Nuclear Instruments and Methods in Physics Research最新文献

We present the discovery of the enhanced transfer of electrons produced in avalanches through metal grids in Penning gas mixtures.

Measurements are presented showing the enhancement which is particularly dramatic at very low transfer fields. With electric fields on either side of the grid in the ratio of 0.015 the transfer ratio is 0.11, representing an increase of a factor of 7 over that predicted and observed in a conventional gas by Bunemann et al. [1]

We propose a model for this phenomenon and we show that both a uniform electric field and the Penning effect [2] are necessary for this transfer to occur. We also identify a possible reason why other workers have not obtained similar results in Penning mixtures.

The application of this “Penning transfer” to a position sensitive X-ray detector (the Penning Gas Imager or PGI [3]) is discussed.

A source for ²³Na⁻ ions is described. A thermal atomic ²³Na beam is ionized on a W-O surface. By use of charge exchange in Cs vapour beam currents up to 500 nA of ²³Na⁻ ions were injected into the Heidelberg MP-tandem accelerator.

By coupling a simple, passive shield of tin with a thin NaI (Tl) detector the atmospheric background counting rate in the energy range 15–95 keV can be attenuated by a factor of approximately 12.5. Including the weight of the collimator, a telescope built with this combination would have a total surface density of only ∼ 3 g cm⁻², allowing a sensitive area of ∼ 30 m² to be constructed within the normal restrictions of weight on a balloon experiment. To measure the of effectiveness of different types of shielding, the atmospheric gamma radiation was simulated using a tank of water containing radioactive sources.

Four vapours with low ionisation potentials: triethylamine (TEA), N,N-dimethylaniline (DMA), N,N-diethylaniline (DEA) and tetrakis (dimethylamino) ethylene (TMAEO) have been studied to produce controlled amounts of laser induced ionisation in proportional counters. DMA, DEA, and TMAE produce large amounts of ionisation at the nitrogen lase wavelength (337 nm) but TMAE has proved difficult to control. At 337 nm TEA does not ionise significantly but initial tests show that it also becomes suitable as a seeding agent at shorter wavelengths.

A spherical positron emission tomograph (S-PET) is proposed and its performance analyzed. Computer modelling and simulation studies indicate that a high resolution and high sensitivity positron emission tomograph having a spatial resolution of 2–3 mm fwhm can be realized with acceptable imaging time and sensitivity.

Multilayer S-PET having 16–24 layers is specifically analyzed and compared with its corresponding cylindrical counterpart. The result shows that the sensitivity gain of the spherical system is a factor 3 over the cylindrical system with identical true to scatter and random ratios.

The performance of an 18 module silica aerogel Cherenkov detector, with a total sensitive surface of 2.3 m², situated in the European Hybrid Spectrometer at the CERN SPS, is described. The light yield for β = 1 particles is on the average 7.5±0.3 photoelectrons. Particle identification with this detector is discussed. Fi3st results on the ageing effects of silica aerogel are presented.

A high-speed data acquisition system has been designed which collect digital data from one- and two-dimensional position sensitive X-ray detectors at a maximum average data rate of 1 MHz. The system consists of two separate fast buffer memories, a 64K word by 20-bit main storage, two timers, a display controller, a computer interface and a keyboard, controlled by a specially designed microprogrammable microprocessor. Data collection is performed by executing a microprogram stored in the control storage; data coming from a detector are first accumulated in a small but fast buffer memory by hardware and transferred to the main storage under control of the microprogram. This design not only permits time-resolved data collections but also provides maximum speed, flexibility and cost-effectiveness simultaneously. The system also accepts data from integrating detectors such as TV cameras. The system has been designed for use in experiments at conventional and synchrotron X-ray sources.

A study has been conducted on the determination of the thoron daughter working level, WL(Tn), by one gross α-count, and of WL(Tn) and the concentrations of ThB, [ThB], and ThC, [ThC], by two gross α-counts. For the one gross α-count the relationship between the gross α-count rate per unit of volume of air sampled and WL(Tn), denoted the F-factor, has been investigated as a function of sampling time, elapsed time from the end of the sampling period, i.e., waiting time, and the thoron daughter disequilibrium ratio [ThC]/[ThB]. It has been found that F depends on both the waiting time and [ThC]/[ThB]. If α-count measurements are made at least 300 minutes after the end of sampling, F changes by less than 10% over the full range of theoretical values of [ThC]/[ThB], i.e., from 0 to 1. The F-factor is independent of [ThC]/[ThB] at approximately 215 min after the end of sampling. This feature can be used to determine WL(Tn) with higher accuracy and at least 1.5 h earlier than is commonly done using other one gross α-count methods reported in the literature. An optimization method based on standard statistical techniques has been followed for the two gross α-count method. Optimum combinations of sampling, waiting and counting times compatible with experimental constraints have been obtained. A sampling time of 10 min and equal counting times of 15 min for the first and second α-counts situated 2 min and 92 min, respectively, after the end of sampling have been used in conjunction with a thoron daughter calibration facility. Comparison of the two gross α-counts method investigated here with other methods shows excellent agreement.

A high-throughput high-resolution curved crystal spectrometer for measurements in the soft X-ray range on fusion magnetic devices is described. The spectrometer uses a large useful area (8 × 5 cm²) quartz lamina bent at a curvature radius 385 cm long as the dispersing element. The detector is a one-dimensional space resolving multiwire proportional counter. The spectral resolving power is R = 18 000, permitting to obtain adequate ion temperature measurements, down to 700 eV, from the Doppler broadening of resonance lines of highly ionized metallic impurities.

The features of the invariant mass distribution obtained from combinations of particles from different events, i.e. event mixing, is studied in detail. It is shown that special care is needed in order to reproduce the shape of the uncorrelated invariant mass distribution. A way to determine the number of resonances and its statistical error using an event mixing technique is described.