Cross talk experiment of array-type CdTe detector for BNCT-SPECT. Experimental design

BNCT is a new radiation therapy which can destroy only tumor cells and will not damage healthy cells. This therapy is not yet established as a usually utilized treatment at present, because it has some very serious unsolved problems. One of them is that the treatment effect cannot be known during BNCT in real time. We are now developing a SPECT system to measure it, named BNCT-SPECT, with a cadmium telluride (CdTe) semiconductor detector. BNCT-SPECT can obtain a three-dimensional image for the BNCT treatment effect by measuring 478 keV gamma-rays emitted from the excited state of ⁷Li nucleus created by ¹⁰B(n,α) reaction. In the previous study, we investigated the feasibility of the BNCT-SPECT system. As a result, the estimated count rate of 478 keV gamma-rays was sufficiently large being more than the target value of over 1000 counts/hour. However, the ancillary target value of S/N ratio did not meet the target value (S/N >1). We then tried to improve the S/N ratio, because we confirmed that deterioration of the S/N ratio was caused by the influence of Compton scattering especially due to capture gamma-rays of hydrogen. To solve this problem, we produced an arrayed detector with two CdTe crystals to test anti-coincidence detection technique. We then carried out anti-coincidence measurement with a standard gamma-ray source and confirmed possibility of reduction of noises formed by Compton continuum. However, it was difficult to discuss the result appropriately, because agreement was not sufficient between experiment and analysis. For more precise analysis for the anti-coincidence detection, we designed and made a collimator having a similar performance to the real BNCT-SPECT. The collimator size was designed to be 14 cm at maximum and the hole diameter is 2 mmφ. Now we are carrying out experiments with the collimator to precisely examine the effect of cross talk of scattering gamma-rays between CdTe elements. (author)