10 Questions You Should to Know about Scintillation Crystal

Greeting
I have some questions:

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1- Does Geant4 have the ability to calculate the intrinsic energy resolution of scintillating crystals?

Optical/LXe example does not do this, in that example we should enter a multipiler manually, AddConstProperty(“RESOLUTIONSCALE”, 1.0);
.

2- What about total energy resolution including the effect of pmt and etc?

3- if Geant4 can’t do this, is there any other software suite for this purpose(FLUKA, or any other)

4-can you introduce any reference which have the reliable experimental energy resolution (total energy resolution) for common scintillation crystals (15-20 case)

Thanks very much!
Mohammad

I don’t think Geant4 is the right approach to this. The intrinsic energy resolution of a scintillator is determined by chemical, optical, molecular, etc. properties which is not the point of a physics package such as this. Even the optical is something Geant handles with empirical values that are from the same papers you could find online. I suppose you could account for crystal size and light transport in the optics libraries but the actual rise time, fall time, index of refraction, radiation length, etc. will all be empirical.

As independent noise sources, the PMT noise and electronic noise you have would just add in quadrature with the intrinsic energy resolution. It also will depend on which PMT and your electronics. The PMT would likely be from the manufacturer since a proper accounting for the fields would require sophisticated EM modeling software. The electronics is just easiest to measure with a pulse generator.

You can do parts of these with different Monte Carlo codes. Perhaps the easiest from an out of the box perspectrive would be GATE, which uses the reported resolution of electronics, PMT, timing, crystal material, etc. for system level. GATE is a wrapper for Geant4 intended for medical imaging so might be challenging to use in high energy physics, for example.

This is something you can find in most textbooks now including Knoll “Radiation Detection and Measurement”. Or any scintillator overview paper such as this one. Intrinsic energy resolution does not change with technological developments.

To add to what the others have said. While you can’t use Geant4 to simulate the intrinsic energy resolution, you can simulate the lowest values that the intrinsic energy could be. Note, you do need empirical data to do this though.

Intrinsic energy may be defined as a function of two terms, the non-unique manner that x/gamma rays deposit their energy and the non-unique manner that the secondary electrons deposit their energy. The former contribution is known as the non-proportionality term and may be estimated by convoluting Geant4 simulation results with the electron response curve of the scintillator of interest. This method was first performed using MCNP, Light yield nonproportionality of CsI(Tl), CsI(Na), and YAP | IEEE Journals & Magazine | IEEE Xplore, but folk have done this in Geant4 too [.] The Energy Response of LaBr3(Ce), LaBr3(Ce,Sr) and NaI(Tl) Crystals for GECAM.

This does require that you have measured the electron response curve for your materials of interest, this curve does somewhat depend on the manufacturing technique of the scintillator so you would expect it to vary depending on your manufacturer, Measurements of NaI(Tl) Electron Response: Comparison of Different Samples | IEEE Journals & Magazine | IEEE Xplore. Unfortunately, manufacturer’s do not provide the electron response curves in spec. sheets so you may be out of luck depending on what scintillator you want to model.

To reiterate, the papers linked simulate the non-proportionality term, NOT the intrinsic energy resolution, however you can somewhat capture the shape of intrinsic energy resolution as a function of energy by calculating the non-proportionality term.

The above image is taken from Intrinsic energy resolution of NaI(Tl) - ScienceDirect. Do be aware that you can’t actually directly measure intrinsic energy resolution, Moszynski had to make various assumptions to get the measured points in the above image.

Oh and if you are interested in trying to understand the various processes which contribute to pulse height variation I suggest you take a look at Non-proportionality in the scintillation response and the energy resolution obtainable with scintillation crystals | IEEE Journals & Magazine | IEEE Xplore.

John,

I may end up butchering the explanation but I will do my best, for an indepth explanation I would suggest looking at the first paper I linked.

In inorganic scintillators like NaI(Tl) or CsI(Tl) the number of scintillation photons produced is not proportional to the deposited energy. The electron response curve represents the relative number of photons produced for an electron of a given energy, and is measured using the Compton Coincidence technique, this curve represents the nonproportional response to energy.

The above plot from The light yield nonproportionality component of scintillator energy resolution | IEEE Journals & Magazine | IEEE Xplore.

The manner that x/gamma rays deposit their energy determines the number and energies of the secondary electrons produced. For example we expect a different number and energy of electrons from events where only photoelectric absorption occurred vs a case where the x/gamma ray Compton scattered then underwent photoelectric absorption. If the electron response was constant then the secondary electron distribution would not matter, however, since this is not the case the number of scintillation photons produced is somewhat dependent on how the x/gamma deposited it’s energy.

The intrinsic energy resolution accounts for this. Intrinsic energy resolution specifically refers to converting the energy of charged secondaries (mostly electrons) into light. The origin of the non-proportionality (or this) is the difficulty in accounting for the competing non-radiative transitions. The Bethe-Bloch formula leads to charged particles having very high LET near the end of their range. This ionization density is significantly higher than at any other point in the track(s) that cause distortions in the phase space for transitions. For organic scinatillors this would be breaks in carbon rings and in inorganic scintillators in the creation of highly localized traps. In both cases, quenching.

Put another way, there are not enough radiative centers near the end of the track and/or you are effectively destroying them. This effect is always present but diminishes at higher energies because the loss (on average) becomes a smaller fraction of the total deposited energy. This is what your plots show as well. Stephen Derenzo has decades of work modelling this but there are still required empirical inputs.

Justin,

I have been reading through the first paper you linked, and am happy to have done so! I won’t be needing to fit arbitrary functions to non-linearity curves anymore and can use a physics based one instead! Please correct me if I am wrong but it appears to me the first paper had a terminology change, it appears that what Stephen defined as the non-proportionality term is equivalent to the intrinsic energy resolution as defined in other literature.

In the first paper I linked, Valentine defined two intrinsic energy resolutions, the photon and electron intrinsic energy resolution. Perhaps this is the source of confusion?

In the equation for detector energy resolution listed in the Dorenbos paper I linked, the intrinsic energy resolution term represents the photon intrinsic energy resolution. Hence, I always thought that when folks discussed the intrinsic energy resolution they were talking about the photon intrinsic energy resolution. So maybe I was working with different definitions?

Sure. I guess that definition works but it will cause confusion to name it “photonic” such as @allison. Photonic typically refers to atomic states and gamma to nuclear states. There are thousands or tens of thousands of photonic states being accessed with every incident single gamma ray (from one source nuclear state transition). The number of primary electrons pushes the electrons to “start” further down the curve you have above. And since it is nonlinear, 2 photons will lose more energy than a single photon with the energy of both.

I’ll need to think on x-rays, I think PE absorption is so strong for most materials that the effect is even less pronounced.

Mechanical, optical and scintillation properties

The most widely used scintillation material for gamma-ray spectroscopy NaI(Tl) is hygroscopic and is only used in hermetically sealed metal containers to preserve its properties. All water soluble scintillation materials should be packaged in such a way that they are not attacked by moisture. Some scintillation crystals may easily crack or cleave under mechanical pressure whereas others are plastic and only will deform like CsI(Tl).

In table 3.1 below, the most important aspects of commonly used scintillation materials are listed. The list is not extensive and new materials are developed regularly.

Physical properties of the most common scintillation materials

Material Density

(g/cm3)

Emission

Maximum

(nm)

Decay

Constant

(1)

Refractive

Index

(2)

Conversion

Efficiency

(3)

Hygroscopic NaI(Tl) 3.67 415 0,23 µs 1.85 100 yes CsI(Tl) 4.51 550 0,6/3.4 µs 1.79 45 slightly CsI(Na) 4.51 420 0.63 µs 1.84 85 yes CsI(Undoped) 4.51 315 16 ns 1.95 4-6 no Cs2LiYCl6:Ce

(CLYC)

3.31 275-450 nm 1,50, ns 1.81 30-40 yes CaF2(Eu) 3.18 435 0.84 µs 1.47 50 no LaCl3:Ce(0.9) 3.79 350 70 ns 1.90 95-100 yes SrI2(Eu) 4.60 450 1-5 µs 1.85 120-140 yes LaBr2.85Cl0.15:Ce (LBC) 4.90 380 35 ns 1.90 140 yes 6Li-glass 2.6 390/430 60 ns 1.56 4-6 no Cs2LiLaBr4.8

Cl1.2 Ce (CLLBC)

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4.08 420 120 ns
500 ns 1.90 84 yes 6Li(Eu) 4.08 470 1.4 µs 1.96 35 yes BaF2 4.88 315

220

0.63 µs/

0.8 ns

150

1.54

16

5

no CeBr3 5.23 370 18 ns 1.9 130 yes YAP(Ce) 5.55 350 27 ns 1.94 35-40 no LYSO:Ce 7.20 420 50 ns 1.82 70-80 no BGO 7.13 480 0.3 µs 2.15 15-20 no CdWO4 7.90 470/540 20/5 µs 2.3 25-30 no PbWO4 8.28 420 7 ns 2.16 0.20 no Plastics(*) 1.023 375-600 ns range 1.58 25-30

no

(1) Effective average decay time for γ-rays.
(2) At the wavelength of the emission maximum
(3) Relative scintillation signal at room temperature for γ-rays when coupled to a photomultiplier
tube with a bi-alkali photocathode.
(*) approximate data

Each scintillation crystal has its own specific application. For high resolution γray spectroscopy, NaI(Tl), or CeBr3 (high light output) are often used. For high energy physics applications, the use of bismuth germanate Bi4Ge3O12 (BGO) crystals (high density and Z) or Lead Tungstate (PbWO4) improves the lateral confinement of the shower. For the detection of β-particles, CaF2(Eu) or YAP:Ce can be used instead of plastic scintillators (higher density).

Scintillation materials and their most common applications

​

Material Important properties Major Application NaI(Tl) Very high light output, good energy resolution General scintillation counting, Health Physics, environmental monitoring, high temperature use CsI(Tl) Non-hygroscopic, rugged Particle and high energy physics, general radiation detection, photo diode readout, CsI(Na)  High light output, rugged Geophysical, general radiation detection CsI(Undoped) Fast, non-hygroscopic Physics (calorimetry) CaF2(Eu) Low Z, high light outputβ detectors, α/β phoswiches β detectors, α/β phoswiches Cs2LiYCl6:Ce
(CLYC) Neutron detection capability High resolution Nuclear identifiers, Physics LaCl3:Ce(0.9) Very high light output, very good energy resolution High resolution scintillation spectroscopy, Health Physics environmental monitoring CeBr3 Very high light output, very good energy resolution, low background High resolution spectroscopy, low background applications 6Lil(Eu) High neutron cross-section, high light output Thermal neutron detection and spectroscopy LaBr2.85Cl0.15:
Ce (LBC) Bright, high resolution scintillator (La-138 background) High resolution gamma spectroscopy Cs2LiLaBr4.8
Cl1.2 Ce
(CLLBC) High resolution scintillator with neutron capabilities Physics, security SrI2(Eu) Bright, high resolution scintillator High resolution gamma Spectroscopy 6Li-glass High neutron cross section, non hygroscopic Physics, security BaF2 Ultra-fast sub-ns UV emission Thermal neutral detection YAP(Ce) High light output, low Z, fast Positron life time studies,physics, fast timing LYSO High density and Z, fast Mhz-X-ray spectroscopy, synchrotron physics BGO High density and Z Physics resarch, PETT, High Energy Physics CdWO4 Very high density, low afterglow. Slow decay times Particle physics, geophysical research PET, anti- Compton spectrometers. PbWO4 Fast, high density, low afterglow DC measurement of   x-rays (high intensity), readout with photodiodes, Computerized Tomography (CT) Plastics Fast, low density and Z high light output Physics research (calorimetry). General counting, particle and neutron detection.

NaI(Tl) scintillation crystals are used in a great number of standard applications for detection of γ-radiation because of their high light output and the excellent match of the emission spectrum to the sensitivity of photomultiplier tubes, resulting in a good energy resolution. In addition NaI(Tl) is a relatively inexpensive scintillator. NaI(Tl) crystals show a distinct non proportionality (see below) which results in a limitation of the energy resolution at 662 keV to about 6% FWHM, NaI(Tl) crystals can be grown to large dimensions (400 mm diameter) in ingots of many hundreds of kg. The material can be cut in a great variety of sizes and shapes and cleaved in small diameters.

CsI(Tl) has the advantage that it not really hygroscopic (its surface however is influenced by humidity on the long term),and does not cleave or crack under stress. It is a relatively bright scintillator but its emission is located above 500 nm where PMTs are not that sensitive. However due to this property it can effectively be read out by silicon photodiodes or SiPms. Thanks to its different decay times for charged particles having a different ionizing power, CsI(Tl) crystals are frequently used in arrays or matrices in particle physics research.

CsI(Na) is a hygroscopic high light output rugged scintillator Like CsI(Tl) mainly used for applications where mechanical stability and good energy resolution are required. Below 120 oC it is an alternative to NaI(Tl). CsI(Na) has its emission peaking at 400 nm like NaI(Tl).

Undoped (pure) CsI is an intrinsic scintillator with same density and Z as CsI(Na). It has en emission at approx. 300 nm and since it intensity is strongly thermally quenched at room temperature it is relatively fast (ns decay time). There is a slow component present in this crystal that makes up at least 10% of the total light yield. The emission spectra below show how the emission spectrum of a scintillator can be influenced by its type of activation.

CaF2(Eu) , Europium doped calcium fluoride is a rather old low density scintillation crystal . Thanks to its low Z value it is well suited for the detection of electrons (beta particles) with a high efficiency (low backscatter fraction). CaF2(Eu) is a relatively slow scintillator that is not hygroscopic and inert to many chemicals. It is brittle and cleaves relatively easy.

(6) LiI(Eu) is used for the detection of thermal neutrons via the reaction

The total Q-value of the alpha and the triton is 4.78 MeV. The resulting thermal neutron peak can be found at a Gamma Equivalent Energy larger than 3 MeV. This allows to separate neutron interactions from gamma events (< 2.6 MeV). Since the typical absorption length (90%) of thermal neutrons in 6-LiI(Eu) crystals is only 3 mm the efficiency for gamma rays can be made small. LiI(Eu) crystals are grown up to 25 mm in diameter.

6-Li glass scintillators offer the same possibility as 6LiI(Eu) crystals to detect thermal neutrons. However, The light output is much lower than of LiI(Eu) scintillators and therefore the neutron peaks are relative broad. In addition the scintillation efficiency for the resulting particles is low so that the neutron peak appears at a location of approximately 1.6 MeV in the gamma energy spectrum. 90% of thermal neutrons are absorbed in only1 mm of material.

All 6-Li containing scintillators can also be used for the detection of fast neutrons but the efficiency of the nuclear reaction is smaller.

Further details on neutron detection can be found in the application note “neutron detection with scintillators”.

Barium Fluoride (BaF2) is a non-hygroscopic scintillator with a very fast decay component located at 220 nm. To detect this component, light detectors with quartz windows are used.
Barium fluoride detectors allow fast sub-nanosecond timing for example for positron life time measurements. It is a weak scintillator with a modest energy resolution at 662 keV (typically about 10-12 % FWHM @ 662 keV.

BGO (Bi4Ge3O12) has the extreme high density of 7.13 g/cm3 and has a high Z value which makes these crystals very suited for the detection of natural radioactivity (U, Th, K), for high energy physics applications (high photo fraction) or in compact Compton suppression spectrometers. Since the light output of BGO is modest, the energy resolution is inferior to that of the the standard alkali halides like NaI(Tl) or CsI(Tl).

YAP:Ce (YAlO3:Ce) is a high density (5.5 g/cm3) oxide crystal with a decay time about 10 times shorter than NaI(Tl) (23 ns) It is used in detectors for high count rate (up to several MHz) The non-hygroscopic nature of this material allows the use of thin mylar entrance windows. YAP:Ce can withstand gamma doses up to 104 Gray.

High resolution (proportional) scintillators

Currently there is an increased better understanding of the properties of scintillators and what determines their intrinsic energy resolution. A number of materials have been developed that exhibits a more proportional response to gamma rays than the classic alkali halides (NaI(Tl), CsI(Tl) etc). This has resulted in the availability of a class of proportional scintillators. New materials are being developed constantly and the list below is not extensive.
Bright proportional scintillator scan have energy resolutions around 3-4 % at 662 keV gamma rays under optimum light detection conditions. Just as other scintillators each have some advantages and disadvantages. Some typical proportionality curves are shown below:

Ref. W. Mengesha, T.D. Taulbee, B .D. Rooney, and J.D. Valentine.Light Yield
Nonproportionality of CsI(Tl), CsI(Na), and YAP IEEE Trans. Nucl. Sci. vol 45, no. 3,
() pp. 456–461

Proportional scintillators only offer their superior performance in energy resolution when the light detection is optimized by covering the largest possible area with light detector (PMT or SiPm).

LBC (Lanthanum BromoChloride) LaBr2.85Cl0.15:Ce scintillators have similar properties to the well-known LaBr3:Ce crystals. Energy resolutions around 3.0% FWHM (662 keV) are standard and the material is mechanically a little stronger than LaBr3. LBC crystals suffer from the same La-138 background as LaBr3

CeBr3 (Cerium Bromide) scintillators are characterized by a relatively high density and Z and a proportional response to gamma rays. Typical energy resolutions are 4% FWHM for 662 keV.

The material exhibits a fast decay of typical 20 ns (for 51 mm crystals) with a negligible afterglow. CeBr3 is highly hygroscopic and provides the best performance when integrally coupled to PMTs. Thanks to its fast light pulse rise time, CeBr3 detectors can provide sub nanosecond time resolutions, slightly worse than BaF2 detectors. With CeBr3 scintillators the 609 and 662 keV gamma lines from respectively radium and Cs-137 can easily be separated.

Cs2LiYCl6:Ce (CLYC) scintillation crystals offer a reasonable density of 3.3 g/cc. This proportional crystal offers an energy resolution of 4.5 – 5 % FWHM for 662 keV gamma rays. The thermal neutron peak due to the n-6Li reaction produces a narrow peak at approximately 3.3 MeV. Its fast scintillation component is not excited by neutrons which opens PSD capabilities or further improve the neutron/gamma separation. CLYC has some slower emission components so larger signal shaping times are required. To absorb 90% of thermal neutron 12.5 mm of crystal is needed.

Cesium Lanthanum Lithium BromoChloride) CLLBC , Cs2LiLaBr4.8Cl1.2:Ce scintillators have properties to the well-known LaBr3:Ce crystals. Energy resolutions around 3 % FWHM (662 keV) are standard. In addition, thanks to the presence of Lithium, the material can be used for neutron detection with a sharp thermal neutron peak between 3.1- 3.2 MeV. In addition, CLLBC offers excellent neutron / gamma discrimination using PSD.

SrI2(Eu), Europium doped strontium iodide Is a very bright relatively slow scintillator with a very good proportionality. Typical energy resolutions are 3.5% @ 662 keV and 6% @ 122 keV. The material is quite radiopure. Due to its intrinsic self-absorption (small stokes shift), the crystal requires some special surface preparation techniques. The long decay time requires very long (digital) shaping time constants (> 10 µs) which complicates high count rate behavior. The self-absorption limits the maximum size of the crystal to approx. 4 cm.

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Organic (plastic) scintillators

Organic scintillators (also called “plastic scintillators”) consist of a transparent host material (a plastic) doped with a scintillating organic molecule (e.g. POPOP : pbis [2(5phenyloxazolyl)] benzene). Radiation is absorbed by the host material, mostly via Compton effect because of the low density and Z value of organic materials. Therefore, plastic scintillators are mostly used for the either detection of β and other particles or when very large volumes are needed since their material cost is relatively low.

Plastic scintillators are mainly used when large detector volumes are required e.g. in security or health physics applications. The cost of plastic scintillation detectors (per volume) is much smaller than that of e.g. NaI(Tl) detectors; plastic scintillators can be manufactured in several meter long slabs.

There exists a large number of different organic scintillators each with specific properties, the materials listed on the SCIONIX web site here are a direct copy of the ELJEN website . SCIONIX is the European representative of ELJEN Technology.

Organic scintillators can be doped with specific atoms like 6-Lithium (EJ-270) or Boron (EJ254) to make them neutron sensitive or with Pb (EJ-256) to improve the response at lower energies (tissue equivalent). This influences the scintillation properties.

Also, plastic scintillators exist that can be used to discriminate gammas from fast neutrons via pulse shape analysis which is used in physics research and in some security applications. An example is EJ-276 (successor of EJ-299-33). See the datasheet on these materials.

Liquid scintillators

Also doped liquids are used as scintillators. Some liquid scintillators like EJ301 or EJ309 offer fast neutron/ gamma discrimination properties based on their scintillation pulse shape. Using proper electronic techniques (digitizers), neutron pulses can be discriminated from gammas.

Liquid scintillation detectors need provisions to allow expansion of the liquids under temperature variations. For further information see the technical datasheet of liquid scintillators.