July 27th, 2022 - Cerium Bromide and Sodium Iodide Detectors

Cerium Bromide (CeBr₃) and Sodium Iodide (NaI[Tl]) are very popular scintillator detectors when used in room temperature spectroscopy systems. Both of these detectors have similar attributes such as:

• Exceptionally low background which results in excellent sensitivity.

• Available in large volumes up to 3x3 inch detectors and much larger.

• Good efficiency with high Z material which results in good stopping power for gamma spectroscopy.

• Good room temperature energy resolution making hand-held radioisotope identification devices (RIIDs) very effective.

• Both these detectors can be packaged in small, light-weight enclosures for ease in handling.

Low Background

CeBr₃ and NaI(Tl) detectors exhibit very Low background from small amounts of actinium (Ac) contamination.  Typical contamination results in background levels of about 0.02 counts/sec/cm³.  Further reduction of Ac gives an additional background reduction of about 0.001 counts/sec/cm³.  This low background material yields excellent sensitivity in the detection of gamma-rays.  Detector materials composed of lanthanum can exhibit background levels exceeding 100 times these levels which substantially lowers sensitivity across the entire spectrum.

Large Volume Detectors

Sensitivity can also be improved with the employment of large detectors.  Today NaI(Tl) detectors are readily available in sizes of 4x4x16 inches and larger.  CeBr₃ detectors are available in 3x3 inch sizes and larger.  These high-performance detectors are available through BNC.

Gamma Stopping Power

The high density of these detector materials allows good absorption of gamma-rays which is related to the atomic number (Z).  The gamma stopping power is directly related to Z².  Many detector materials are a compound, so the term “effective Z” is generally used.  For example, CeBr₃ has an effective Z of 45.9 giving a stopping power of 2107.  NaI(Tl) has an effective Z of 50.8, giving a stopping power of 2581.

Energy Resolution

Good energy resolution is always important for good radioisotope identification algorithms. CeBr₃ detectors give about 4% energy resolution FWHM, at 662 keV, whereas NaI(Tl) is typically 7%.  NaI(Tl) performs well at high energies and where photopeaks are not closely spaced.  Therefore, NaI(Tl) is best used when high sensitivity, efficiency and lower cost are most important – especially when very large detectors are required.  It is not unusual to maintain 7% resolution on very large NaI(Tl) detectors (e.g., 4x4x16 inch detectors).

Below is an example of CeBr₃ resolution showing separation of the ²¹⁴Bi, 609 keV peak from ¹³⁷Cs, 662keV peak. These closely spaced energy lines cannot be separated with NaI(Tl) detectors.

RIIDs

These room temperature detectors lend themselves to light-weight packaging for ease in handling.  This reduction in size is made possible by the use of a Silicon Photo-Multiplier (Sipm).  Replacing the large Photomultiplier tubes has been accomplished for some years, usually for alpha and beta counters.  More recently efficient Sipm scintillators for gamma spectroscopy are available making possible hand-held products like the SAM 940+ shown below.  In addition, the packaging can be kept small by adding other special attachments (e.g., the alpha pancake detector as shown below).