J. Klarmann

Abundances of ultraheavy elements in the cosmic radiation: results from HEAO 3

We report here an analysis that, for the first time, systematically normalizes the data from the HEAO 3 Heavy Nuclei Experiment on .the cosmic-ray abundances of all the elements heavier than germanium to that of .iron. In the range of atomic number Z, 33 ≤Z ≤60, the analysis yields abundances of odd-even element pairs. These abundances are consistent with a cosmic-ray source having a composition similar to that of the solar system, but subject to source fractionation correlated with the first ionization potential (FIP) of each element. For Z > 60, the analysis yields abundances of element groups. For these heaviest nuclei, we find an enhancement of the abundance of the platinum group, elements with 74 ≤ Z ≤ 80, relative to that in a propagated solar system source, and a corresponding increase in the abundance of the largely secondary elements in the range 62 ≤ Z ≤ 73. These abundances suggest that there is an enhancement of the r-process contribution to the source nuclei in the Z > 60 charge region. Over the entire region of charge, standard leaky box models of propagation satisfactorily model secondary production.

Rapid two-dimensional dose measurement in brachytherapy using plastic scintillator sheet : linearity, signal-to-noise ratio, and energy response characteristics

Because of the large dose gradients encountered near brachytherapy sources, an efficient, accurate, low-atomic number areal detector, which can record dose at many points simultaneously, is highly desirable. We have developed a prototype of such a system using thin plates of plastic scintillator as detectors. A micro-channel plate (MCP) image intensifier was used to amplify the optical scintillation images produced by radioactive 125I and 137Cs sources in water placed 0.5-5.7 cm distance from the detector. A charge-coupled device (CCD) digital camera was used to acquire 2-D light-intensity distributions from the image intensifier output window. For both isotopes, a small area (2 x 3 mm2) PVT detector yields a CCD net count rate that is linear with respect to absorbed dose rate within +/- 3% out to 5.7 cm distance. Acquisition times range from 1.5-400 sec with a reproducibility of 0.5-5.5%. If a large-area (6 x 20 cm2) PVT detector is used, a four-fold increase in count rate and large deviations from linearity are observed, indicating that neighboring pixels contribute light to the signal through diffusion and scattering in PVT and water. A detailed noise analysis demonstrates that the image intensifier reduces acquisition time 10000-fold, reduces noise relative to signal 200-fold, and reduces amplifier gain noise as well.

Development of a second-generation fiber-optic on-line image verification system☆

PURPOSE We have previously reported the development of a fiber-optic fluoroscopic system for on-line imaging on radiation therapy machines with beam-stops because of space limitation. While the images were adequate for clinical purposes in most cases, an undesirable grid artifact existed and distracted visualization. The resolving power of the system, limited by the 1.6 mm x 1.6 mm dimension of the input fibers, appeared insufficient in some cases. This work identifies solutions to reduce grid artifact and to improve the resolution of the system. METHODS AND MATERIALS In the clinical system, it was found that the scanning mechanism of the newvicon camera was deflected differently at various gantry positions because of the different orientation of the earths magnetic field. The small image misregistration produced grid artifact during image normalization, particularly near boundaries of the fiber bundles. One approach taken to reduce magnetic field effects was to shield the camera with mu-metal. Alternatively, a charged-coupled-device camera was used instead of the newvicon camera. As for improving spatial resolution, fibers with smaller input dimension were used. A 20 cm x 20 cm high resolution fiber-optic prototype consisting of 250 x 250 fibers, each with an input dimension of 0.8 mm x 0.8 mm was constructed. Its performance was tested using several phantoms studies. RESULTS Both shielding the newvicon camera with mu-metal or replacing it with a charge-coupled-device camera reduced grid artifact. However, optimal shielding could not be made for our clinical system because of the space limitation of its housing. High contrast resolution was improved, the 30% value of the modulation transfer function occurred at 0.3 linepairs per mm for the clinical system and at 0.7 linepairs per mm for the high-resolution prototype. However, because of the larger degree of transmission non-uniformity of the prototype, it was less effective using the current setup in detecting low contrast objects. CONCLUSIONS The results are encouraging and demonstrate successful reduction of grid artifact and improvement of high contrast spatial resolution using the proposed methods. The less effective low contrast detection was related to reduced light collection efficiency due to use of prototype fibers whose productions were not closely monitored. The findings are being considered in our construction of a second generation clinical fiber-optic on-line image verification system.

Cosmic-ray abundances of elements with atomic number 26 less than or equal to 40 measured on HEAO 3

Individual elements in the cosmic radiation of even atomic number (Z) in the interval 26-40 have been resolved and their relative abundances measured. The results are inconsistent with a cosmic-ray source whose composition in this charge interval is dominated by r-process nucleosynthesis. The ratios of cosmic-ray source abundances to solar system abundances in this interval follow the same general correlation with first ionization potential as for the lighter elements, although there are deviations in detail.

Lead, platinum, and other heavy elements in the primary cosmic radiation - HEAO 3 results

An observation of the abundances of cosmic-ray lead and platinum-group nuclei using data from the HEAO-3 Heavy Nuclei Experiment (HNE) which consisted of ion chambers mounted on both sides of a plastic Cerenkov counter is reported. Further analysis with more stringent selections, inclusion of additional data, and a calibration at the LBL Bevalac, have allowed obtaining the abundance ratio of lead and the platinum group of elements for particles that had a cutoff rigidity R sub c 5 GV.

Scintillator-fiber charged-particle track-imaging detector

A scintillator-fiber charged-particle track-imaging detector was developed using a bundle of square cross section plastic scintillator fiber optics, proximity focused onto an image intensified charge injection device (CID) camera. The tracks of charged particle penetrating into the scintillator fiber bundle are projected onto the CID camera and the imaging information is read out in video format. The detector was exposed to beams of 15 MeV protons and relativistic Neon, Manganese, and Gold nuclei and images of their tracks were obtained. Details of the detector technique, properties of the tracks obtained, and preliminary range measurements of 15 MeV protons stopping in the fiber bundle are presented.

Large area pulse ionization chamber for measurement of extremely heavy cosmic rays

Abstract Parallel-plate ionization chambers with nearly 1 m 2 active area have been constructed and successfully operated in a series of high-altitude balloon flights. The chambers are used for identification of relativistic cosmic-ray nuclei with charge greater than or equal to 20. For relativistic iron nuclei (charge 26) the charge resolution (fwhm) achieved is 5.5 percent (i.e., less than 1.5 charge units).

A scintillating optical fiber track imaging detector

Abstract In this paper we describe a prototype of the scintillating optical fiber isotope experiment (SOFIE) and give results of a Bevalac calibration using iron nuclei to study the measurement precision in range and trajectory which can be obtained. We have measured the range of iron nuclei with approximate energy 500 MeV/amu entering the SOFIE instrument to a precision of 200–300 μm, and their transverse position coordinate to better than 35 μm. These results indicate that scintillating optical fibers are very useful as a range detector and as a high precision hodoscope for heavy nuclei.

Cosmic-ray abundances of individual elements in the Z interval between 26 and 30

The relative abundances of Fe, Co, Ni, Cu, and Zn in the cosmic rays have been measured using a large-area balloon-borne electronic detector system. The abundance ratios Ni/Fe and Zn/Fe are 5.0 +- 0.2% and 0.06 +- 0.01%, respectively. The Zn abundance is low (40%) compared with the Cameron (Cl) solar system, and is best consistent with the solar system C2 meteorite abundances. The ratios Co/Fe and Cu/Fe, extrapolated to the top of the atmosphere, are 0.68 +- 0.14% and 0.066 +- 0.030% respectively; since charge peaks are not resolved at Co and Cu, these results are taken as upper limits 0.8% and 0.1%, respectively. The Co upper limit is consistent with complete decay of /sup 57/Co at the source, and implies a lower limit of 2 years for the time between nucleosynthesis and acceleration of these nuclei.

Interaction mean-free-path of cosmic-ray Fe in air

The interaction mean-free-path of cosmic-ray iron in air has been determined by observing the change in flux as a function of atmospheric depth. A large detector and a large variation in altitude during the balloon flight result in a high precision measurement. The result is in agreement, though slightly larger than the mean-free-path deduced from the measurements of interactions of iron on different targets at the Bevalac accelerator.