Daniel J. OSullivan

HIGH RESOLUTION STUDY OF NUCLEONIC COSMIC RAYS WITH Z ≥ 34

Abstract Preliminary results of the analysis of large area Lexan polycarbonate and nuclear emulsion sandwich stacks flown from Sioux Falls between 1971 and 1974 are given. The total exposure was ⋍120 m 2 days at ⋍3.8 g/cm 2 atmospheric depth and 284 tracks of nuclei with Z⩾34 have been found to date, of which 97 have Z>65. The charge distribution features a Platinum peak, a marked Actinide gap and a high Uranium group flux, but no example of a super heavy nucleus (Z>110) was observed. The energy spectrum of nuclei with Z>65 is “normal” confirming our earlier results1).

The low energy particle detector SLED (∼30 keV-3.2 MeV) and its performance on the PHOBOS Mission and its moons

Abstract A low energy particle detector system (SLED) is described which was designed to measure the flux densities of electrons and ions in the energy range from ≈30 keV to a few MeV in (a) the varying solar aspect angles and temperatures pertaining during the Cruise Phase of the Phobos Mission and (b) in the low temperature environment (reaching −25° C) pertaining during Mars Encounter. Representative data illustrating the excellent functioning of SLED during both phases of the mission are presented.

New results on the investigation of the variation of nuclear track detector response with temperature

Abstract Track response of polymers is now known to depend on the temperature of the detector during passage of the ionising particle (the registration temperature effect). This has serious consequences for cosmic ray composition studies carried out in earth orbit or on high altitude balloons. Further studies are reported for Lexan, plain CR-39 and doped CR-39 exposed to 245 MeV/N Fe ions. It was found that the relative signal strengths continue to increase down to -137°C, the minimum temperature employed in this experiment. The stack temperatures were cycled between 20°C and -137°C but no evidence of a hysteresis effect. was observed.

Preliminary results on the registration temperature effect for uranium ions in various solid state nuclear track detectors

Abstract The variation of track response with registration temperature for Uranium ions in solid state nuclear track detectors has been investigated for the first time. Preliminary results are presented for Tuffak polycarbonate, Lexan polycarbonate and three types of doped CR-39 detectors exposed to 960 MeV/N Uranium beams at registration temperatures of -78°C and +18°C. The fractional change in signal strength (VT) per °C was found to be considerably greater than that for nuclei of lower ionisation. This result has very significant implications for future ultra heavy cosmic ray experiments employing solid state nuclear track detectors. In addition, it can explain apparent anomalies in the data of previous ultra heavy cosmic ray experiments.

Energetic particle studies at Mars by SLED on Phobos 2

A preliminary overview of particle records obtained by the SLED instrument on Phobos 2, February–March, 1989 during Mars encounter, is presented. Data obtained while in close elliptical orbit around the planet (pericenter < 900 km), in both spin and three axis stabilised mode, display evidence of energy related particle shadowing by the body of Mars. This effect was also observed, under favourable conditions, in certain circular orbits (altitude 6330 km above the planet). Flux enhancements, inside the magnetopause, in the approximate range 30–350 keV, recorded in the same general location at < 900 km above Mars over an 8 day period during three consecutive elliptical orbits, are described. Possible explanations of these enhancements include the presence of quasi-trapped radiation at the planet and the detection of the propagation of accelerated particles along the boundary of the magnetopause from the day to the night side of Mars. Large anisotropic ion flux increases (1–1.5 orders of magnitude) in the approximate range 30–200 keV recorded in front of the bow shock (inbound and outbound) during certain circular orbits, provide evidence that the spacecraft traversed strongly anisotopic jets of energetic particles. These are suggested to have constituted O+ ions. The pickup process would have been sufficient to accelerate such ions to their observed energies in the prevailing solar wind conditions. Alternatively, they might have comprised particles that had leaked from inside the magnetopause, perhaps undergoing shock drift acceleration in the process. Significant flux enhancements were also sometimes identified in the magnetotail (approximate energy range 30–50 keV). These are suggested to represent the signatures of O+ beams, impelled by acceleration processes similar to those associated with terrestrial ion beams.

The dependence of track response on registration temperature in Lexan and CR-39

ABSTRACT The track response of Lexan polycarbonate and CR-39 track detectors has been found to vary significantly with the polymer temperature during registration. This temperature effect may cause errors in cosmic ray composition experiments employing detector arrays exposed by means of high altitude balloon flights or earth orbiting spacecraft with passive temperature control. Outline results are presented for a series of exposures to high energy Fe beams at the LBL Bevalac during 1980 covering registration temperatures from −189°C to +48°C and using detector stacks sealed in an environment of dry air. Comparison is made with results from unsealed stacks and from detectors exposed in vacua.

Low energy charged particles in near Martian space from the SLED and LET experiments aboard the Phobos-2 spacecraft

Abstract The charged particle detector SLED on the Phobos-2 spacecraft has recorded, during a number of circular orbits about Mars, significant fluxes of ions with energies up to 200 keV in close spatial association with the Martian bow shock. The observed characteristics of these enhancements suggest that different shock acceleration mechanisms were operative in producing individual events

The ionisation dependence of the registration temperature effect in solid state nuclear track detectors

Abstract The Registration Temperature Effect in solid state nuclear track detectors has been investigated over a wide range of ionisation, using two types of polycarbonate detector and three types of doped allyl diglycol carbonate (CR-39) detector exposed to beams of 960 MeV/N uranium ions and 300 MeV/N iron ions at different temperatures. Beam spreaders were employed to provide a range of energies for both ions in each detector stack. The fractional change in signal strength per unit temperature change is found to be a strong, increasing function of ionisation for a given solid state nuclear track detector polymer at a given registration temperature. This fractional change in signal strength also appears to be an increasing function of charge for a given value of ionisation . In the case of both uranium ions and iron ions in polycarbonate track detectors, it is found that the mean fractional change in etch rate per degree centigrade, P, may be represented by an expression of the form P = KJM, in the energy regions studied, where K is a constant specific to the ion, J is given by Z2eff/β2 and the index, M, is ⋍ 0.75. These results constitute a major hardware “design driver” for future heavy nucleus experiments involving extended duration exposures of solid state nuclear track detectors on spacecraft.

THE FORM OF THE CR-39 RESPONSE CURVE AND ITS EFFECT ON PARTICLE RESOLUTION AND AREAS OF APPLICATION

ABSTRACT The form of the response curve for CR-39 throughout its useful ionisation range has been established. The important features are discussed and some examples of its application given.

Overview of recent analysis of the energetic particle observations recorded in situ by the EPONA instrument on the Giotto mission to Comet Halley

Abstract Highlights of recent results obtained through analyzing data recorded by the energetic particles experiment EPONA on the Giotto Mission to Comet Halley (March 12–15, 1986) from ∼7 × 10 6 km upstream of the bow shock (inbound) to ∼6 × 10 6 km upstream of the bow shock (outbound) are presented. The identities of the ions recorded, which displayed energies in excess of those attributable to the pickup process acting alone, are discussed and evidence is presented to indicate that these ions were of the water group. Indications of the local influence of various acceleration processes in energizing particles along different sections of the Giotto Encounter trajectory are provided. Spin modulation of the outgassing rate of the nucleus may have been responsible for quasiperiodic variations in the flux levels observed inbound and outbound. A dramatic burst of energetic particles deemed to be of cometary origin, recorded while the spacecraft traversed the magnetic pileup region and the magnetic cavity, is an unexpected feature of the in situ observations, uniquely recorded by the EPONA instrument.