LaBr3(Ce) gamma-ray detector for neutron capture therapy

Results of developing of a gamma-ray detector based on LaBr3(Ce) scintillation crystal for neutron capture therapy are presented. An energy resolution of the detector measured by photomultiplier tube Hamamatsu R6233-100 is showed. It was 2.93% for gamma line 662 keV from a source 137Cs. For radiative capture gamma line of isotope 10B (478 keV) and annihilation line (511 keV) the values were 3.33 and 3.24% respectively. Data analysis of gamma spectra for an estimation of energy resolution threshold required for visual identification gamma lines 478 and 511 keV was made.


Introduction
At the present time an increasing of cancer patients is observed. For their treatment different methods of radiotherapy based on the use of radiation sources with high ability are developing dynamically. During treatment session a maximum therapeutic effect is reaching by means of special pharmaceutical product and thermal neutron flux.
Every treatment session requires accurate dosimetry control of absorbed dose. For this purpose a gamma-detector based on scintillation LaBr 3 (Ce) crystal was designed for registration of gamma-rays with energy 478 keV. The gamma-rays appear as a result of interaction of thermal neutrons and the pharmaceutical product based on isotope 10 B. At the same time, high energy gamma-rays appear due to interaction of thermal neutrons with surrounding materials (for example, material detector and etc.). Positrons, appearing as a result of high energy gamma-rays interaction, annihilate with electrons and produce gamma-rays with energy 511 keV. Gamma-ray detector with high energy resolution is required for visual identification of the gamma lines with 478 and 511 keV.

Neutron capture therapy
The special pharmaceutical product based on isotope 10 B (the isotope has large thermal neutron capture cross section about 3800 barns [1]) is administered into the patient. After reaching of optimum concentration of preparation in tumour tissue it is exposed by thermal neutron flux irradiation. Result of interaction between boron nucleus and neutron is showed below:

The test results of the gamma-ray detector
Tests of the gamma-ray detector were carried out using different radiation sources from a set of standard calibration gamma-ray sources. The main purposes of this experiment were determination of energy resolution using scintillation crystal LaBr 3 (Ce) for gamma-rays with 478keV energy and estimation of capability detector to divide gamma lines with energies 478 and 511 keV. As results of experiment gamma spectra were obtained from isotopes: 137 Cs (figure 1), 60 Co, 133 Ba, 241 Am. Energy resolution for 137 Cs is (2.93±0.03)%. Based on results of experiment energy resolution as function of gamma-rays energy was fitted (figure 2).  Experimental data was approximated by the function: where a = 0.87±0.10 , b = 43±3 and c = 224±25. Energy resolutions of the gamma-ray detector for gamma-rays with energies 478 and 511 keV are (3.33±0.03)% and (3.24±0.03) % respectively. Monte-Carlo simulation of energy peaks from gamma-rays with energies 478 and 511 keV carried out to estimate the energy resolution threshold of gamma-ray detector required to identify overlapping gamma peaks [4]. Two Gaussian distributions were created with standard deviations with values obtained during the testing of the detector. Amplitude distributions were equal intensity for both gamma-ray lines with energies 478 and 511 keV. To estimate energy resolution threshold standard deviations is gradually increased with a step of 1.1%. After that new construction of the spectrum with updated standard deviations for Gaussian distributions was obtained (figures 3 and 4).  Further increasing of full width at half maximum for the peak (as a consequence of the deterioration of the energy resolution) was allowed to observe (figure 4) overlapping two gamma peaks. Finally visual identification without additional mathematical treatment is not possible. Threshold values of energy resolution for both gamma lines were (6.68±0.06) % and (6.58±0.06) % respectively.

Conclusions
According to the experiment the energy resolutions of the gamma-ray detector based on the scintillation crystal LaBr 3 (Ce) at energy 662 keV from gamma source 137 Cs are 2.93% and for energies 478 and 511 keV 3.33 and 3.24 % respectively. Energy resolution thresholds were obtained by Monte-Carlo simulation and are equal 6.68 and 6.58 % respectively. The gamma-ray detector based on scintillation crystal LaBr 3 (Ce) for neutron capture therapy allows to visually identify both gamma lines.