Nuclear Instruments and Methods in Physics Research最新文献

Novel techniques used in PAD studies of high spin states populated by conventional fusion evaporation reactions are described. The interaction of the recoiling ions with the exit surfaces of multiple tilted carbon foils induced polarization of the ionic shells. In turn, the hyperfine interaction during flight in vacuum between successive foils affects the nuclear spins in a controlled manner. Systems have been designed for measuring g-factors and for inducing nuclear polarization of high spin isomers. The technique was applied to 10⁺ isomers in ⁵⁴Fe and ¹⁴⁴Gd and nuclear polarizations of 0.18(5) and 0.10(4), respectively, were observed.

A compact channel plate detector used as a start detector in heavy ion reaction studies is described. In order to avoid spurious structures in the energy spectra of the detected ions the usual electron accelerating grid is replaced by five electrodes which provide a uniform electric field. The start detector has an intrinsic resolution of 85 ps and efficiency of 90% for medium mass nuclei. The overall time resolution with a solid state detector as stop is 105 ps.

A small (30 cm × 30 cm) model of a proposed large aperture positron camera has been developed at Rutherford Appleton Laboratory. Based on multiwire proportional counter technology it uses lead foil cathodes which function simultaneously as converters for the 511 keV gamma rays and readout electrodes for a delay line readout system. The detectors have been built up into a portable imaging system complete with a dedicated computer for data taking, processing and display. This has permitted evaluation of this type of positron imaging system in the clinical environment using both cyclotron generated isotopes (¹⁵O, ¹¹C, ¹⁸F, ¹²⁴I) and available isotopic generator systems (⁸²Rb, ⁶⁸Ga). At RAL we provided a complete hardware system and sufficient software to permit our hospital based colleagues to generate useful images with the minimum of effort. A complete description of the system is given with performance figures and some of the images obtained in three hospital visits are presented. Some detailed studies of the imaging performance of the positron camera are reported which have bearing on the design of future, improved systems.

An x−y position-sensitive detector system which provides simultaneous mass and charge identification with discrete nuclide resolution is described. This system is specially suitable for low-energy, medium-mass (E₁ ≈ 1–10 MeV/mu; A ≈ 10–60) heavy tons. Charge identification is determined from a two-element gas-ionization counter coupled with a semiconductor stop detector. The gas-ionization counter employs a simple electron drift time technique to obtain two-dimensional position information. Mass identification is derived from measurement of the fragment time-of-flight with a pair of channel-plate time devices and the fragment energy, as provided by the ΔE_y – ΔE_x – E telescope. This system yields mass and charge resolution ⩽0.7 units in both A and Z and gives position resolutioon <0.5 mm for fragments with A ≲60, as determined from measurements performed with a 7.2 MeV/u ⁵⁶Fe beam incident on ²³⁸U.

In the last few years picture analysis methods have evolved into a powerful technique for measurements of nuclear tracks in plastic detectors. We have developed a microprocessor-based picture analysis system for automatic track measurements. The video pictures of particle tracks seen through a microscope are digitized in real time and the picture analysis is done by software. The microscope is equipped with a stage driven by stepping motors, which are controlled by a separate microprocessor. A PDP 11/23 supervises the operation of all microprocessors and stores the measured data on its mass storage devices. This paper describes the hard- and software of the system and the methods of picture analysis which are used for the track identification and measurement.

A general-purpose computer code MONTHY has been written to perform Monte Carlo simulations of physical systems. To achieve a high degree of flexibility the code is organized like a general purpose computer, operating on a vector describing the time dependent state of the system under simulation. The instruction set of the “computer” is defined by the user and is therefore adaptable in the particular problem studied. The organization of MONTHY allows iterative and conditional execution of operations.

We have studied the performance of limited streamer drift tubes made of conductive plastic. Spatial resolution and rate capability obtained are 100–150 μm and 2–3 kHz/cm, respectively.

The process of deconvolution is helped by the introduction of an inverse resolution function. Its use, however, forces one to perform analytic continuation of data measured along the real axis into the complex plane. We examine the viability of this scheme in combination with the reference resolution function scheme. Our results indicate that this is a viable alternative scheme for deconvolution.

The method of fixed dead-time per event was used to correct for pulse losses due to random summing in the amplifier and dead-time of the multichannel analyser (MCA) when a sample containing a mixture of short- and long-lived radioisotopes is assayed. The monostable was used as an on-and-off switch of the input linear gate of the ADC thereby keeping it closed for 50 μs each time a pulse was detected. This time was found adequate for the conversion of all pulse heights encountered in this work.

The correction curve, which will be discussed below, was then used in the analysis of some NBS reference standards irradiated in the experimental reactor having a thermal flux of 2.2×10¹² n/cm²s. This method was found credible for analysis of samples with activities as high as 5×10⁴ cps.

Organic scintillator solutions with decay times as fast as 500 ps and with relatively high conversion efficiencies have been developed. The intramolecular quenching was achieved through the novel approach of adding a bromine atom to the 3- or 4-position of para-oligophenylenes, the fluorescent solutes in these binary solutions. The bromine serves to enhance singlet-to-triplet intersystem crossing in the chromophore, causing a reduction in the scintillation yield and a conconitant reduction in the decay time. The very fast value given above probably also involves some intermolecular self-quenching at high concentration. In addition, the bromine reduces the symmetry of the molecules, thereby increasing their solubility. Finally, an alkyl chain on the opposite para position further increases the solubility and also increases the immunity of the chromophore to quenching.

The following solutes were studied in binary liquid solutions and to a limited extent in plastics: 4-bromo-4″-(5-hexadecyl)-p-terphenyl: (4-BHTP), 3-bromo-4″-(5-hexadecyl)-p-terphenyl: (3-BHTP), 4-bromo-p-terphenyl: (4-BTP), 3-bromo-p-terphenyl: (3-BTP), 4-bromo-4‴-(5-hexadecyl)-p-quaterphenyl: (4-BHQP). The decay times for binary liquid solutions in toluene (at the indicated concentrations) were 0.51 ns for 4-BHTP (0.14 M), 0.75 ns for 3-BHTP (0.14 M), 0.57 ns for 3-BTP (0.14 M), and 1.3 ns for 4-BHQP (0.06 M). Binary plastics with 4-BHTP as the solute in concentrations up to 0.14 M were cast in polystyrene. The shortest decay time, 0.40 ns, was measured for the 0.14 M concentration. A plastic scintillator containing 3-BTP (0.11 M in polystyrene) had a decay time of 0.85 ns. These results compare favorably with the plastic scintillator BC-422 whose decay time is about 1.4 ns.