R. Brenner

GENERAL PURPOSE ANALOG PULSE HEIGHT COMPUTER

Experimental techniques in nuclear physics often call for arithmetic operations on pulses whose amplitudes convey the experimental data. An instrument capable of pulse height multiplication, division, exponentiation, as well as pulse height addition and subtraction is described. These operations are performed in about 0.5 μsec with a precision of ±0.1% of full scale over a pulse height range of 20:1. At room temperature the computer exhibits a drift of about 0.05%/C° or ±0.3%/day. With an input rate of 10 000 counts/sec the pulse height distribution shifts 0.2%, and with 25 000 counts/sec it shifts 1%. Multiplication, division, and exponentiation are performed by using logarithms and antilogarithms. The log and antilog function generators are based on the fact that the emitter‐base voltage of a silicon planar transistor is proportional to the logarithm of its collector current. Addition and subtraction are performed by using linear operational amplifiers. Results of experiments are shown in which the comp...

High Resolution Ge (Li) Spectrometer for High Input Rates

A system was developed for obtaining γ‐ray spectra with high resolution at high input rates. For rates up to 100 000 γ‐ray events/sec in a 3.5 cm3 (13 mm thick) Ge(Li) detector, the FWHM at 1.33 MeV is 2–3½ keV and the spectral shift is no more than 0.1%. The amplifying chain, consisting of a cooled FET preamplifier and a simple main amplifier, is pole‐zero compensated throughout. The amplifier produces unipolar pulses shaped with equal integrating and differentiating time constants of 1.2 μsec. The amplifier is followed by a two‐diode baseline restorer, a pileup rejector, a linear gate, and an analog to digital converter. The salient features of the system are described and performance data are presented and discussed.

Resolution and Linearity of Anger-Type Neutron-Position Detectors as Simulated with Different Signal Processing and Optics

The spatial linearity and resolution of Angertype neutron-position scintillation detectors are studied using a semi-empirical model. Detector optics with either an air gap or optical grease between the scintillator and the dispersive light guide are considered. An air gap focuses the scintillation light on the photomultiplier tubes nearest the scintillation point. Four signal processing methods which truncate signals from photomultipler tubes distant from the scintillation are compared with the linear resistive weighting method. Using linear processing, air-gap optics yield a 25% improvement in resolution distance and an 80% reduction in integral nonlinearity relative to grease-coupled optics. With either optics, using signal truncation instead of linear processing improves the resolution distance 5-15%.

Computer Controlled CAMAC Systems at Argonne

Several computer controlled CAMAC systems are presently being developed at Argonne for data acquisition in low-energy nuclear physics. This paper discusses four of these. The systems are designed around 8K, 16-bit, Lockheed MAC-16 computers. A typical system includes two CAMAC crates and a complement of 15 modules. The crate controller in each crate serves as interface between the computer I/O bus and the CAMAC dataway. Up to 14 individual crates can be addressed in addition to a Teletype and other peripheral devices. The CAMAC modules being developed include Crate Controller, Dual ADC Controller, ADC Coincidence Unit, Dual Stabilizer, Quad Pre-Scaler, Clock, Display Controller, Readout Selector, Magnetic Tape Controller and Disk Controller. Most units are built in double-width modules using computer controlled wire-wrap construction.

Cathode Uniformity of New Square Photomultiplier Tubes

Square photomultiplier tubes (PMTs) should have better light collection than round PMTs when used in square or rectangular Anger-type ¿-ray or neutron position detectors. Photocathode response uniformity of new RCA square PMTs type S83003E (51.5×51.5 mm2) was measured and compared with conventional round PMTs. The PMTs were scanned with a small scintillation source while the anode signal amplitudes were processed in a computer-based multichannel analyzer. The analyzer was programmed in a mode which virtually eliminates effects due to statistical fluctuations in source rate and output amplitude. Pulse heights as a function of source position on the PMT face are shown for typical tubes. The response uniformity of 64 square PMTs and 21 round ones was evaluated. It is shown that the center region of the square PMTs is as uniform as that of the round ones and that the response in the corners is comparable to that in the center. Thus square PMTs appear to have an advantage for use in square or rectangular position detectors.

STABLE 1 pF CHARGE INJECTION CAPACITOR FOR NUCLEAR PULSE PREAMPLIFIERS.

Servostabilizers used with Ge(Li) γ‐ray spectrometers are frequently referenced to pulser peaks obtained by injecting the output of a stable pulse generator into a charge sensitive preamplifier via a small capacitor. A 1 pF charge injection capacitor has been developed that meets the requirements of high resolution spectrometers. The three terminal air capacitor has a coaxial structure with electrodes made of Kovar. It has a temperature coefficient ranging from −1 to +6 ppm/°C and a long term stability of ±20 ppm, i.e., less than ±0.1 channel out of 4000. It adds only 1 pF of stray capacitance to ground at the preamplifier input and the noise added with 3 kV across it is negligible.

Compton Scatter in Germanium and Its Effect on Imaging with Gamma-Ray Position-Sensitive Detectors

The spatial spread due to Compton scatter in Ge was measured to study the reduction in image contrast and signal-to-noise ratio (S/N) resulting from erroneous readout in Ge position-sensitive detectors. The step response revealing this spread was obtained by scanning with a 122 keV ¿-ray beam across a boundary of two sectors of a slotted coaxial Ge(Li) detector that is 40 mm diameter by 22 mm long. The derived line-spread function at 140 keV (99mTc) exhibits much shorter but thicker tails than those due to scatter in tissue as observed with a NaI detector through 5.5 cm of scattering material. Convolutions of rectangular profiles of voids with the Ge(Li) line-spread function show marked deterioration in contrast for voids less than 10 mm across, which in turn results in even greater deterioration of the S/N. As a result, the contrast for voids in Ge images is only 20-30% higher than that in NaI and the S/N is only comparable for equal detector areas. The degradation in image contrast due to scatter in Ge detectors can be greatly reduced by either using thin detectors (~5 mm), where scatter virtually does not exist, or by using thicker detectors and rejecting scatter electronically. To reduce the effects of scatter on the S/N as well as on contrast, the erroneous position readouts must actually be corrected.

Intrinsic efficiency of germanium - a basis for calculating expected detector efficiency

A method is presented whereby the intrinsic efficiency of Ge is utilized to calculate the expected peak efficiency of detectors having a wide range of sizes. The intrinsic efficiency of Ge, which is the probability for total absorption, was measured at 122 and 136 keV in Ge(Li) coaxial detectors and HPGe planar detectors having an effective thickness ranging from 5 to 50 mm. At 136 keV it is 64% for a thickness of 10 mm and 82% for 20 mm, after which it levels off reaching 89% at 50 mm. It is shown that the peak efficiency of a detector is a product of only the intrinsic efficiency and the solid angle, once losses due to edge escape and detector imperfections (surface channels and high dislocation densities) are determined. The absolute and relative [to NaI(T1)] peak efficiency of a sample detector, calculated on the basis of intrinsic efficiency, are in good agreement with measured values. This method should find applications in the design of new detector systems particularly those for diagnostic imaging with 99Tc (140 keV).

Resonance Neutron Radiography Using a Pulsed Neutron Source and a 2-Dimensional Scintillation Detector

Resonance neutron radiography is an imaging technique whereby a single exposure to neutrons produces several radiographs each of which shows a distribution of a different isotope in the object. The technique has been shown before to be feasible but it has not been practicable due to the lack of an intense pulsed epithermal neutron source and a suitable active imaging detector. An accelerator-based pulsed spallation source has recently been completed at Argonne National Laboratory (ANL). For energies higher than 1 eV, its peak flux is several orders of magnitude more intense than that of a chopped beam from a high-flux reactor. A 2-dimensional neutron-position scintillation detector was also recently developed here. It consists of a thin (1-2 mm) Li glass scintillator coupled to an array of photomultipliers. The prototype detector has a FWHM resolution of 3 mm and a sensitive area of 30×30 cm2. The neutron source and detector were used in a resonance radiography experiment in which a foil sample consisting of indium and gold were radiographed. Two images were obtained; one due to resonance absorption of 1.46 eV neutrons in 115In showing the extent of indium in the sample, and the other due to absorption of 4.91 eV neutrons in 197Au showing the extent of gold. A radiography experiment on nuclear fuel pellets showed resonances of the uranium and plutonium isotopes in the range of 0.3–10 eV. The area of potential applications of resonance radiography below 20 eV includes isotopes (Z>~40) of fission products, rare earths, heavy metals, and actinides. Resonance cross sections are generally 10–1000 times larger than thermal cross sections thereby providing higher isotopic detection sensitivity and better image contrast than in thermal radiography.