Comparison and observations on the performance of LaBr3 and CeBr3 scintillation detectors *

When LaBr3(Ce) crystals hit the market a few years ago we were intrigued from its properties and resolution. We have invested quite some work to specify these detectors and we can now quantitatively deconvolute the peaks in the spectra. We "know" LaBr3(Ce) detectors. However, going into applications it turned out that these detectors are actually not so very useful.

The naturally radioactive isotope La-139 creates some peaks in the spectrum together with complex structures which come from the Beta-Minus (negatron) decay path. One prominent peak sits exactly at 1461 keV where also is the only peak from K-40. This natural nuclide must be quantified in most surveys from natural materials and the activity of K-40 is frequently taken as an internal standard. One cannot decompose K-40 from the La-139 peak (actually it is two peaks very close to 1460 keV) by any means, so the only possibility to quantify the K-40 contribution is to subtract the well-known activity of La-139. This procedure provides correct count rates for K-40, however, with a very large uncertainty. That is not tolerable for a value which is taken as an internal standard!

Luckily another scintillator material hit the market shortly after - CeBr3. 

Cerium Bromide scintillators feature very high light yields, fast response, and high density properties. The key advantage to the material, when compared to other high resolution scintillators, is its very low intrinsic background noise.CeBr3 is also fast without any slow components.The scintillators are hygroscopic and are available from BNC encapsulated with an entrance window, integrally coupled to a light sensor such as a PMT or SiPM, or fully integrated in detector assemblies with light sensor and front-end electronics. Sizes ranging from pixels for arrays to volumes as large as 102 x 127 mm are currently available.**

Whereas the resolution of LaBr3(Ce) is typically 3.9% (FWHM at 662 keV) which is a vast improvement over 7% for NaI(Tl), the CeBr3 material has typically 4.1% resolution, which is almost as good! CeBr3 has no intrinsic radioactive isotope in the crystal material and therefore the K-40 line is easily measured without any disturbance. The other properties of CeBr3 are almost identical to LaBr3(Ce), i.e. the signals are also very fast, the photon yield is much less temperature dependent than NaI(Tl), the crystal is hygroscopic, the efficiency of CeBr3 is almost twice the efficiency of NaI(Tl), and many others.

And: CeBr3 material is less expensive than LaBr3(Ce)! in particular for smaller detectors

Example: 1.5"x1.5" CeBr3 - 6064 €    /    1.5"x1.5" LaBr3(Ce) - 10200 €

The equilibrium question:

There is basically no seriously disturbed equilibrium in the Th deay chain. The halflife of Rn-220 is only one minute (55 sec) in which time not much radon gas emanates from a natural source. Normally you can take all daughter lines from Th-232 as being indicative of the thorium contents of the sample.

In the uranium decay chain, the half life of Rn-222, however, is very long (3.8 days) and in that long time plenty radon gas can emanate from a natural source. Peaks in the spectrum that one measures  from a U-containing source are almost all from nuclides that come after Rn-222 in the decay chain, i.e. they are affected by emanation. The only peaks which are not affected by emanation are the 186 keV line from Ra-226 and the 1001 keV line from Pa-234m. Thus, one can measure the efficiency function of the detector using all lines (daughters and Ra-226) and check by how much the 186 keV peak from Ra-226 is too high. This activity excess of the 186 keV peak, or in other words the activity deficiency of the other peaks, is indicative of the radon loss through emanation. We have made such determinations with  High Resolution Germanium (HPGe) detectors but never with a scintillator detector. That might be an interesting experience.

In the uranium decay chain there can actually be two separating points at which dis-equilibrium may occur. The first one is Ra-226 with its 1600 years half life. Radium behaves chemically like barium or calcium and it may chemically separate from the uranium-containing ore by weather or other chemical influence. The half life of 1600 years leaves ample time for chemical reactions and out washing of radium. Sometimes one finds radium-containing tails but no uranium in them. The other separating nuclide is Rn-222 where separation occurs through emanation of the noble gas from the source (see above). The only peak where one can try to see separation of radium from uranium is the 1001 keV peak from Pa-234m. Unfortunately this peak is very weak (0.82% only) and in scintillator spectra one does barely see it at all.

The precision of detection in nuclear measurements (i.e. statistical processes) is a function of measuring time. Longer measuring time will increase statistical precision and quality of data. There is no way how one can generalize statements on precision, except one defines the source strength, the detector efficiency and the measuring time. Then one may be able to estimate precision however, with a very large margin of uncertainty.

* The bulk of the material below is based on a study done by Dr. Wolfram Westmeier who is a leading authority in Germany in the area of Gamma and Alpha spectroscopy and from technical data and photos from Berkeley Nucleonics Corporation (BNC) in California, USA.

** Berkeley Nucleonics Corporation (BNC)