Alpha-particle Spectrometer Research Articles

IBIC analysis of gallium arsenide Schottky diodes

Semi-insulating (SI) gallium arsenide (GaAs) devices operating as a reverse biased Schottky diode offer an attractive choice as radiation detector at room temperature both in high energy physics experiments and as X-ray image sensors. However, SI GaAs devices contain a high concentration of traps, which decreases the charge collection efficiency (cce), and affects the energy resolution of such detectors working as nuclear spectrometers. In this paper we present a detailed investigation of the spatial uniformity of the cce carried out by analysing ion beam induced charge (IBIC) space maps obtained by scanning a focused 2 MeV proton microbeam on a SI n-GaAs Schottky diode. The microbeam irradiated both the front (Schottky) and back (ohmic) contacts in order to evaluate the transport properties of both electrons and holes generated by ionisation. The IBIC space maps show a clear non-uniformity of the cce. The poor energy resolution previously observed in such detectors working as alpha particle spectrometers is ascribed to the presence of two different “phases” in the material, which produce two distinct collection efficiency spectra. Such “phases” show different behaviour as a function of the applied bias voltage which is most likely due to the different electric field dependence of the relevant capture cross sections of the trapping centres for both charge carriers.

Mixed alpha sources preparation by electrodeposition

This work presents a technical procedure to prepare mixed electroplated alpha sources of ( 233U, 241Am and 244Cm) and ( 233U, 238Pu and 244Cm), to carry out the energy calibration curve of an alpha particle spectrometer, an option to the use of individual standards, maintaining the same experimental conditions. The chemical processing of the standard radionuclide solutions were made with the purpose to find the best pH range of the electrolytic solution, resulting in a yield deposition of 90%.

The Lunar Prospector Discovery Mission: mission and measurement description

Lunar Prospector, the first competitively selected planetary mission in NASA's Discovery Program, is described with emphasis on the radiation spectrometer instrumentation and anticipated scientific data return. Scheduled to be launched in January 1998, the mission will conduct a one year orbital survey of the Moon's composition and structure. The suite of five instruments are outlined: neutron spectrometer, alpha particle spectrometer, gamma-ray spectrometer, electron reflectometer and magnetometer. Scientific requirements and measurement approach to detect water/ice to a sensitivity of 50 ppm (hydrogen), measure key elemental constituents, detect radioactive gas release events and accurately map the Moon's gravitational and magnetic fields are given. A brief overview of the programmatic accomplishments in meeting a tightly constrained schedule and budget is also provided.

State-of-the-Art Instruments for Measuring Radon/thoron and Their Progeny in Dwellings-A Review

Radon and radon progeny measurements are conducted world-wide for the assessment of the radiation dose to workers and the general public. In the last 10 years, a variety of instruments have been developed that utilize different principles of detection for different measurement applications. The various instruments and methods used today depend on whether radon or radon/thoron progeny are being measured, the type of radiation being detected, and also the duration of the measurement. Other important design criteria are applicability, portability, convenience, reliability, and cost considerations. The instruments under consideration use the following detection systems: pulse ionization chambers, electret ionization chambers, scintillation detectors with zinc sulfide ZnS(Ag), alpha particle spectrometers with silicon diodes, surface barrier or diffused junction detectors, registration of nuclear tracks in solid-state materials, and gamma-ray spectometry with NaI(TI) scintillation crystals or germanium lithium (GeLi) semiconductors. Discussed in this paper are the advantages and disadvantages of the various portable instrumentation used for measuring radon, thoron, and their progeny. Also provided is guidance for their application in the field.

Enhancement in the energy resolution of cellulose nitrate track detectors for alpha particles

Different irradiation and etching modes have been investigated to improve the energy resolution properties of cellulose nitrate track detectors when used as alpha particle spectrometer. In this connection CA80-15 and LR-115 plastics were employed for the separation of alpha particle peaks from 239Pu and 238U sources. Enhancement in the energy resolution has been observed when the detectors are etched from the reverse side, or if some degrading foils of optimum thickness are interposed between the source and the detector surface to be analysed. In both the above mentioned modes of enhancement in energy resolution, the regions of maximum damage (the Bragg peaks) occuring along the particle trajectories are attacked immediately with the starting of the etching process. The peaks in the Bragg curve occur at different places when different energy alpha particles pass through the detector. Drastically different rates of energy losses at these places are responsible for the diametric separation of the damage trails.

Radon emanation as an indicator of current activity of the moon

An interpretation of previously reported measurements of the Apollo 15/16 alpha-particle spectrometer on the distribution of 222Rn and 210Po across the lunar surface suggests that continuation of these measurements is a method of monitoring current activity on the moon. Since the two isotopes are relatively short-lived with effective half-lives of 3 days and 21 years, respectively, the activity detected has had to have been released during this current epoch. Changes in the rate of lunar emanation can be measured on three different time scales: (1) of a few days or less by detecting 222Rn at discrete sites such as the crater Aristarchus; (2) of a month by measuring 222Rn activity at the sunrise terminator; (3) of a few years by measuring 210Po activity at various locations. These observations could be carried out very effectively from a lunar polar orbiting satellite.

Detection of Radon Emission at the Edges of Lunar Maria with the Apollo Alpha-Particle Spectrometer

The distribution of radioactive polonium-210, a decay product of radon-222, shows enhanced concentrations at the edges of lunar maria. Enhancements are seen at the edges of Mare Fecunditatis, Mare Crisium, Mare Smythii. Mare Tranquillitatis, Mare Nubium, Mare Cognitum, and Oceanus Procellarum. The observation is indicative of the transient emission of radon gas from the perimeters of lunar maria.

Radon emanation from the moon, spatial and temporal variability

Observations of the lunar surface with the orbiting Apollo Alpha Particle Spectrometer during the Apollo 15 and Apollo 16 missions have shown spatial and temporal variations in radon emission. There are a number of well localized features in the spatial distribution of lunar222Rn and her daughter210Po which apparently correlate with sites of reported transient visual events. There are sources at Aristarchus, Grimaldi and possibly Tsiolkovsky. Activity of210Po shows enhancement at most maria edges at rates far in excess of222Rn activity. This demonstrates unequivocally the presence of time varying radon activity at the maria edges, taking place at the present time. The increased radon emission is probably caused by sporadic internal activity. In analogy to terrestial processes, radon may be merely a trace component accompanying the release of larger quantities of more common gases to the lunar surface.

Detection of a Nonuniform Distribution of Polonium-210 on the Moon with the Apollo 16 Alpha Particle Spectrometer

The polonium-210 activity of the lunar surface is significantly larger than the activity of its progenitor radon-222. This result establishes unequivocally that radon emanation from the present-day moon varies considerably within the 21-year half-life of lead-210, the parent nuclide of polonium-210. There are large variations and well-localized enhancements in polonium-210 activity over much of the moon's surface.

Detection of Radon Emanation from the Crater Aristarchus by the Apollo 15 Alpha Particle Spectrometer

The alpha particle spectrometer aboard the Apollo 15 command/service module was designed to detect alpha particles from radon decay and to locate regions with unusual activity on the moon. A significant increase in radon-222 activity was detected from a region containing the crater Aristarchus. The result is interpreted as probably indicating internal activity at the site. By analogy with terrestrial processes, increased radon emanation may be associated with the emission of other volatiles.

The Apollo Alpha Spectrometer

Located in the Science Instrument Module of Apollo 15 and 16, the Alpha Particle Spectrometer was designed to detect and measure the energy of alpha particles emitted by the radon isotopes and their daughter products. The spectrometer sensor consisted of an array of totally depleted silicon surface barrier detectors. Biased amplifier and linear gate techniques were utilized to reduce resolution degradation, thereby permitting the use of a single 512 channel PHA. Sensor identification and in-flight radioactive calibration were incorporated to enhance data reduction.

A position sensitive surface barrier array type detector

A position sensitive array type surface barrier detector system has been constructed and mounted in the focal surface of the Argonne double focussing magnetic alpha particle spectrometer. The array is 9″ long, and consists of 224 detector elements, each 0.040″ wide by 1″ high. The elements are made in blocks of 16 on single silicon crystals 0.640″ wide and 1″ long by evaporating the gold through a grid of parallel wires. The energy of the detected alpha particles is uniquely determined by the position of the detecting element in the array, and background rejection is accomplished by pulse height selection. The immediate availability of the data pulse makes possible coincidence counting experiments, thus enhancing the versatility of the instrument. The details of the fabrication and the analysis of the performance of the detector system are discussed.

Large bandgap mercury and lead compounds for nuclear particle detection

Research has been done on mercury and lead compounds with emphasis on better detection efficiency (due to higher absorption coefficients) and room temperature application (due to large bandgaps) Crystal growth and detection properties were studied. PbO and PbI 2 were used as alpha particle spectrometers (resolution about 30%) and as gamma quanta detectors (no resolution). HgI 2 was used as a gamma spectrometer (resolution about 10%).

High resolution alpha particle spectroscopy at very low specific activities

This paper describes an alpha particle spectrometer which is suitable for use with materials of very low specific activities (the order of 10 −12 C/g) and which provides a resolution of 50 keV. The instrument is a pulse ionisation chamber with a gridded cylindrical electrode arrangement, providing a source area of 1500 cm 2. For convenience in dealing with small, “weightness” sources, a four-position, conventional parallel plate gridded electrode system is available as an alternative to the cylindrical arrangement, using the same gas chamber and electronic system. The instrument has a number of applications in the study of biological and geological aspects of natural radioactivity and also in health physics.

Performance of Germanium and Silicon Surface Barrier Diodes as Alpha-Particle Spectrometers

The characteristics of a germanium surface barrier diode operated at room temperature make it particularly useful as an alpha-particle spectrometer. The small size, stability, energy resolution, and relative insensitivity to β and γ radiation of the units suggest applications in medical and nuclear research. Studies were made on Au–Ge and Au–Si surface barriers with a barrier width (≅1 μ) less than the range of the incident alpha particles. In the germanium units, the pulse-height response to alpha particles increased linearly with energy up to 7.5 Mev and then increased monotonically but more slowly as the energy rose to 12 Mev. In silicon the deviation from linearity occurred around 6 Mev. The observed linearity between pulse height and energy depends simply on the fact that the number of carriers excited is proportional to the particle energy and that the time for the carriers to be collected at the barrier is less than the circuit time constant. The general theory of the transient response of the diode was developed and the calculated response compared with the observed behavior. The observed values of the energy required to create a hole-electron pair, measured over the energy range of linear response of the junction, were ε(Ge)=2.96±0.1 ev and ε(Si)=3.9±0.3 ev.

Aberrations and fringing effects in a 180° double focussing magnetic spectrometer

Second and third order effects, including fringing, are considered for a 180° symmetrical double focussing spectrometer. The optic axis and focal points for particles of a definite momentum are found. Values are calculated for β and γ, the coefficients of the quadratic and cubic terms in the expansion of the magnetic field, which eliminate second and third order aberrations due to radial motion. The linear dispersion and the orientation of the focal plane for paraxial trajectories of slightly different momenta are determined. Numerical values are presented for a high transmission alpha-particle spectrometer designed for this laboratory.

Alpha-Particle Spectrograph

An alpha-particle spectrometer, using a 60° sector magnet capable of focusing alpha-particles of energies up to 14 Mev, is described. Description of the instrument, sample technique, focusing properties, and current supplies are discussed. Transmission is about 0.01 percent of the total solid angle of the sample and with a resolution of 8 kev.