10 Questions You Should to Know about Light Guide For Scintillator Array

Best practices on DDA construction that contribute to achieving the best quality image possible

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Introduction

Digital detector arrays (DDAs) are becoming more prevalent in industrial digital radiography (DR) inspection processes. Although several factors in radiographic techniques, such as dose level or scatter control measures, have a profound impact on the radiographic process results, the type of scintillator used with a DDA determines the overall achievable image quality. In order to guide our selection for the best scintillator options, we investigated DRZ High, DRZ Plus, DRZ Standard, and DRZ Fine gadolinium oxysulfide terbium (GOS: Tb) activated scintillators using the same digital detector array panel (Carestream HPX-DR DDA). Three X-ray beam conditions were utilized for exposure: RQA-5 (70 kVp, 21 mm aluminum), RQA-9 (120 kVp, 1 mm copper and 4 mm aluminum), and NDT (220 kVp, 8 mm copper). DR image quality-related parameters such as detective quantum efficiency (DQE), modulation transfer function (MTF), sensitivity, and interpolated basic spatial resolution (iSRb) were determined for each scintillator and beam condition. We chose two common beam conditions from medical imaging that are used for DQE and MTF and also chose an NDT beam condition from ASTM E.

Before anything, what is a scintillator?

ASTM E Standard Terminology for Nondestructive Examinations defines a scintillator as “a detector that converts ionizing radiation to light.” Those detectors have direct use not only in the manufacturing of DDAs for digital radiography devices but also in radiology applications where real-time images are produced. Amorphous silicon detector arrays by themselves do not react to X-rays.  Amorphous silicon is only sensitive to visible light.  Therefore, the need for a scintillator that converts X-rays to visible light that the array can actually detect is relevant, and hence the importance of choosing the proper scintillator for your intended application. Once a choice of scintillator is made, it can’t be substituted by a different one because it is permanently built into the DDA. Scintillators utilized for DDAs have phosphors that are prompt emitting, meaning that when X-ray or gamma rays expose the scintillator, it immediately glows with visible light.

ASNT’s Nondestructive Testing Handbook, Vol. 3: Radiographic Testing, fourth edition expands the fundamental definition above with guidelines related to their desirable properties in the following terms: “Scintillators are materials that convert X-rays, gamma ray photons, or neutrons into visible-light photons, which are then converted to a digital signal using technologies such as amorphous silicon arrays, CCDs, or CMOS devices together with an analog-to-digital converter. Since there are various stages of conversion involved in recording the digital image, it is very important to ensure that minimal information is lost during conversion in the scintillator. The desired properties for ideal scintillators include the following: 1) High stopping power (high absorption and attenuation) for the desired radiation. A high percentage of heavy elements is typically required to achieve this for X-rays and gamma rays, 2) High X-ray to light conversion efficiency light yield), 3) Matched emission spectrum of the scintillator to the spectral sensitivity of the light collection device, 4) Low afterglow and burn in to avoid lag and ghosting in subsequent images or scenes, 5) Stable, linear performance with X-ray dose, 6) Temperature independence from light output, and 7) Stable mechanical and chemical properties.” Ghosting or detector image lag refers to the latent image that remains on the DDA for the current exposure derived from a previous exposure.

DDA construction and image quality principles 101

DDA technology has increasing acceptance within the NDE industry due to a series of workflow advantages. In parallel, nearly all medical radiological applications have converted to indirect DDAs.

DDAs are electronic imaging devices that convert X-rays or gamma rays to light, which is stored as a voltage and is subsequently sampled to form the digital image. [See top left in the first image in Figure 1(a)]. There are two common types of scintillators for radiology and radiography: cesium iodide thallium (CsI:Tl) and gadolinium oxysulfide terbium (GOS: Tb). CsI-type scintillators are not commonly used for industrial radiography because of ghosting issues above 150 kV; therefore, GOS-type scintillators are almost always chosen for NDT applications.

GOS-type scintillators are optically coupled to a piece of glass that contains a patterned array of pixels. Each pixel contains a photodiode, which senses the light from the scintillator, and a thin film transistor (TFT), which stores the charge created by the photodiode. The thickness and phosphor size of the scintillator has a profound influence on the overall quality of the digital image, which is a combination of brightness, sharpness, and noise. Although this is an important influence, scintillator type is often overlooked, as it determines whether or not the DDA can meet inspection requirements.

Detective quantum efficiency (DQE) provides a quantitative way to measure the detection capability of an imaging system for given exposure parameters and is a measure of image quality as a function of spatial frequency. Spatial frequency is a characteristic of any structure that is periodic across positions in space. The spatial frequency is a measure of how often sinusoidal components of the structure repeat per unit of distance (as determined by the Fourier transform). The SI unit of spatial frequency is cycles per meter (m). In image-processing applications, spatial frequency is often expressed in units of cycles per millimeter (mm) or equivalently line pairs per millimeter.

Mathematically, DQE is defined by the following equation (If you perceive it as challenging, please review the note on the inclusion of mathematical equations in articles describing practical applications in the white paper, “Imaging Plate Use for Radiographic Nondestructive Evaluation — Best Practices Directed to Achieve the Best Quality Image Possible Derived from our Hands-on Practical Experience”):

In this equation:

• DQE(f) denotes that the detective quantum efficiency is a function of spatial frequency,

• q is the density of incident quanta per unit area at the detector (flux),

• g is the system gain, that is related with the amplification of the received signal

• T is the MTF1, and

• NNPS is the normalized noise power spectrum.

1MTF is a way to measure the achievable detail that a system can obtain. The MTF describes the contrast of an image as a function of spatial frequency. MTF is calculated by measuring the edge response of a tungsten phantom with a very sharp angle. A line profile is drawn in the radiograph across the sharp angle, resulting in an edge spread function (ESF). Taking the derivative of the ESF results in a rate of change across the angled edge, which is known as the line spread function (LSF). An oversampled LSF has a Fourier transform applied to it to yield the contrast modulation as a function of spatial frequency, as the Fourier transform decomposes the LSF into the frequency domain.

Basic spatial resolution (SRb) is a measure of the amount of detail that can be seen in an image with a duplex wire gauge placed directly on the detector. The duplex wire gauge consists of several elements. Each element has two wires with a specific diameter and spacing between them. In an image, a line profile is drawn perpendicular to the elements. The element is said to be resolved if the intensity difference is greater than 20% of the wires against their background.

Description of the experiment and results obtained

Four different GOS-type scintillators were investigated. Three different X-ray beam conditions and four different scintillators were used. Table 1 presents GOS scintillator types.

The experiment aimed to explore a palette of several practical usages. The DRZ Fine, Standard, Plus, and High were investigated. The DRZ Plus screen is chosen for DDAs that utilize GOS for low-resolution applications. The DRZ Standard screen is utilized for some NDT applications; however, the DRZ Fine screen has been introduced for DDAs with smaller pixel pitch. We wanted to determine, through scientific analysis, the best scintillator for our HPX-DR DDA panel, which has a pixel pitch of 139 μm.

The four GOS-type screens were placed in pressure contact with a TFT photodiode array. Exposures were performed with standardized beam conditions, RQA-5 and RQA-9 from IEC -1. IEC is the International Electrotechnical Commission—which are used in medical radiology, and NDT, which is commonly used for industrial radiography. Individual images were acquired for the dark calibration, gain calibration, flat-field noise power spectrum, slanted-edge MTF target, and duplex wire gauge. The MATLAB software suite was utilized for the analysis of the DQE and MTF.

For the DQE comparisons, the exposure was adjusted to match the DRZ Plus signal level and the results obtained are described in the series of graphics that are included in Figure 2.

The overall image quality was measured by DQE analysis. Figures 2(a), 2(b), and 2(c) present the DQE results for the four GOS screens at the three beam conditions. The DRZ Standard scintillator had the best DQE above 2 cycles/mm. Below 2 cycles/mm, the DRZ Plus scintillator was the best choice as the DRZ High lacked sufficient sharpness for consideration. The DRZ Fine screen did not have suitable image quality except at very high spatial frequencies. As the kilovoltage increased, the overall achievable image quality decreased.

Figures 3(a), 3(b), and 3(c) present the MTF results for four different screens at the three beam conditions. The sharpness of the DRZ Fine scintillator was clearly the best across all spatial frequencies, followed by the DRZ Standard, DRZ Plus, and the DRZ High. The sharpness decreased at higher kilovoltages for all GOS screens.

Table 2 presents the interpolated basic spatial resolution results for the duplex wire gauge method. The four scintillator screens were tested using the three beam conditions.

Resolution below the pixel pitch of the detector was achieved with the DRZ Fine screen. The DRZ Standard screen gave a resolution near or below the pixel pitch, whereas the DRZ Plus screen resulted in a resolution above the pixel pitch. The DRZ High screen had a resolution much higher than the pixel pitch of the detector. In general, the sharpness became worse as the kilovoltage was increased.

The DRZ Standard GOS scintillator screen was the best choice for NDT applications utilizing the Carestream HPX-DR digital detector array panel. The choice of the DRZ Standard screen resulted in the best overall image quality above 2 cycles/mm, and with images that had sharpness that was at or below the pixel pitch of the detector.

Where can I innovate in my everyday work? Guidelines to select optimum scintillator type

The practical implications of the results obtained from this series of experiments can be expressed in the following terms [10]: “As the thickness and phosphor size of the GOS scintillator is changed, the brightness, sharpness, and noise of the image changes dramatically. Most radiographers do not realize that the scintillator choice determines the image quality of the DDA, and it is often overlooked and taken for granted. Thinner GOS scintillators with smaller phosphors will result in images that are sharper, with reduced brightness and improved noise uniformity. Likewise, thicker GOS scintillators with larger phosphors will result in images that have reduced sharpness, with increased brightness and degraded noise uniformity. The balance of the sharpness and the signal-to-noise ratio will determine the overall image quality. Therefore, the proper choice of scintillator determines whether or not the DDA can meet inspection requirements. The brightness of the scintillator helps to determine the amplification or gain of the system. Brighter scintillators can result in improved image quality if all else is equal. Relative to the DRZ Plus, the DRZ Fine was 2.0 times less sensitive, the DRZ Standard was 1.27 times less sensitive, and the DRZ High was 1.40 times more sensitive for the RQA-5 beam condition.

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Author

Brian S. White, Carestream NDT, Rochester, NY. USA,

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References

1. ASNT, , Nondestructive Testing Handbook, Vol. 3: Radiographic Testing, fourth edition, Chapters 9 and 11, Columbus, OH, American Society of Nondestructive Testing.

2. ASTM, , ASTM E – 22a: Standard Terminology for Nondestructive Examinations, West Conshohocken, PA, ASTM International.

3. ASTM, , ASTM E-22: Standard Practice for Determining Image Unsharpness and Basic Spatial Resolution in Radiography and Radioscopy, West Conshohocken, PA, ASTM International.

4. ASTM, , ASTM E-22: Standard Practice for Manufacturing Characterization of Digital Detector Arrays, West Conshohocken, PA, ASTM International.

5. ASTM, , ASTM E-21: Standard Practice for Radiographic Examination Using Digital Detector Arrays, West Conshohocken, PA, ASTM International.

6. ASTM, , ASTM E-17(): Standard Guide for Digital Detector Array Radiography, West Conshohocken, PA, ASTM International.

7. ASTM, , ASTM E-10(): Standard Practice for Digital Detector Array Performance Evaluation and Long-Term Stability, West Conshohocken, PA, ASTM International.

8. Dainty, J. C. and Shaw, R., Image Science: Principles, Analysis and Evaluation of Photographic Type Imaging Processes, Academic Press, New York, NY, .

9. White, B., M. Shafer, W. Russel, E. Fallet, J. Roussilhe, and K. Toepfer, , “Digital detector array image quality for various GOS scintillators,” ASNT Annual Conference Proceedings.

10. White, B., M. Shafer, W. Russel, E. Fallet, J. Roussilhe, and K. Toepfer, , “Imaging Plate Use for Radiographic Nondestructive Evaluation,” ASNT Annual Conference Proceedings.

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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)

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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)

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

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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.

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