.
One indicator to express these relations is proposed here. It is the ratio R which
is defined as the quotient, i.e. photopeak counts by B / photopeak counts by A.
They are apparently proportional to probability of photoelectric absorption: 
10. New Gamma-Ray Directional Detectors with Different Types of Scintillators
Yoshiyuki Shirakawa
Keywords: gamma ray, NaI(Tl) scintillator, BGO scintillator, photomultiplier, photopeak, coming direction, energy
The tandem detector, which positively increases directional sensitivity to
incident gamma rays, mainly consists of two different types of scintillators,
a photomultiplier tube and electronic devices such as a preamplifier. In the detector,
a cylindrical front scintillator A, the same-sized back scintillator B and a photomultiplier
tube fitted with scintillators are combined optically in this order in tandem.
Since path length of gamma rays through each scintillator is dependent on incident
directions, the probability of photoelectric absorption events occurring in each
scintillator will also be changed. Hence the photopeak counts in a spectrum collected
by the front scintillator A, and those by the back scintillator B have some relations
with the incident direction .
One indicator to express these relations is proposed here. It is the ratio R which
is defined as the quotient, i.e. photopeak counts by B / photopeak counts by A.
They are apparently proportional to probability of photoelectric absorption:
R = photopeak counts by B / those by A = Pb / Pf (1)
where Pb and Pf mean the probability of photoelectric absorption in the B
scintillator and that in the A scintillator, respectively. We expect that R()
has a value in the range between R(0) and R(90), and the value monotonously increases
with .
Here requirements for both scintillators A and B should be considered. The most
important characteristic is that scintillation efficiencies of both scintillators
are sufficiently apart to distinguish photopeaks completely for stable counting.
Another desired feature is that each scintillator has a high density to detect
gamma rays efficiently for a short counting time. From these considerations, it
is reasonable to chose a NaI(Tl) scintillator as the front one of A and a BGO
scintillator as the back one of B. There tandem detectors were designed and made
for experiments. They have 1) the diameter of 50 mm and thickness of 25 mm for
the NaI(Tl) scintillator and the BGO scintillator; 2) the diameter of 50 mm and
thickness of 50 mm for the NaI(Tl) scintillator and the BGO scintillator; and
3) the diameter of 25 mm and thickness of 50 mm NaI(Tl) scintillator and the BGO
scintillator. Using these detectors, we carried out experiments to confirm the
measurement principle and to examine the performance. A137Cs source
of 3.7 MBq was selected as representative of gamma ray sources because of its
widespread use. Experimental procedure is described with reference to Fig. 12.
The source was set 100 cm in front of the detector (=0).
Gamma rays coming from the source and reaching the detector were counted for 60
seconds and the ratio R was calculated from observation of photopeaks in the spectrum.
Then the source was moved in 10-degree intervals towards the side of the detector
(=90)
and the same procedure was repeated. These curves were expressed by
R()=-4.42x10-63+5.86x10-42-3.53x10-2+1.620
(2)
R()=-4.26x10-63+3.82x10-42+2.31x10-2+0.491 (3)
R()=-2.70x10-63+7.20x10-42+7.20x10-2+0.633 (4)
where eq.(2), eq.(3) and eq.(4) were applied for experimental data by the detectors with the 50 mm x 25 mm scintillators, the 50 mm x 50 mm scintillators, and the 25 mm x 50 mm scintillators, respectively. These fitting curves agree well with experimental data points and the curve for the detector with the 25 mm x 50 mm scintillators is more sensitive to the change of than curves for the other detectors. In practical applications, R() is given by calculation using photopeak count data after measurements and then the direction , which is an objective parameter, must be solved. From this viewpoint, the third order polynomial is suitable for this application because the equation can be solved analytically and we can regularly obtain the result . Characteristics of other parameters, i.e. energies and counts are confirmed to be the same as those by conventional NaI(Tl) and BGO scintillation detectors. Hence the simulations and experiments show that the proposed detectors have a potential for measuring three parameters simultaneously.
Publications:
1) Shirakawa, Y.: Radioisotopes, 50 [4], 117-122 (2001).
2) Shirakawa, Y.: JASP, Radiation Science, 26 [4], 67-73 (2001).
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