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Selecting Gamma Detector

The choice of gamma ray detector depends on a number of factors including their efficiency (determined in part by their density) and their peak resolution (determined in part by their light outut). In addition, other detector material properties are important to consider such as whether the detector is rugged to thermal and mechanical shock and whether it is hygroscopic (absorbs water). Of course, price may also be a consideration in selecting a detector type.


For gross counting only, there are several choices of detectors. Some applications require large area detectors such as large stationary systems. Here, the only choices are PVT plastic scintillator or NaI(Tl). A smaller NaI(Tl) detector can provide equivalent sensitivity to a PVT detector at higher energies because of the NaI(Tl) detectors better energy resolution allowing the use of regions of interest to improve the signal to background. However, at lower energies, the larger area detector will always provide a better sensitivity. As NaI(Tl) is more expensive than PVT, to have the same geometric efficiency may be cost prohibitive. On the other hand, if nuclide identification is needed, the NaI(Tl) detector will be required. Nucsafe can provide recommendations for the type and size of gamma ray detector needed for your application.


For portable systems, Nucsafe uses NaI(Tl), BGO and CsI detectors. Each has advantages and disadvantages. Detector resolution and intrinsic efficiency are the most important considerations if size is equal. Energy resolution of a scintillator detector is a function of the intrinsic crystal resolution which is related to its light output, but is also affected by the photomultiplier tube and its photocathode which has a variable probability that a visible photon will produce a photoelectron that can be collected in first dynode stage of the PMT.


Detector Selection Table

 

NaI(Tl)

BGO Bi4GeO12

CsI(Tl)

CsI(Na)

PVT

Portable Radiation Search Tool

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PRST with nuclide ID

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Transportable Radiation Monitors

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TRMS with nuclide ID

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Continuous Radiation Monitors

 

 

 

 

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CRMS with Nuclide ID

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Handheld Gamma Spec

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NaI(Tl) is recommended for all nuclide identification applications because it provides the best currently available energy resolution for gamma rays in a room temperature detector that is relatively inexpensive and available in a wide variety of sizes. BGO is heavier and thereby has higher intrinsic efficiency at higher energies, useful for measuring the 2.6 MeV gamma ray associated with highly enriched Uranium (HEU). CsI has good light output but poorer resolution than NaI, however it can be used with electronics that provide very low power consumption for portable applications. The detector selection table provides some physical parameters for these detectors.


Detector Parameters Table

 

NaI(Tl)

BGO Bi4GeO12

CsI(Tl)

CsI(Na)

PVT

Density [g/cm³]

3.67

7.12

4.51

4.51

1.03

Melting point [k]

924

1050

894

894

75

Hardness (mho)

2

5

2

2

0

Hygroscopic

yes

no

slightly

yes

no

Wavelength of emission max. [nm]

415

480

565

420

423

Refractive index at emission max.

1.85

2.15

1.80

1.80

1.58

Primary decay time 1/e [µs]

0.23

0.30

1.00

0.63

0.0024

Afterglow (after 6 ms) [%]

0.3-0.5

0.005

0.5-5.0

0.5-5.0

0.01

Typical resolution % FWHM 137Cs

6

10

8

9

180

Light yield [photons/MeVy]

38,000

8200

52,000

39,000

10,000


For gross counting applications only, the following table provides a comparison of NaI(Tl) and BGO for varying gamma ray energies of interest for 1” and 2” thick detectors. Note the biggest difference is in the higher energy gamma rays because both types of detectors are nearly 100% efficient at lower gamma ray energies.


Calculated Photopeak Efficiency for NaI(Tl) vs BGO

 

NaI(Tl) 1"thick

BGO 1"thick

NaI(Tl) 2"thick

BGO 2"thick

186keV

97%

99%+

99%+

99%+

1.46MeV

35%

62%

58%

85%

2.614MeV

30%

53%

51%

78%


Organic Plastic Scintillator


Polyvinyl Toluene (PVT) is a type of organic scintillator. PVT is not the only organic plastic scintillator, but clearly the most commonly employed for radiation measurement systems. It is dissolved in a solvent and subsequently polymerized forming the equivalent of a solid solution. These “plastic” scintillators are easy to shape and fabricate and are often the only practical choice if very large volume solid scintillators are required. When making very large detectors, the self-absorption of light may no longer be negligible and must be accounted for in the design of the system. For good quality PVT, detectors of more than a meter in length can be made as a single detector. Some designs requiring uniform response to radiation over a given height and width may require designs using more than one detector to ensure constant count rates independent of the radiation source position.


Photon production in organic scintillator is a molecular process in which the energy deposited by a charged particle or photon, not dissipated as heat, results in an excited state in which the excess energy is carried away as an emitted photon. This fluorescent emission produces approximately 1 photon per every 100 eV of the energy deposited. Because the energy required to produce an excited state exceeds that carried away by photons, the probability for re-absorption of the emitted photon is small; i.e., the scintillator is transparent to the light that it generates.


Secondary scintillators are frequently used to "shift" this emitted light to longer wavelengths, near the peak of a Photomultiplier (PMT) spectral-response curve. These secondary fluors have a high absorption cross section at the wavelengths generated by the primary scintillator, and respond to the energy deposited in exactly the same manner as described above, except that all energy levels are somewhat lower (wavelengths are longer) making the emitted light more compatible with the response of the PMT.


PVT plastic scintillator has a polyvinyl toluene base with characteristics as follows:


Density - 1.032 g/cm3
Electrons/cc - 3.39 x 1022
H atoms/cc - 5.28 x 1022
C atoms/cc - 4.78 x 1022
Index of refraction - 1.581
Attenuation length (l/e) - ~ 43 cm
Wavelength of emission - 423 nm
Rise time - 0.9 ns
Decay time - 2.4 ns
Luminescent efficiency - ~15% of NaI(Tl)

Inorganic Scintillators


Scintillation in inorganic materials depends on the crystal lattice of the detector material.In order to scintillate (create light in response to energy deposited from radiation interactions), electrons in the material must be able to move from their discrete energy bands. In some cases the energy difference between these bands is too large and the emitted photon would not be in the visible wavelengths. The two energy bands are called the valence band in which electrons are essentially bound to the lattice atoms and the conduction band where electrons are free to move through the material. There exist intermediate bands of energies called the forbidden band. Electrons in the inorganic lattice cannot be in these bands in a pure crystal.


To enhance the efficiency of light emission and to ensure the emitted photon is of the appropriate wavelength, small amounts of an impurity element can be added to the crystal, these impurities are called activators. Activators create special sites in the lattice at which the normal energy band gap is modified allowing energy states in the forbidden band through which an electron can de-excite back to the valence band emitting a visible flash of light ( a scintillation).


Energy Resolution


An important aspect of a g -ray spectrometer is the ability to discriminate between g-rays with slightly different energy. This quality is characterized by the so-called energy resolution which is defined as the width (FWHM) of the photopeak at certain energy.


Besides by the g -ray energy, the energy resolution is influenced by:

The light output of the scintillator,
The size of the scintillator (light collection),
Photomultiplier characteristics (quantum efficiency and photocathode homogeneity)


At low energies where photoelectron statistics dominate the energy resolution, the energy resolution is roughly inverse proportional to the square root of the g-ray energy.


The energy resolution of a scintillation detector is a true detector property, limited by the physical characteristics of the scintillator and the PMT or other readout device.


A typical energy resolution for 662 keV g-rays absorbed in small NaI(Tl) detectors is 7.5 % FWHM. At low energies, e.g. at 5.9 keV, a typical value is 45 % FWHM. At these low energies, surface treatment of the scintillation crystal strongly influences the resolution. It is clear that especially at low energies, scintillation detectors are low resolution devices unlike Si(Li) or HPGe detectors.


Peak-to-valley Ratio


A sensitive way to check the energy resolution of a scintillation detector is to define a so-called peak-to-valley (P/V) in the energy spectrum. This criterion does not depend on any possible offsets in the signal. Either the peak-to-valley between two gamma peaks is taken or the ratio between a low energy peak and the PMT / electronics’ noise. A good P/V ratio for a 76 x 76 mm NaI (Tl) crystal is 10 : 1. This is equivalent to an energy resolution of7.0 % at 662 keV.

 

Sodium Iodide( Nal(Tl)) Detectors

 

NaI is an alkali halide inorganic scintillator with a relatively high Z from iodine (53). This results in high efficiency for gamma-ray detection. A small amount of TI is added in order to activate the crystal, so that its formula is typically shown as Nal(TI)l. Energy resolution of about 6% for the 662-keV gamma ray from 137Cs is achievable for a 3-in.-diameter by 3-in.-long crystal and slightly worse for smaller or larger sizes.


The light decay time constant in Nal is about 0.23 µs. Typical charge-sensitive preamplifiers translate this into an output pulse rise time of about 0.5 µs. Fast coincidence measurements cannot achieve the very short resolving times that are possible with plastic, especially at low gamma-ray energies. Nucsafe offers various sizes of Nal(TI) detectors from 2x2” to 4x4x16” arrays depending on your application.


Bismuth germanate (BGO)


BGO is a scintillation crystal that features high density due to the high Z of Bismuth (83). Its high intrinsic efficiency at high gamma ray energies make it an excellent alternative to NaI for gross counting applications. BGO (bismuth germanate oxide) has a very fast response and little or no afterglow. Because of its speed of response and high density it is ideal for high energy and high rate gamma ray measurements. The only disadvantage of BGO is its low conversion efficiency (i.e. low light output) which results in poorer energy resolution than NaI detectors; making it less desirable for nuclide identification applications.


Cesium (CsI) Iodide


Like NaI, CsI is another alkali halide inorganic scintillator. CsI also requires a small amount of thallium or sodium activator for proper operation. Each activator results in a scintillation detector with varying properties. CsI has a higher intrinsic efficiency than NaI but less than BGO. Csl has poorer energy resolution than NaI. However, CsI has much faster pulse light decay and finds use in timing applications but the wavelength of its light is poorly matched to most PMTs. It is also less brittle than NaI and thereby may be subjected to more severe shock and vibration conditions than NaI detectors. It is less hygroscopic than NaI but more than BGO.

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