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).
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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)
EmissionMaximum
(nm)
DecayConstant
(1)
RefractiveIndex
(2)
ConversionEfficiency
(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 Cs2LiLaBr4.8Cl1.2 Ce (CLLBC)
4.08 420 120 ns220
0.63 µs/0.8 ns
1501.54
165
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 25-30no
(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:CeNaI(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.
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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.
Connect a signal generator (BK 2MHz function generator B) with a BNC “T” to the oscilloscope (you might want to ask for help at this point to get familiar with the oscilloscope) and to the input of the ADC at the back of the PC. Set the signal size to 1V and the rate to 100Hz. On the PC start the Maestro program and click on the start symbol to start taking data. You should see the channel contents growing. When you click on the stop symbol the data taking stops. Take about events. Change the signal size by 1 V and take data without clearing the spectrum. Continue to do this until you reach 5 V in steps of 1 V. You should see now a total of five sharp peaks, each corresponding to the various signal sizes. This allows you to calibrate the MCA in terms of signal size / channel. It also allows you to check the linearity of the ADC.
Fit a quadratic function ( using polyfit)
to your data. The quadratic term is a measure of the non-linearity of
the instrument. How much does it contribute and how large
is its uncertainty ? Make a plot of your experimental data and the
fitted result.
Now you are ready to take data with the scintillator and the PMT.
The photo multiplier requires high voltage (HV) in order to accelerate the secondary electrons to the anode. It is supplied by the Ortec 556 module. Before you turn it on, make sure the knobs are set to 0. Turn the unit on and then set it to 700-800V. The output of the PMT should be connected to the input of the Ortec 590A amplifier. Connect the output of the amplifier with a BNC ”T” to an Oscilloscope and continue the connection to the input of the ADC at the back of the PC. Place a 137Cs source on the scintillator and check if you see pulses on the oscilloscope. Make sure the signals are in the range of 2 – 5 V. What you see is the histogram (spectrum) of pulses described previously.
You should save this spectrum using from the menu. Make sure that you select to save an ASCII spectrum with the extension. You can load this file back into Maestro but you can also use it in LabTools for further analysis (look in the documentation). Identify the photo peak and the Compton edge. Replace the \(^{137}Cs\) source with a \(^{60}Co\) source. And take data for about 4 minutes.
Remove all the sources and take data for 5 minutes. The spectrum you obtained in such a way is a background spectrum and is present in all the spectra you took so far. You can subtract the background from the spectra you took if you know how long each spectrum was accumulated. You need to adjust your background spectrum to correspond to the same acquisition time as the one you took with the source. You can perform this subtraction either using the Maestro program (see the manual), or using the LabTools: working with MCA data
Show your subtracted spectra in your report.