) is the physical process where a material, called a scintillator , emits ultraviolet or visible light under excitation from high energy photons ( X-rays or gamma rays ) or energetic particles (such as electrons , alpha particles , neutrons , or ions ). See scintillator and scintillation counter for practical applications. Scintillation

8165-501: The different experimental results in most conventional WIMP-DM scenarios is actually disfavored, but it is strongly dependent on the DM particle and halo models considered. A comparison using the same target material, NaI (Tl), is more direct and almost model-independent. Source: ANAIS-112 experimental setup consists of 112.5 kg of NaI (Tl), distributed in 9 cylindrical modules, 12.5 kg each and built by Alpha Spectra Inc., arranged in

8280-468: The emission is due to radiative decay of self-trapped excitons. The scintillation process in GaAs doped with silicon and boron impurities is different from conventional scintillators in that the silicon n -type doping provides a built-in population of delocalized electrons at the bottom of the conduction band. Some of the boron impurity atoms reside on arsenic sites and serve as acceptors. A scintillation photon

8395-407: The energy conversion process in scintillation: photoelectric absorption , Compton scattering , and pair production , which only occurs when E γ {\displaystyle E_{\gamma }} > 1022 keV, i.e. the photon has enough energy to create an electron-positron pair. These processes have different attenuation coefficients , which depend mainly on the energy of

8510-464: The excitons. The delayed de-excitation of those metastable impurity states again results in scintillation light (slow component). BGO ( bismuth germanium oxide ) is a pure inorganic scintillator without any activator impurity. There, the scintillation process is due to an optical transition of the Bi ion, a major constituent of the crystal. In tungstate scintillators CaWO 4 and CdWO 4

8625-401: The experiment, ϕ b k g {\displaystyle \phi _{bkg}} is the probability distribution function (PDF) in time of any non-modulated component, S m is the modulation amplitude, ω is fixed to 2π/365 d = 0.01721 rad d, t 0 to −62.2 d (time origin has been taken on 3 August and then the cosine maximum is on 2 June), M is the total detector mass, ∆E

8740-433: The fast (or prompt) and the slow (or delayed) decay constants. Many scintillators are characterized by 2 time components: one fast (or prompt), the other slow (or delayed). While the fast component usually dominates, the relative amplitude A and B of the two components depend on the scintillating material. Both of these components can also be a function of the energy loss dE / dx . In cases where this energy loss dependence

8855-541: The features expected for the signal induced by weakly interacting dark matter particles ( WIMPs ) in a standard galactic halo . This modulation is produced as a result of the Earth rotation around the Sun. A modulation with all the characteristic of a Dark Matter (DM) signal has been observed for about 20 years by DAMA/LIBRA, but it is in strong tension with the negative results of other DM direct detection experiments. Compatibility among

8970-576: The former two processes, enabling the detection of the full energy of the gamma ray. If the gamma rays are at higher energies (>5 MeV), pair production dominates. Since the neutron is not charged it does not interact via the Coulomb force and therefore does not ionize the scintillation material. It must first transfer some or all of its energy via the strong force to a charged atomic nucleus . The positively charged nucleus then produces ionization . Fast neutrons (generally >0.5 MeV ) primarily rely on

9085-487: The hall B of the LSC on 3 August 2017 under 2450 m.w.e. rock overburden. The "live time" of the experiment, useful for analysis, is more than 95%, allowing for the high duty cycle achieved. Down time is mostly due to the periodical calibration of the modules. A background understanding has been achieved, except in the [1-2] keV energy region, where the background model underestimates the measured event rate. Crystal bulk contamination

9200-469: The harnessing of gamma-ray energy through the photovoltaic effect, for example in a nuclear battery . The use of a scintillator in conjunction with a photomultiplier tube finds wide use in hand-held survey meters used for detecting and measuring radioactive contamination and monitoring nuclear material. Scintillators generate light in fluorescent tubes, to convert the ultra-violet of the discharge into visible light. Scintillation detectors are also used in

9315-482: The history section), CdS(Ag), ZnO(Zn), ZnO(Ga), CdS(In), ZnSe(O), and ZnTe(O), but none of these are available as single crystals. CdS(Te) and ZnSe(Te) have been commercially available in single crystal form, but their luminosity is partially quenched at room temperature. GaAs(Si,B) is a recently discovered cryogenic semiconductor scintillator with high light output in the infra-red and apparently no afterglow. In combination with ultra-low noise cryogenic photodetectors it

9430-417: The incident radiation, the average atomic number of the material and the density of the material. Generally the absorption of high energy radiation is described by: where I 0 {\displaystyle I_{0}} is the intensity of the incident radiation, d {\displaystyle d} is the thickness of the material, and μ {\displaystyle \mu }

9545-582: The inelastic scattering of photons by bound electrons, often also leading to ionization of the host atom, becomes the more dominant conversion process. The linear attenuation coefficient contribution for Compton scattering is given by: Unlike the photoelectric effect, the absorption resulting from Compton scattering is independent of the atomic number of the atoms present in the crystal, but linearly on their density. At γ-ray energies higher than E γ {\displaystyle E_{\gamma }} > 1022 keV, i.e. energies higher than twice

9660-495: The ionizing radiation needs greater photo-fraction; this allows for a compact detector. High operating speed is needed for good resolution of spectra. Precision of time measurement with a scintillation detector is proportional to √ τ sc . Short decay times are important for the measurement of time intervals and for the operation in fast coincidence circuits. High density and fast response time can allow detection of rare events in particle physics. Particle energy deposited in

9775-403: The lowest triplet state (T 0 ), and its excited levels (T , T , ...). A fine structure corresponding to molecular vibrational modes is associated with each of those electron levels. The energy spacing between electron levels is ≈1 eV; the spacing between the vibrational levels is about 1/10 of that for electron levels. An incoming particle can excite either an electron level or

9890-399: The luminescent center, and then the electrons and hole recombine radiatively . The exact details of the luminescence phase also depend on the type of material used for scintillation. For photons such as gamma rays, thallium activated NaI crystals (NaI(Tl)) are often used. For a faster response (but only 5% of the output) CsF crystals can be used. In organic molecules scintillation is

10005-489: The material of a scintillator is proportional to the scintillator's response. Charged particles, γ-quanta and ions have different slopes when their response is measured. Thus, scintillators could be used to identify various types of γ-quanta and particles in fluxes of mixed radiation. Another consideration of scintillators is the cost of producing them. Most crystal scintillators require high-purity chemicals and sometimes rare-earth metals that are fairly expensive. Not only are

10120-413: The material. This is probably one of the most critical phases of scintillation, since it is generally in this stage where most loss of efficiency occur due to effects such as trapping or non-radiative recombination . These are mainly caused by the presence of defects in the scintillator crystal, such as impurities, ionic vacancies, and grain boundaries . The charge transport can also become a bottleneck for

10235-505: The materials an expenditure, but many crystals require expensive furnaces and almost six months of growth and analyzing time. Currently, other scintillators are being researched for reduced production cost. Several other properties are also desirable in a good detector scintillator: a low gamma output (i.e., a high efficiency for converting the energy of incident radiation into scintillation photons), transparency to its own scintillation light (for good light collection), efficient detection of

10350-406: The molecules in the S state, which then decays to S 0 with the release of a scintillation photon. Since the T 0 -T 0 interaction takes time, the scintillation light is delayed: this is the slow or delayed component (corresponding to delayed fluorescence). Sometimes, a direct T 0  → S 0 transition occurs (also delayed), and corresponds to the phenomenon of phosphorescence . Note that

10465-414: The most dominant conversion process above E γ {\displaystyle E_{\gamma }} ~ 8 MeV. The μ o c {\displaystyle \mu _{oc}} term includes other (minor) contributions, such as Rayleigh (coherent) scattering at low energies and photonuclear reactions at very high energies, which also contribute to the conversion, however

10580-664: The naked eye measurement with the newly developed PMT . This was the birth of the modern scintillation detector. Scintillators are used by the American government as Homeland Security radiation detectors. Scintillators can also be used in particle detectors , new energy resource exploration, X-ray security, nuclear cameras, computed tomography and gas exploration. Other applications of scintillators include CT scanners and gamma cameras in medical diagnostics, and screens in older style CRT computer monitors and television sets. Scintillators have also been proposed as part of theoretical models for

10695-419: The necessity to seal the solution in an oxygen-free, airtight enclosure. The term "plastic scintillator" typically refers to a scintillating material in which the primary fluorescent emitter, called a fluor, is suspended in the base , a solid polymer matrix. While this combination is typically accomplished through the dissolution of the fluor prior to bulk polymerization, the fluor is sometimes associated with

10810-576: The number of emitted scintillation photons N in a single scintillation event can often be described by linear superposition of one or two exponential decays. For two decays, we have the form: N = A exp ⁡ ( − t τ f ) + B exp ⁡ ( − t τ s ) {\displaystyle N=A\exp \left(-{\frac {t}{{\tau }_{f}}}\right)+B\exp \left(-{\frac {t}{{\tau }_{s}}}\right)} where τ f and τ s are

10925-413: The number of events in the time bin t i (corrected by live time and detector efficiency), σ i is the corresponding Poisson uncertainty, accordingly corrected, and μ i is the expected number of events at that time bin, that depends on the background model and can be written as: μ i = [R 0 φ bkg (t i ) + S m cos(ω(t i − t 0 ))]M∆E∆t. Here, R 0 represents the non-modulated rate in

11040-430: The observational difference between delayed-fluorescence and phosphorescence is the difference in the wavelengths of the emitted optical photon in an S  → S 0 transition versus a T 0  → S 0 transition. Organic scintillators can be dissolved in an organic solvent to form either a liquid or plastic scintillator. The scintillation process is the same as described for organic crystals (above); what differs

11155-420: The order of 1 ps, which is much faster than the average decay time in photoluminescence . The second stage of scintillation is the charge transport of thermalized electrons and holes towards luminescence centers and the energy transfer to the atoms involved in the luminescence process. In this stage, the large number of electrons and holes that have been generated during the conversion process, migrate inside

11270-425: The organic-inorganic and all inorganic Pb-halide perovskites have various interesting scintillation properties. However, the recent two-dimensional perovskite single crystals with light yields between 10 000 and 40 000 ph/MeV and decay times below 10 ns at room temperature will be more favorable as they may have much larger Stokes shift up to 200 nm in comparison with CsPbBr 3 quantum dot scintillators and this

11385-431: The perimeter free-electron model (Platt 1949). This model is used for describing polycyclic hydrocarbons consisting of condensed systems of benzenoid rings in which no C atom belongs to more than two rings and every C atom is on the periphery. The ring can be approximated as a circle with circumference l. The wave-function of the electron orbital must satisfy the condition of a plane rotator: The corresponding solutions to

11500-411: The petroleum industry as detectors for Gamma Ray logs. There are many desired properties of scintillators, such as high density , fast operation speed, low cost , radiation hardness , production capability, and durability of operational parameters. High density reduces the material size of showers for high-energy γ-quanta and electrons. The range of Compton scattered photons for lower energy γ-rays

11615-426: The polymer directly, either covalently or through coordination, as is the case with many Li6 plastic scintillators. Polyethylene naphthalate has been found to exhibit scintillation by itself without any additives and is expected to replace existing plastic scintillators due to higher performance and lower price. The advantages of plastic scintillators include fairly high light output and a relatively quick signal, with

11730-476: The radiation being studied, a high stopping power , good linearity over a wide range of energy, a short rise time for fast timing applications (e.g., coincidence measurements), a short decay time to reduce detector dead-time and accommodate high event rates, emission in a spectral range matching the spectral sensitivity of existing PMTs (although wavelength shifters can sometimes be used), an index of refraction near that of glass (≈1.5) to allow optimum coupling to

11845-423: The resolution of the detector (the efficiency is the ratio of detected particles to the total number of particles impinging upon the detector; the energy resolution is the ratio of the full width at half maximum of a given energy peak to the peak position, usually expressed in %). The light output is a strong function of the type of incident particle or photon and of its energy, which therefore strongly influences

11960-465: The rest-mass energy of the electron, pair production starts to occur. Pair production is the relativistic phenomenon where the energy of a photon is converted into an electron-positron pair. The created electron and positron will then further interact with the scintillating material to generate energetic electron and holes. The attenuation coefficient contribution for pair production is given by: where m e {\displaystyle m_{e}}

12075-404: The result. The performance of a large set of Monte Carlo pseudo-experiments sampled from the background model guarantees that the fit is not biased. A frequency analysis have also been conducted, and the conclusion is that there is no statistically significant modulation in the frequency range searched in the ANAIS-112 data. ANAIS-112 sensitivity limitation is mostly due to the high background in

12190-451: The same energy deposited by electrons. Measurements of Quenching Factors (QF) in NaI scintillators are affected by strong discrepancies.  ANAIS-112 detectors QF are being determined after measurements at TUNL. In addition, a complete calibration program for the experiment using neutron sources onsite is being developed. ANAIS-112 published results are available in open access at the webpage of

12305-414: The same group, and others. In 2020,(C38H34P2)MnBr4 was reported to have a light yield up to 80 000 Photon/MeV despite its low Z compared to traditional all inorganic. Impressive light yields from other 0D OMHH have been reported. There is a great potential to realize new generation scintillators from this material class. However, they are limited by their relatively long response time in microseconds, which

12420-824: The scintillation of the base and then emit at larger wavelength, effectively converting the ultraviolet radiation of the base into the more easily transferred visible light. Further increasing the attenuation length can be accomplished through the addition of a second fluor, referred to as a spectrum shifter or converter, often resulting in the emission of blue or green light. Common fluors include polyphenyl hydrocarbons, oxazole and oxadiazole aryls, especially, n-terphenyl (PPP), 2,5-diphenyloxazole (PPO), 1,4-di-(5-phenyl-2-oxazolyl)-benzene (POPOP), 2-phenyl-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD), and 2-(4’-tert-butylphenyl)-5-(4’’-biphenylyl)-1,3,4-oxadiazole (B-PBD). Inorganic scintillators are usually crystals grown in high temperature furnaces , for example, alkali metal halides , often with

12535-482: The scintillation properties of 3-D and 2-D layered perovskites under X-ray excitation. MAPbBr 3 ( CH 3 NH 3 PbBr 3 ) emits at 550 nm and MAPbI 3 ( CH 3 NH 3 PbI 3 ) at 750 nm which is attributed to exciton emission near the band gap of the compounds. In this first generation of Pb-halide perovskites the emission is strongly quenched at room temperature and less than 1 000 ph/MeV survive. At 10 K however intense emission

12650-450: The scintillator. Vacuum photodiodes are similar but do not amplify the signal while silicon photodiodes, on the other hand, detect incoming photons by the excitation of charge carriers directly in the silicon. Silicon photomultipliers consist of an array of photodiodes which are reverse-biased with sufficient voltage to operate in avalanche mode , enabling each pixel of the array to be sensitive to single photons. The first device which used

12765-441: The sensitivity projection. ANAIS-112 results support the prospects of reaching a sensitivity above 3σ in 2022, within the scheduled 5 years of data taking. Several consistency checks have been carried out (changing the number of detectors entering into the fit, considering only the first two years or the last two years, or changing the time bin size), concluding that there is no hint supporting relevant systematical uncertainties in

12880-441: The spectral sensitivity range of a particular PMT, or B to increase the neutron detection efficiency of the scintillation counter itself (since B has a high interaction cross section with thermal neutrons ). Newer approaches combine several solvents or load different metals to achieve identification of incident particles. For many liquids, dissolved oxygen can act as a quenching agent and lead to reduced light output, hence

12995-669: The temperature. This dependence can largely be ignored for room-temperature applications since it is usually weak. The dependence on the temperature is also weaker for organic scintillators than it is for inorganic crystals, such as NaI-Tl or BGO. Strong dependence of decay time on the temperature in BGO scintillator is used for remote monitoring of temperature in vacuum environment. The coupled PMTs also exhibit temperature sensitivity, and can be damaged if submitted to mechanical shock. Hence, high temperature rugged PMTs should be used for high-temperature, high-vibration applications. The time evolution of

13110-423: The timing of the scintillation process. The charge transport phase is also one of the least understood parts of scintillation and depends strongly on the type material involved and its intrinsic charge conduction properties. Once the electrons and holes reach the luminescence centers, the third and final stage of scintillation occurs: luminescence. In this stage the electrons and holes are captured potential paths by

13225-418: The type of scintillation material to be used for a particular application. The presence of quenching effects results in reduced light output (i.e., reduced scintillation efficiency). Quenching refers to all radiationless de‑excitation processes in which the excitation is degraded mainly to heat. The overall signal production efficiency of the detector, however, also depends on the quantum efficiency of

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