Soft Γ-ray Research Articles

Sigma/GRANAT soft gamma-ray observations of the X-ray nova in Musca - Discovery of positron annihilation emission line

The day after its discovery by the Watch instrument, the X-ray nova GRS 1124-684 in Musca was detected by the soft γ-ray telescope SIGMA at the limit of its field of view. SIGMA pointed the source seven other times between 1991 January and February, and GRS 1124-684 has always been detected up to 300 keV, showing it was one of the hardest objects of the sky. After the flare of January 9 the average spectrum is well fitted by a power law of index 2.38, and the light curve shows a slower decrease than observed at low energy with superposed variability on time scales of several hours

Systematic treatment of the photopeak efficiency

The method introducing the effective interaction depth for the photopeak efficiency proposed for small detectors was applied to three large germanium detectors and found feasible for large detectors too, except for soft γ-rays measured at small source-to-detector distances. These exceptional cases, are caused by attenuation by the absorption layer, or by ineffective core in the case of coaxial Ge(Li) detectors. Coincidence summing effects are also discussed in order to extract a simple relationship between the magnitude of the coincidence summing and the source-to-effective-volume distance regardless the details of individual cascade relations. This enabled us to devise a correction method for the coincidence summing even when the decay scheme is unknown for the relevant γ-ray.

Possible models for the high-energy transient GB790107

The discovery of rapid repetitive soft γ-ray burst events on time-scales as short as seconds from GB790107 by Laros and co-workers and Atteia and co-workers1–3 may have important implications for our understanding of the γ-ray burst mechanism. Although there is a general consensus that the high-energy transient phenomenon in the X-ray range known as Type I X-ray bursts can be successfully explained in terms of a thermonuclear flash in the accreted surface layers of a neutron star4,5, the theory for γ-ray bursts is still controversial. In fact, it is possible (perhaps even likely) that there is more than one mechanism by which the γ-ray burst phenomenon can be understood. The properties of GB790107 place severe constraints on the viability of models proposed for the typical γ-ray burst events as applied to this soft γ-ray repeater. Here we review the various models proposed for γ-ray bursts and show that a model involving a comet cloud around a neutron star is consistent with the observational data.

Evolution and use of nuclear detectors and systems

A survey is given of the current status of theoretical and experimental studies of elastic photon-atom scattering, with particular emphasis on the soft γ-ray energy regime 59.5 keV–1.33 MeV. Basic assumptions of theory, simple approximations, and best available calculations are described. The focus is on the Rayleigh scattering amplitudes for elastic scattering off the bound atomic electrons, but it is also necessary to discuss Delbrück and nuclear amplitudes. The numerical code of Kissel is central to the results which are obtained. A new reference data set of theoretical predictions is presented for a grid of 10 elements (Z = 13–103) and 7 energies (59.5 keV–1.33 MeV) at 55 scattering angles (0–180°).Basic considerations of the experiment are described, including sources and detectors and experimental arrangements. All modern experiments (using Ge detectors) are summarized, and older experiments are listed for reference. Finally, the status of comparison between theory and experiment is examined. All experiments available on the reference grid of theoretical calculation are compared with the predictions; agreement appears generally satisfactory. The situations at higher energies (focusing on Delbrück scattering) and lower energy X-ray scattering (anomalous scattering) are also discussed.

A New Nuclear but Non Radioactive Method for Rapid Elemental Analysis of Clays

AbstractA method using the bombardment of samples with protons and other positive ions of energy in the MeV range is described: the prompt atomic (γ-rays) and nuclear (γ-rays and charged particles) events generated during the bombardment, are detected with energy sensitive solid state detectors: cooled Ge(Li) for γ-rays and hard γ-rays, cooled Si(Li) detectors for soft γ-rays (3–20 keV) and Si-barrier detectors for charged particles. These 3 types of detectors can be simultaneously used for multipurpose experiments. These techniques are particularly useful for simultaneous detection of all the elements of interest in the sample. Comparisons are made with other microtechniques. Analyses of Li, Na, K, Ca, Rb, Cs and Cl exchanged in appropriate chloride solutions have been achieved for several clays (Geisenheim, Isola di Ponza, Layton). A new phenomenon relative to the anion contribution in cation exchangers is observed. A method of profile analysis of Na by resonant nuclear reactions is discussed.

An extension of classical bone mineral measurements

Cameron's concept of bone mineral measurement using soft γ-rays has been extended to determine the area and shape of the compacta cross-section and the corresponding mean linear absorption coefficient. By repeating the linear scanning process at the same section for different directions a polygon is defined which circumscribes the outer contour of the bone section. With the aid of a PDP 11–45 computer and an iterative technique the compacta area and the mean linear absorption coefficient are evaluated. In addition the computer synthesizes iteratively an image of the actual compacta cross-section. Thanks to the redundancy of the absorption data the values for the total mineral content, the compacta area and the mean density can be determined as many times as the scanning process has been carried out. The fluctuations of these values from their averages can apparently be interpreted as a measure of the consistency of the data. To validate the method and assess the accuracy and statistical fluctuations of the measurements an aluminum tube with circular cross-section has been used as a model for a bone. The data indicate that the total mineral content, the compacta area and the mean density can be determined with an error on the order of 1%. Data from measurements on excised human femurs have been obtained and the results are equally consistent as those of the aluminum model.

Emission of soft bremsstrahlung?-rays in p + p ? p + p +? and ??+p ? ?0+n +? reactions

Low's theorem [1] on the emission of soft γ-rays is applied to a study of p + p→ p+p+γ and π− + p→ π0+ n + γ reactions. The first two terms in the expansion of the differential cross sections for these processes in powers of the photon energy are obtained (for proton scattering in the ultrarelativistic limit).

Cerebral blood flow estimated by 133 Xe clearance technique.

RADIOACTIVE XENON (133Xe) inhalation and clearance techniques for the measurement of cerebral blood flow have been described previously.1-3It is the purpose of this paper to present in detail our experiences with such a method and to examine the limitations of the method more fully. Principle of Method When the inert gas xenon is mixed with air and breathed, it crosses the pulmonary alveolar membrane and is conveyed by the arterial blood to all parts of the body. The isotope of xenon,133Xe, emits a soft γ-ray of energy equal to 81 kiloelectron volts; thus, cerebral radioactivity, due to the presence of this isotope, can be measured by external scintillation counting. It has been stated4,5that the blood flow in the scalp and the scattered radiation from the chest do not contribute significantly to the radioactivity detected in this way. It is suggested that cerebral

Chemical Use of the Absorption of Soft γ-Rays, β-Ray-excited X-Rays and K-Capture X-Rays. III. Application to Column-chromatographic Separation of Halogen Compounds

Abstract The absorption technique of soft γ-rays, β-ray-excited X-rays and K-capture X-rays was successfully applied to column-chromatographic separation and purification of halogen compounds. By this technique, the behavior of the iodine- and bromine-containing compounds in the column was determined in situ, and important information for column chromato-graphy, including the retention volume, was also satisfactorily applied to the frontal analysis of halogen compounds. An excellent method for the preparation of pure 4-iodobiphenyl by the use of the in-situ determination was provided.

Chemical Use of the Absorption of Soft γ-Rays, β-Ray-excited X-Rays and K-Capture X-Rays. II. Application to the Analysis of a Binary Alloy and the Determination of a Heavy Element in a Multi-component System

Abstract The absorption phenomenon of soft γ-rays, β-ray-exited X-rays and K-capture X-rays was applied to the following two kinds of analyses (1) Analysis of solder (simultaneous determination of the composition of a binary system and the sample thickness), and (2) Determination of an element of high atomic number in a multi-component system). Two photon-beams of energies suitable for each purpose were chosen, and for the letter purpose the retionship between the absorption coefficient of an element and the photon-beam energy was used. The results were in good agreement with those obtained by usual chemical anaiysis.

Chemical Use of the Absorption of Soft γ-Rays, β-Ray-excited X-Rays and K-Capture X-Rays. I. Improvement in Fundamental Technique and Introduction of a Differential Absorption Method

Abstract Studies for the improvement of fundamental techniques for the chemical use of the absorption of soft γ-rays, β-ray-excited X-rays and K-capture X-rays were undertaken. The exponential absorption curve, which is essential for the measurement of the accurate absorption coefficient and for many quantitative works, was obtained by the use of a proper detection method and the differential absorption technique, even in the presence of some photons having various energies. The relationship between the absorption coefficient thus obtained and the photon energy proved to agree with the theoretical result.

Spectra of soft γ-rays from Cu63, Cu65, Ag107, Ag109 and In115 resulting from thermal neutron capture

Spectra of soft γ-rays from Cu63, Cu65, Ag107, Ag109 and In115 resulting from thermal neutron capture

Soft γ-rays from the capture of thermal neutrons by nuclei

Abstract A luminescent spectrometer was used to measure the energies and absolute intensities of the γ-quanta emitted by Co59, Rh103, I127, Sm149, Au197 and Hg199 nuclei following the capture of thermal neutrons. It is shown that the γ-lines in the 50–500 keV energy range detected in the present work can be ascribed to transition from the lower excited states of the investigated nuclei. A description of the apparatus and method of measurement is presented.

Radiobromine

Don C. De Vault and W. F. Libby1 and E. Segrè;, R. S. Halford and G. T. Seaborg2 have shown that the 4.5-hr. radiobromine transforms into the 18-min. isotope which is β-active, by the emission of soft γ-rays. Le Roux, Lu and Sugden3 find that this is the method of disintegration of at least 90 per cent of 4.5-hr. radiobromine. Segrè, Halford and Seaborg2 predicted that the γ-rays would be soft, and a large proportion would be internally converted.

Chemical Separation of Isomeric 80Br Atoms

FOR rather more than a year, we have been studying the concentration of radiobromine by modifications of the method of Szilard and Chalmers1, and have been using concentrates to follow the kinetics of exchange reactions between radiobromine ions and alkyl bromides. In the course of this work, we have encountered a number of puzzling phenomena which are now completely explained by the remarkable discovery made by Segre, Halford and Seaborg2 and DeVault and Libby3 that the isomer of 80Br with a half-life of 4.5 hours decays by a γ-ray change to an isomeric 80Br which emits β-rays with a half-life of 18 minutes. The recoil from the soft γ-ray emitted by the parent nucleus (and its internal conversion) has been found sufficient to break a chemical bond in the BrO3 anion or to activate tho C–Br bond in tertbutyl bromide and facilitate the hydrolysis of this compound. These reactions have been used to separate the isobaric 18-min. product from its parent 4.5-hour isomer with which it was previously in radioactive equilibrium. The β-ray activity of the daughter isotope in the reaction product was found to decay with a half-life of 18 min., whilst the activity of the residual bromine in the original reaction increased with time as the equilibrium concentration of the 18-min. isomer was restored by decay of the parent.

Selection Rules in Nuclear Radiation

AbstractIn the first part of this paper we discuss the radiation from a single charged particle moving in an arbitrary central field of force and obeying Dirac's equation. We consider the electric quadripole and magnetic dipole radiation as well as the electric dipole. We derive the selection rules for the magnetic dipole radiation and collect together for reference the corresponding selection rules for the electric dipole and quadripole radiations. In the second part we discuss the relative intensities of the various types of radiation, treating in detail the cases where the selection rules for magnetic dipole and electric quadripole are simultaneously satisfied. Finally we show that these results have an important bearing on the theory of internal conversion of γ-rays. The internal conversion of soft γ-rays occurs with such high probability that the theory is unable to account for the experimental results unless it is assumed that the radiation is largely magnetic dipole in character. On the other hand, Fisk and Taylor (loc. cit.) were unable to account for the presence of magnetic dipole radiation in appreciable amounts. We show that this is due to the fact that, of the two possible transitions (a and e of § 2) in which both magnetic dipole and electric quadripole radiation can be emitted, Fisk and Taylor considered only the second. In the case of the second, corresponding to a transition between two distinct terms, we show that Fisk and Taylor were correct in predicting a negligible amount of magnetic dipole radiation, but in the case of the first, corresponding to a transition between two levels of one multiplet term, we find that there is indeed a high percentage of magnetic dipole radiation.

The calculation of internal conversion coefficients of γ-rays

The purpose of this paper is the calculation of theoretical values for the internal conversion coefficient I, of γ -rays converted K-and L 1 -shells. Hulme has obtained values for I in the K-shell assuming the radiating nucleus to emit the field of a dipole; while Taylor and Mott have assumed a quadripole field. The internal conversion coefficient has here been calculated for a number of values of hv for the L I -shell applying the theory developed by Taylor and Mott; the previous results for the K-shell have been extended and slight errors in the region of soft γ-rays have been corrected. Finally, the limiting values for very soft γ -rays have been obtained for both K-and L I -shells, with both quadripole and dipole fields. The calculations have been carried through using a value of the charge number Z = 84. The correction for RaB (Z = 82) would be small. Recently Taylor and Mott have extended their theory to account for the interaction between nucleus and the extranuclear electrons. They have shown that the "internal conversion coefficients" as calculated in H II and TM I are not a measure of the ratio Number of β-particles ejected in time dt /Number of γ -quanta leaving the nucleus in time dt , but, apart from a factor in general effectively unity, give I ≡ Number of β-particles ejected in time dt /Number of γ -quanta escaping from the system in time dt .

Speculations concerning the α-, β- and γ-Rays of Ra B, C, C.- Part I. A revised theory of the interval absorption coefficient

1. Introduction .- It is well known that the γ-ray emitted by radioactive nuclei are often very strongly absorbed in a species of photoelectric effect by the planetary electrons of the parent atom, thus giving rise to the sharp lines of the β-ray spectrum. The recent work of Ellis and Aston provides numerical values of this “internal conversion coefficient” of the γ-rays from Ra BC and Ra CCD-data which bring out more clearly than hitherto the curiosities of this coefficient (see Table I below). The present paper is the outcome of repeated discussions between Dr. Ellis and the author about possible explanations of the data. All the preliminary work was carried out in collaboration, and it was our original intention to publish our results jointly. This has proved impossible, so that the responsibility for the corrections of the calculations of this paper rests entirely on me. But the work could not have been attempted without Dr. Ellis’s help. Ellis and Aston find that this coefficient behaves so differently for different classes of β-ray line that one is even sometimes tempted to suspect a different mechanism of emission. The rather soft γ-rays of Ra BC are converted by the K-level electrons in fractions of the surprising size of 10 percent. To 25 percent., varying in a normal manner with the frequency. The harder γ-rays of Ra CC’D fall, however, into three groups. The main γ-rays of energy from 6·12 X 10 5 to 12·48 X 10 5 volts have a practically constant conversion fraction of about 0·006; those from 13·90 X 105 to 22·19 X 10 5 volts a similar constant fraction now about 0·006; those from 13.90 X 10 5 to 22·19 X 10 5 volts a similar constant fraction now about 0·0015. All these γ-rays, though most easily studied via their derived natural β-ray lines undoubtedly can be studied as γ-rays outside their parent atom. But there is a third group containing one outstanding γ-ray of 14·26 X 10 5 volts energy whose existence has been inferred only from certain associated β-ray lines, among the strongest in the spectrum. (if is it a γ-ray) is so strongly converted in the parent atom that it is doubtful if it has ever been observed as a γ-ray at all. It would be consistent with (though not necessitated by) the evidence, to assert that it is a γ-ray with an internal conversion factor unity.