Abstract:
A photomultiplier tube (PMT) electronic instrument was designed and
fabricated. High voltage (-2000 V) and low voltage supplies (+8, ± 12 V) were also
designed and constructed for the instrument. 40 PMTs were tested for voltage
gains to determine whether they were good or bad using photoelectric effect
principle. Good and bad PMTs were identified during the measurement of their
gains. The dark currents of the PMTs were measured with a Keithy Electrometer
and PMTs with high dark current (10k ~ Ix jJA) were declared bad alongside the
bad PMTs and were rejected.
PMTs that were identified as good during the testing were then sorted by
their gains and arranged on the NaI (Tl) detector of the gamma camera, The PMT
with the highest gain was placed at the centre, and PMTs with the lowest gains
were placed at the outer circle (edge) of the detector. The power supplies and the
signal processing boards of the gamma camera were then aligned (calibrated) to
bring their offset voltage to zero (±O.OOI V) and the PMTs HV pots were adjusted
to bring their individual energy to the same photo-peak (140 Kev of the 99mTe
radionuclide).
Quality control pictures which were taken after alignment and fine-tuning
the PMTs showed some high improvement of image quality. The central field of
view (CFOV) Integral Uniformity was 1.64 %; and the Differential Uniformity
was 1.13 %. The Useful field of view (UFOV) Integral and Differential
Uniformities were 1.67 % and 1.26 %, respectively.
The results compared to NEMA showed that the Gamma camera uniformity
and linearity were highly improved .
