Gamma-ray Continuum Research Articles

Study of continuum gamma rays following low angular momentum reactions

The continuum γ-rays following the reaction 166Er(α, 2n) 168Yb have been studied at bombarding energies of 21, 27 and 36 MeV. The shape of the statistical γ-rays below 1 MeV was deduced. A low energy bump at ∼300 keV was observed. This may arise from transitions along bands at and above the pairing gap.

Non-Collective Properties of the Continuum Gamma Rays Feeding Two High-Spin Isomers in 152Er

The decay and feeding of two yrast isomer (I ∼ 26ℏ and I ∼ 36ℏ respectively) in 152Er is measured. The isomeric delays are used to separate the spectrum of γ-rays from the highest angular momentum states from the spectrum due to the decay of low spin states. Strong dipole as well as quadrupole radiation dominates the yrast cascades which feed into the I ∼ 36ℏ isomer, and the correlation spectrum for the feeding transitions does not indicate any collective structure for spins below 50ℏ. It is tentatively concluded that the near magic nucleus 152Er remains spherical or oblate up to this high spin value.

Observations with the SMM gamma-ray spectrometer - The impulsive solar flares of 1980 March 29

Gamma-ray continuum emission from 0.3 to 1 MeV was observed with the gamma-ray spectrometer on the Solar Maximum Mission satellite during two impulsive solar flares on 1980 March 29, from active region 2363 at 0918 UT and from active region 2357 at 0955 UT. Evidence is presented for a hardening of the spectrum during the impulsive phase of the flares. The photon intensity greater than 100 keV appears to decay at a slower rate than that at lower energies. Time-integrated photon spectra for both flares are incompatible with a single-temperature thermal-bremsstrahlung model. Upper limits for prompt and delayed gamma-ray lines are presented.

Gamma-ray lines and continuum radiation from the galactic center direction

Our balloon-borne germanium ..gamma..-ray telescope has been reflown over Alice Springs, Australia. The detection of a sharp 511 keV electron-positron annihilation line from the galactic center (GC) direction has been confirmed at a flux level compatible with our previous result. No evidence for several other candidate lines was found, indicating that they were statistical fluctuations in the previous data. Evidence for a significant change in the GC inverse power-law continuum has been obtained, suggesting that its origin lies in the many unresolved and highly variable pointlike X-ray sources in the field of view.

HEAO 1 observations of gamma-ray lines from a solar flare

HEAO 1 observed gamma radiation, including the 2.223 MeV line of deuterium and the 4.43 MeV line of C-12, from a white-light solar flare of 1978 July 11. Line strengths over a 4 minute integration were 1.00 + or - 0.29 and 0.18 + or - 0.07 photons/sq cm s, respectively, and the continuum in the 1-5 MeV range fitted a spectrum 10 E to the -3rd photons/sq cm s keV. The 2.2 MeV line lagged 94 + or - 30 s behind the gamma-ray continuum, which itself was delayed about 20 s from the hard X-ray (not less than 40 keV) and microwave fluxes. This is the second flare for which both MeV-range lines and continuum have been observed, and the first for which simultaneous white-light observations exist. The prompt gamma-ray line (4.43 MeV) can be directly interpreted as an energy deposition of not greater than 7 x 10 to the 26th ergs per sec by energetic protons at photospheric depths. This is insufficient to maintain the white-light continuum by normal photospheric emission mechanisms.

GAMMA‐RAY OBSERVATIONS OF SEYFERT GALAXIES AND THE PROBLEM OF THE EXTRAGALACTIC GAMMA‐RAY BACKGROUND

Annals of the New York Academy of SciencesVolume 336, Issue 1 p. 267-277 GAMMA-RAY OBSERVATIONS OF SEYFERT GALAXIES AND THE PROBLEM OF THE EXTRAGALACTIC GAMMA-RAY BACKGROUND V. Schönfelder, V. Schönfelder Max-Planck-Institut für Physik und Astrophysik Institut für Extraterrestrische Physik 8000 Munich 40 Federal Republic of GermanySearch for more papers by this author V. Schönfelder, V. Schönfelder Max-Planck-Institut für Physik und Astrophysik Institut für Extraterrestrische Physik 8000 Munich 40 Federal Republic of GermanySearch for more papers by this author First published: February 1980 https://doi.org/10.1111/j.1749-6632.1980.tb15935.xAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Volume336, Issue1Ninth Texas Symposium on Relativistic AstrophysicsFebruary 1980Pages 267-277 RelatedInformation

Predictions of Induced Background Radiations at Gamma/X-Ray Experiment Envelopes in NASA Spacecraft

This work seeks to predict secondary radiation levels induced in spacecraft structures by space protons. The radiations analyzed are secondary neutrons from spallation and evaporation reactions and gamma and beta rays from the decay of induced radioactivity, as sources of interfering background to spaceborne measurements of galactic and planetary gamma rays below 10 MeV. The spacecraft considered are the Multi-Mission Spacecraft (MMS) and the Space Shuttle, modeled as spherical shells. The proton environment is that of the South Atlantic Anomaly, as well as cosmic ray protons. The induced radioactivity is analyzed in terms of its interference with various gamma-ray lines of astrophysical interest, as well as its contribution to several spectral regions of the gamma-ray continuum. The buildup of the line and continuum radioactivity background is predicted for a period of nearly 9 months in orbit (~4100 orbits). In addition, background contributions from cosmic ray electron bremsstrahlung and earth gamma-ray albedo are estimated.

Average Lifetimes of Collective Transitions in the Spin-(30-50) Region

A method is developed to determine average lifetimes of continuum gamma rays following heavy-ion compound-nucleus reactions. The resulting enhancement factors for $E2$ transitions depopulating states in the spin-(30-50) regions are of the same order of magnitude as for ground-state rotational bands of deformed nuclei. This shows for the first time that these high-spin states decay through strongly collective bands.

Steady-State and Transient Radiation Effects in Precision Quartz Oscillators

Radiation effects on 5 MHz 5th overtone AT cut Premium-Q quartz resonators are evaluated, comparing sweeping processes and resonator fabrication procedures. The resonators were irradiated at operating temperatures in oven controlled oscillator test beds with pulsed 10 MeV electrons from 200 rads up to 1 Mrad as well as with continuous 60Co gamma rays. Steady-state (permanent) and transient radiation-induced frequency changes were measured, the persistence of radiative preconditioning was investigated and the effect on drift rate was evaluated. From saturation characteristics of the steady-state frequency offset as function of total electron dose, the formation rate cross sections of several radiation-induced crystal defects have been evaluated. Formation of at least one defect is sensitive to differences in the sweeping process. In some resonators, frequency recovery after irradiation is dominated by an exponential decay mechanism with a time constant of 3.7 days. Under continuous gamma ray exposure with dose rates between 10 and 90 rad/hour, the frequency response is characterized by a steep initial rise followed by different patterns of saturation and recovery.

Gamma-Ray Production Cross Sections for Copper from 0.67 to 19.6 MeV

Gamma-ray production cross sections were measured for (n,x..gamma..) reactions in natural copper from 0.68- to 19.6-MeV neutron energy using the IRT Linac pulsed neutron source and a Ge(Li) detection system measuring gamma rays in the energy range from 365 to 6620 keV. Cross sections for 65 gamma rays are given using 17 neutron energy groups. For an additional 21 discrete gamma rays of uncertain origin, cross sections were determined for 11 neutron energy groups. The gamma-ray spectra for 16 neutron energy groups were also unfolded to obtain gamma-ray production cross sections for the sum of both discrete and continuum gamma rays. The cross sections are in general agreement with previous work for both the discrete peaks and the unfolded spectra, except near 15 MeV. Measurements of the discrete lines provide needed experimental data for the neutron energy region from 3 to 20 MeV. 6 figures, 3 tables, 13 references.

Gamma-ray and microwave evidence for two phases of acceleration in solar flares

Relativistic electrons in large solar flares produce gamma-ray continuum by bremsstrahlung and microwave emission by gyrosynchrotron radiation. Using observations of the August 4, 1972, flare, a detailed evaluation is made of the electron spectrum and the physical properties (density, magnetic field, size, and temperature) of the common emitting region of these radiations. Information is also obtained on energetic protons in this flare by using gamma-ray lines. From the electron spectrum, the proton-to-electron ratio, and the time dependences of the microwave emission, the 2.2-MeV line, and the gamma-ray continuum, it is concluded that relativistic electrons and energetic nuclei are accelerated in large solar flares by a mechanism which is different from the mechanism which accelerates electrons with energies not exceeding about 100 keV in flares.

Solar gamma rays

The theory of gamma-ray production in solar flares is treated in detail. Both lines and continuum are produced. The strongest line predicted at 2.225 MeV with a width of less than 100 eV and detected at 2.24±0.02 MeV, is due to neutron capture by protons in the photosphere. Its intensity is dependent on the photospheric 3He abundance. The neutrons are produced in nuclear reactions of flare accelerated particles which also produce positrons and prompt nuclear deexcitation lines. The strongest prompt lines are at 4.43 MeV from 12C and at ∼6.2 from 16O and 15N. These lines result from both direct excitation and spallation. The widths of individual prompt lines are determined by nuclear kinematics. The width of the 4.43 MeV line is ∼100 keV and that of the 6.2 MeV feature is ∼300 keV. Both these lines have been observed from a solar flare. Other potentially observable lines are predicted at 0.845 and 1.24 MeV from 56Fe, at 1.63 MeV principally from 14N and 20Ne, at 1.78 MeV from 28Si, at ∼5.3 MeV from 15O and 15N, and at 7.12 MeV from 16O. The widths of the iron lines are only a few keV, while those of the other lines are about 100 keV. The only other observed line is at 0.511 MeV from positron annihilation. The width of this line is determined by the temperature, and its temporal variation depends on the density of the ambient medium in the annihilation region. Positrons can also annihilate from the 3 S state of positronium to produce a 3-photon continuum below 0.511 MeV. In addition, the lines of 7Li and 7Be at 0.478 keV and 0.431 keV, which have kinematical widths of ∼30 keV, blend into a strong feature just below the 0.511 MeV line. From the comparison of the observed and calculated intensities of the line at 4.4 MeV to that of the 2.2 MeV line it is possible to obtain information on the spectrum of accelerated nuclei in flares. Moreover, from the absolute intensities of these lines the total number of accelerated nuclei at the Sun and their heating of the flare region can be estimated. We find that about 1033 protons of energies greater than 30 MeV were produced in the 1972, August 4 flare. The gamma-ray continuum, produced by electron bremsstrahlung, allows the determination of the spectrum and number of accelerated electrons in the MeV region. From the comparison of the line and continuum intensities we find a proton-to-electron ratio of about 10 to 102 at the same energy for the 1972, August 4 flare. For the same flare the protons above 2.5 MeV which are responsible for the gamma-ray emission produce a few percent of the heat generated by the electrons which make the hard X-rays above 20 keV.

Gamma-Ray Production Cross Sections for Iron from 0.86 to 16.7 MeV

Gamma-ray production cross sections were measured for (n, xγ) reactions in natural iron from 0.86- to 16.74-MeV neutron energy using the IRT Linac pulsed-neutron source. The 847- and 1238-keV gamma-ray production cross sections are presented with very high neutron resolution. Also, cross sections for 13 principal gamma rays are given using 20 neutron-energy groups. Cross sections were determined for an additional 16 discrete gamma rays using 10 neutron-energy groups. In general, the cross sections for the discrete gamma rays are in agreement with previous measurements. The gamma-ray spectra for the 10 neutron-energy groups were also unfolded to obtain gamma-ray production cross sections for the sum of both discrete and continuum gamma rays. In the present work the continuum component constituted over half the total gamma-ray production cross sections above 8 MeV.

Observations of solar gamma ray continuum between 360 keV and 7 MeV on August 4, 1972

Measurements were made of the time-averaged gamma ray energy loss spectrum in the energy range 360 keV to 7 MeV by the gamma ray detector on the OSO-7 satellite during the 3B flare on August 4, 1972. The differential photon spectrum unfolded from this spectrum after subtracting the background spectrum and contributions from gamma ray lines is best described by a power law with spectral index of 3.4±0.3 between 360–700 keV and by an exponential law of the form exp (-E/E 0) with E 0 = 1.0±0.1 MeV above 700 keV. It is suggested that this spectrum is due to nonthermal electron bremsstrahlung from a population of electrons, with a strong break in the spectrum at 2 MeV. Since the observational data indicates that the matter number density must be n H ⩾ 5 × 1010 cm-3 in the production region, the number of electrons above 100 keV required to explain the results is ⩽2 × 1034.

Gamma-Ray Lines from Solar Flares

We have treated in detail the theory of gamma-ray line production in solar flares. The strongest line, both predicted theoretically and detected observationally at 2.2 MeV, is due to neutron capture by protons in the photosphere. The neutrons are produced in nuclear reactions of flare accelerated particles which also produce positrons and prompt nuclear gamma rays. From the comparison of the observed and calculated intensities of the lines at 4.4 or 6.1 MeV to that of the 2.2 MeV line it is possible to deduce the spectrum of accelerated nuclei in the flare region; and from the absolute intensities of these lines it is possible to obtain the total number of accelerated nuclei at the Sun. The study of the 2.2 MeV line also gives information on the amount of He3 in the photosphere. The study of the line at 0.51 MeV resulting from positron annihilation complements the data obtained from the other lines; in addition it gives information on the temperature and density in the annihilation region and on the anisotropy of the accelerated electron beam which produces continuum gamma rays at energies greater than about 1 MeV.

Use of a Ge(Li) total-absorption spectrometer in (n, xγ) measurements

An 80 cm 3 Ge(Li) detector, sectioned to operate as a total-absorption spectrometer, has been used in (n, xγ) cross-section measurements using the Linac time-of-flight technique. Use of a computer-controlled data acquisition system allowed the detector to be operated simultaneously as an ordinary Ge(Li) detector (singles mode) and as a total absorption spectrometer (Duode mode). Typical operating characteristics of the detector are given and its performance illustrated with examples of gamma-ray spectra from (n, xγ) reactions. The determination of true continuum gamma-ray intensities by unfolding the spectra measured in the singles mode is described and typical results of this Ge(Li) spectrum unfolding are shown.

Gamma-ray spectra arising from thermal-neutron capture in elements found in soils, concretes, and structural materials

Abstract : Gamma-ray spectra arising from the capture of thermal neutrons are presented for iron, aluminum, copper, zinc, titanium, nickel, silicon, calcium, potassium, sodium, barium, chlorine, sulfur, and stainless steel. The spectral intensities in photons per 100 captures are presented as both individual lines and as sums of these lines over 0.5-MeV intervals which are intended for use in shielding calculations. The latter results have an estimated accuracy of plus or minus 15%, and include the contributions from both discrete and continuum gamma rays. Comparisons of these spectra are made with previously determined spectra, as well as with values reported in the Reactor Handbook, and considerable revision in the latter is concluded to be necessary. The present results do not go below 1 MeV, but when the relatively small contributions from energies less than 1 MeV are added to them, binding energy checks within plus or minus 5% are obtained. (Author)

Compton recoil gamma-ray spectroscopy

A method of continuous gamma-ray spectroscopy based on Compton electron continuum measurements has been developed. Recoil electron energy spectra are observed in lithium-drifted silicon solid-state detectors. Iterative unfolding is used to determine the gamma-ray continuum from the observed electron recoil energy distribution. Limitations of the method are discussed. Absolute measurements in the energy region 0.2–2.2 MeV are possible with an accuracy of approximately 20%. In-core gamma continua observed in a depleted uranium Snell block, a graphite thermal column, and a fast reactor critical assembly are presented.

Magnetic Internal Compton Coefficients in the Born Approximation

The absolute and relative probabilities of a nuclear transition in which there is a simultaneous ejection of a $K$ electron and emission of a gamma ray has been calculated quantum-mechanically for the case in which the virtual radiation field is a ${2}^{L}$ magnetic multipole. The process will be referred to as an internal Compton effect. The Born approximation has been used and hence the calculations are expected to be valid only if the nuclear charge $Z$ is small and the nuclear energy level separation large. The ratio of the number of continuous energy gamma rays to the number of discrete energy $K$ electrons is independent of the nuclear matrix element and decreases as $Z$ or as $L$ increases. This is true for any energy range and any angular range of the continuous energy gamma rays, where the angles are measured with respect to the coincident continuous energy $K$ electron. The angular distribution is most sensitive to $L$ for comparable electron and gamma-ray energies and for large angular separation. These conclusions are not much affected by inclusion of the $L$ shell. In the limit of small gamma-ray energies, the results reduce to those predicted by semiclassical theory.