János Gál

A random tail pulse generator for simulation of nuclear radiation detector signals

Abstract A random tail pulse generator is presented in which the mean rate is adjustable from 10 Hz-1 MHz in decade steps with continuous adjustment within each decade. The time interval distribution is verified to be Poissonian. Minimum spacing between adjacent tail pulses can be smaller than 100 ns. Rise time can be set from 25–1000 ns in six steps. The decay time constant is also adjustable in the range of 10–1000 μ s. Double tail pulses can be produced by using a mixed triggering facility which is described. Spacing between these two pulses is continuously variable from 0.1–100 μ s. The generator can have the average random rate controlled by a periodic source.

Particle discriminator for the identification of light charged particles with CsI(Tl) scintillator + PIN photodiode detector

Abstract A particle discriminator exploiting the ballistic deficit effect for pulse shape discrimination has been developed for CsI(Tl) scintillator + PIN photodiode charged-particle detectors. The method is theoretically investigated and it is shown that the figure of merit of the particle separation is mainly governed by the absolute value of the differential quotient of the rise time dependent ballistic deficit. As the actual particle discriminator contains shaping amplifiers, baseline restorer, pile-up rejector and analog-to-digital converters, it directly accepts signals from a charge-sensitive preamplifier, and its outputs deliver the type and the energy of the particles in the form of eight-bit digital codes. The performance of the particle discriminator is characterised by the figure-of-merit measured as a function of the particle energy.

Realization of charge sensitive preamplifiers using current feedback operational amplifier

Abstract A simple charge sensitive preamplifier circuit has been designed, which uses a CFOPA. Despite its simplicity the preamplifier exhibits good noise and speed parameters.

A Promising Future: Comparable Imaging Capability of MRI-Compatible Silicon Photomultiplier and Conventional Photosensor Preclinical PET Systems

We recently completed construction of a small-animal PET system—the MiniPET-3—that uses state-of-the-art silicon photomultiplier (SiPM) photosensors, making possible dual-modality imaging with MRI. In this article, we compare the MiniPET-3 with the MiniPET-2, a system with the same crystal geometry but conventional photomultiplier tubes (PMTs). Methods: The standard measurements proposed by the National Electrical Manufacturers Association NU 4 protocols were performed on both systems. These measurements included spatial resolution, system sensitivity, energy resolution, counting rate performance, scatter fraction, spillover ratio for air and water, recovery coefficient, and image uniformity. The energy windows were set to 350–650 keV on the MiniPET-2 and 360–662 keV on the MiniPET-3. Results: Spatial resolution was approximately 17% better on average for the MiniPET-3 than the MiniPET-2. The systems performed similarly in terms of peak absolute sensitivity (∼1.37%), spillover ratio for air (∼0.15), spillover ratio for water (∼0.25), and recovery coefficient (∼0.33, 0.59, 0.81, 0.89, and 0.94). Uniformity was 5.59% for the MiniPET-2 and 6.49% for the MiniPET-3. Minor differences were found in scatter fraction. With the ratlike phantom, the peak noise-equivalent counting rate was 14 kcps on the MiniPET-2 but 24 kcps on the MiniPET-3. However, with the mouselike phantom, these values were 55 and 91 kcps, respectively. The optimal coincidence time window was 6 ns for the MiniPET-2 and 8 ns for the MiniPET-3. Conclusion: Images obtained with the SiPM-based MiniPET-3 small-animal PET system are similar in quality to those obtained with the conventional PMT-based MiniPET-2.

OPTIMIZATION OF THE PARTICLE DISCRIMINATOR BASED ON THE BALLISTIC DEFICIT METHOD USING DELAY-SWITCHED GATED INTEGRATOR

Abstract The particle discriminator we developed for CsI(Tl) elements of an ancillary detector array planned to be used together with large γ-ray detector systems utilizes the ballistic deficit effect for particle discrimination. This technique applies a low-ballistic-deficit long-time-constant unipolar semi-Gaussian shaper and a large-ballistic-deficit short-time-constant bipolar shaper. To match the signal processing time of our discriminator with that of the standard electronics for Ge detectors the long-time-constant semi-Gaussian shaper was substituted with the so-called delay-switched gated integrator. The experimental tests of the new arrangement show that the pulse processing time can be significantly shortened. The figure of merit of the particle discrimination improves with increasing the total integration time of the delay-switched gated integrator up to 12 μs, above which the improvement is negligible. Therefore, this 12 μs integration time is considered as a good compromise as for the figure of merit and the dead time of the new arrangement.

ON THE NOISE PERFORMANCE OF A NOVEL DELAY-SWITCHED GATED INTEGRATOR

Abstract For trapezoidal filtering, widely used for diminishing the harmful effect of the ballistic deficit, a modified version of the gated integrator is suggested. In this delay-switched gated integrator the true integration is delayed with respect to the beginning of the pulse, and for the delay period the true integration process is approximated with a low-pass filter. The paper theoretically compares the noise behavior of the delay-switched gated integrator and that of the gated integrator. The noise calculations are based on the concept of the step- and delta-noise residual functions and they are performed in three cases. In the first case the standalone shaping circuits are analyzed, in the second case they are preceded by a fast baseline restorer and in the third case they are preceded by a baseline restorer of finite time constant. It is shown that the novel delay-switched gated integrator preceded by a baseline restorer can be a good alternative to gated integrators.

Simple dead-time and pile-up correction technique using a gated periodic pulse train

Abstract In a pulse-height analysis system using a gated periodic pulse train for the correction of counting losses, it is shown that both the dead-time and pile-up losses can be corrected for if the clock pulses have the same width as the so called true pile-up time of the pulse processing system, and the clock pulses are rejected from the pulse train if a coincidence between the shaped clock pulses and the busy time of the system is detected. The technique was tested both with random-pulse generators and a Ge(Li) detector (60Co and 137Cs sources). At counting rate up to 50 kHz, it is shown that the correction was accurate to ± 1%.

A zero crossing discrimination technique for constant fraction timing

Abstract A zero crossing discrimination technique is given for constant fraction timing. A very simple method makes a walkingless timing possible, independently of delay and fraction settings.

Novel time over threshold based readout method for MRI compatible small animal PET detector

Combined PET-MRI scanners start a new era in medical imaging. However the development of MRI compatible PET detector module is a challenging task. SiPM sensors are insensitive to magnetic field and constitute a promising solution. A drawback is the high dark current. A readout concept for SiPM based small animal PET detector module is presented in this paper. The results show that the readout of the SiPM is possible using only four ADC channels and the position map is comparable to the ideal solution. The detector modules based on the method are feasible solution for MRI compatible PET scanners.

Simulation studies with SiPM arrays and LYSO crystal matrix analyzing a new readout scheme

The concept of implementing Positron Emission Tomography and Magnetic Resonance Imaging in a dual modality imaging system gained high scientific importance recently. SiPM photo detectors became a solution for operation in high magnetic fields instead of the conventional photomultipliers. The method of Anger logics in the SiPM readout of pixelated scintillator crystals would produce significant noise in the readout signals, while single channel readout for each crystal pixel would not be cost effective. Another solution could be defined if we sum the SiPM signals for each row and column and an appropriate algorithm may select the X and Y coordinates of the original position of the scintillation source. It is expected that the highest signal related row and column would indicate correct position. The scope of this work was to calculate the optimal light distribution for different SiPM and scintillator matrix geometries, that would result in the greatest signal difference between the primary and adjacent readout channels and therefore would perform the best reliable selection of the relevant row and column indices.