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Gamma-Ray Transitions Induced in Nuclear Spin Isomers by X-Rays
Abstract
Because of the high density of energy storage and the large cross section for its release, nuclear spin isomers have attracted
considerable recent interest. The triggering of induced gamma emission from them has encouraged efforts to develop intense
sources of short-wavelength radiation. One of the more interesting examples is the 16+ 4-qp isomer of 178Hf which stores 2.445 MeV for a half-life of 31 years meaning that as a material, such isomeric 178Hf would store 1.3 GJ/g. Recently, a sample containing 6.3×1014 nuclei of the isomer of 178Hf was irradiated with X-ray pulses derived from a device operated at 15 mA to produce bremsstrahlung radiation with end point
energies set to values between 60 and 90 keV. Emission of gamma radiation from the sample was increased by 1–2% above the
quiescent value of spontaneous emission. Such an accelerated decay of the 178Hf isomer is consistent with an integrated cross section of 2.2×10−22 cm2 keV if the resonant absorption of the X-rays takes place below 20 keV as indicated by the use of selective absorbing filters
in the irradiating beam. The work reported here describes the current experimental focus and results recently obtained with
the use of coincident detection of emitted gamma photons by several detectors.
... Filtering of the X-rays with Al foils showed that pulsed irradiation by the parts of the Bremsstrahlung with E < 20 keV increased the rate of spontaneous emission of γ photons corresponding to some of the characteristic transitions found in spontaneous decay of 178 Hf m 2 by a few percent. Integrated cross sections of the order of 2.2 × 10 -22 cm 2 keV were reported [12,13]. The enhancement was found to scale linearly [14] with flux at 20 keV around 10 10 photons cm -2 keV -1 s -1 . ...
... The authors attempted to interpret the negative results as "proving" an upper limit on the cross section for accelerated decay of the isomeric 178 Hf m 2 that was considerably smaller than the established value [25] of 1.6 to 2.0 × 10 -3 of the photoionization cross section for L3 electrons. However, from the published data [25], it can be seen that the instrumentation used to search for γ photons from accelerated decay was totally blind at the energy of the "new" line reported [13] to be a unique result of triggered decay. Moreover, the γ spectra shown [25] were collected with a gate set to open seconds after the irradiating SR X-rays were moved off the target. ...
... However, when decay is induced by K-mixing, then "new" γ lines should be expected in the spectrum as larger changes of K are "mixed in" by the coupling to the electron transitions. One such line has been reported [13] by our group near 130 keV to arise from both triggering with Bremsstrahlung X-rays and with monochromatic X-rays from SR sources. Moreover, since the moment of inertia of the Hf nucleus is large and only little affected by quasiparticle excitation, the spacing of levels within bands often repeats. ...
Cited By (since 1996):13, Export Date: 27 April 2013, Source: Scopus
... While only the bands of excited states involved in spontaneous decay are shown in Fig. 1, there are 17 other known bands which are not involved in spontaneous decay because of selection rules. Also shown in Fig. 1 is a K-mixing level ( Collins et al., 2001a) which has been assumed to have energy close to that of the isomeric 16+ state and mixed values of the pure quantum states of the projections of angular momenta upon which the selection rules act to inhibit spontaneous decay. The usual path of decay is to cascade down the 8band until the 8-state at the bandhead is reached. ...
... It has a 4 s half-life that introduces a statistical time lag in further cascading into the ground state band (GSB). In the pulsed Bremsstrahlung experiments with coincidence detection using 4 Ge detectors, we showed that the accelerated decay benefited from a ''short circuit'' around the 8-level because the induced g-photons from the GSB were detected less than 1 ms after the application of the pulse of irradiation ( Collins et al., 2001a). Evidently the involvement of inner shell electrons in the nuclear transitions relaxed the constraints imposed by the selection rules by making available for change the additional angular momentum of those electrons. ...
... We found an excitation function that peaked near the L III and L I edges for photoionization in Hf. Additional evidence for the ''short-circuit'' around the delay associated with the 8-level was found in the emission induced by the X-rays of g-photons in a line near 130 keV, not present in spontaneous decay ( Collins et al., 2001a). ...
A nuclear-XAFS effect occurs during the scattering of electrons or X-ray photons when the excitation of inner shell electrons couples both energy and angular momentum into nuclear channels of excitation. Nuclear transitions can be induced if the density of excited states of the nucleus enclosed by the electrons is high enough. The 31-yr isomeric state, 178m2Hf satisfies the condition for excitation by monochromatic X-rays from synchrotron sources. Strong effects upon such nuclei are studied.
... This final report describes work continued with AFOSR support using SR sources for triggering the release of the energy stored in the ' 78 Iff isomer during the period March 1, 2001 to August 31,2005. However for continuity, it is convenient to recall that positive confirmation and extension of those first results were subsequently reported with a nearly annual frequency [2][3][4][5][6][7][8]. Interleaved with those publications were reports of experiments purported to disprove all of the positive results [9][10][11][12]. ...
Nuclear spin isomers store electromagnetic energy at densities reaching 1.3 GJ/g for shelf lives approaching centuries. The best combination of shelf life and energy storage is found in Hf-178. It has been shown to release its stored energy when triggered by x-ray flashes at modest power levels. In this work trigger pulses were derived from tunable, monochromatic x-rays produced by synchrotron radiation sources. It was proven that the triggering was initiated by photoionization of an electron from the L-shell surrounding the isomeric nucleus. A fraction of 0.16% of those photoionizing events led to triggering of the release of the energy stored by the Hf-178 nuclear spin isomer. Confidence in these positive experimental results reached 92 sigma which is equivalent to 1,824 nines of certainty.
Comparing the spectra acquired during two series of irradiations, it was found that the peak intensities of the 178m2 Hf isomer are enhanced at the end of each series of irradiations. At the same time, the intensities of the ¹⁵⁰ Eu and ¹³³ Ba peaks, which are present in the isomeric target, keep stable. This means that some additional activity is produced during the irradiations. The half-life of this activity seems to be of the order of several days.
High-spin nuclear isomers have been recognized as the most promising way to create a gamma-ray source controlled by low energy X rays. This was recently n proved by triggering the release of the isomeric state energy, with a Hf-178m2 isomer source irradiated by soft X-rays from both a classical bremsstrahlung source and synchrotron radiation. Here, we report on an extension of this approach for the electron excitation of Lu-176 nuclei, the beta decay of which is highly K-forbidden (DeltaK = 7), just as the internal transition of the Hf-178m2 isomer (DeltaK = 8). A Lu-nat target containing 2.6% Lu-176 was irradiated by electrons with an energy of 75 keV and a flux of around 6 x 10(14) electrons s(-1) cm(-2). The fractional enhancement over the normalized spontaneous decay rate was found to be about 16%. The corresponding excitation cross section can be estimated as (1.6 +/- 0.6) x 10(-14) cm(2).
High-spin nuclear isomers have been recognized as the most promising way to create a gamma-ray source controlled by low energy X rays. This was recently n proved by triggering the release of the isomeric state energy, with a 178m2Hf isomer source irradiated by soft X-rays from both a classical bremsstrahlung source and synchrotron radiation. Here, we report on an extension of this approach for the electron excitation of 176Lu nuclei, the β decay of which is highly K-forbidden (ΔK = 7), just as the internal transition of the 178m2Hf isomer (ΔK = 8). A natLu target containing 2.6% 176Lu was irradiated by electrons with an energy of 75 keV and a flux of around 6 × 10 14 electrons s-1 cm-2. The fractional enhancement over the normalized spontaneous decay rate was found to be about 16%. The corresponding excitation cross section can be estimated as (1.6 ± 0.6) × 10-34 cm2.
We considered the NEET (nuclear excitation by electron transition) as a possible triggering mechanism of the isomer 178Hfm2 during ionization of the L3 atomic shell by x-rays. This 16+ isomer is assumed to be excited into an intermediate state 15- by E1 electronic transition between M5 and L3 shells. Simple nonrelativistic formulas are derived for the NEET probability. The estimations show the probability to be less than the experimental data of [1] by one order of magnitude. The intermediate level is found to decay bypassing the isomeric level 16+, if the nucleus attributes a triaxial shape in the 15- state and, besides, there exists a level 13- shifted with respect to 15-- by 400 keV. We have shown also that the NEET cross section σNEET (E) as a function of the energy of x-ray photons E, has to accept constant value above the L 3 photoionization threshold in contrast to narrow peak observed by [1].
Yield curves and cross sections are obtained for the reactions 113In(γ, γ′)113mIn (T1/2 = 1.7h, Jπ = l/2-), 195Pt(γ, γ′)195mPt (T1/2 = 4.1 day, Jπ = 13/2+), and 199Hg(γ, γ′)199mHg (T1/2 = 42.6 min, Jπ = 13/2+). The 113In(γ, γ′)113mIn reaction, the cross section reaches its maximum at the energy about 9.0 MeV, whereas the cross sections for the 195Pt(γ, γ′)195mPt and 199Hg(γ, γ′)199mHg reactions reach the maximum at E ≈ 6.4 MeV. The position of the resonance for all nuclei studied coincides with the thresholds of the corresponding (γ, n) reactions.
178m2Hf isomer production in different spallation reactions with protons, α particles and neutrons at projectile energies up to 100 MeV has been analyzed using both STAPRE and ALICE code simulations. The STAPRE code was used to calculate the isomeric ratios, while the ALICE code was used to simulate the excitation functions of the respective ground states. A number of spallation reactions have been compared taking into account not only 178m2Hf isomer productivity but also, first, the isomeric ratios calculated by the STAPRE code; second, the accumulation of the most undesirable Hf isotopes and isomers, such as 172Hf, 175Hf, and 179mHf; and, third, the production of other admixtures and by-products that could degrade the quality of the produced 178m2Hf isomer sources, including all stable Hf isotopes as well. Possibilities and ways of optimizing 178m2Hf isomer production in spallation reactions at projectile energies up to 100 MeV are discussed. This can be considered a very important preliminary stage for accumulating such exotic nuclear isomers in lab-sized quantities at reasonable cost.
178m2Hf isomer triggering was studied using the upgraded experimental setup developed in Kharkiv National Uni-versity and installed at Kyiv Institute for Nuclear Research. The target presenting a single Ta foil of 300 μm thickness with 178m2Hf isomeric activity of about 100 Bq was irradiated by 30 keV electron beam. The enhanced counting rates of all the ground-state band transitions were observed. Our data are consistent with an estimate for the triggering effect of (2. 9 ± 0. 7)% and corresponding triggering cross-section can qualitatively be estimated as σtrig = 4. 2 × 10-27cm2.
Adopting a proactive approach and focusing on emerging radiation-generating technologies, Health Physics in the 21st Century meets the growing need for a presentation of the relevant radiological characteristics and hazards. As such, this monograph discusses those technologies that will affect the health physics and radiation protection profession over the decades to come. After an introductory overview, the second part of this book looks at fission and fusion energy, followed by a section devoted to accelerators, while the final main section deals with radiation on manned space missions. Throughout, the author summarizes the relevant technology and scientific basis, while providing over 200 problems plus solutions to illustrate and amplify the text. Twelve appendices add further background material to support and enrich the topics addressed in the text, making this invaluable reading for students and lecturers in physics, biophysicists, clinical, nuclear and radiation physicists, as well as physicists in industry.
A recent experiment where the irradiation of an isomeric sample with X-ray pulses resulted in the accelerated decay of the isomeric states and rapid energy release is the basis for this paper. This indication of controlled or triggered decay of the isomer and release of energy suggests that engines and aircraft operating on this triggered release of energy may be possible. The possible use of a triggered isomer energy source in a heat exchanger as part of a hybrid long-endurance, dual-energy source jet engine is described. The application of such an engine in a high-altitude, long-enduance aircraft is discussed. Engine mode changeover operation from conventional combustion to an isomer energy source is described by identifying the switchover condition as an altitude and Mach number. Comparisons of conventional and hybrid engines indicate that a hybrid turbofan engine flying partially on conventional fuel coupled with a switchover to isomer power at altitude could provide weeks of endurance. Takeoff weight could also be reduced due to lower conventional fuel requirement.
Whether or not the 178Hfm2 isomer has the
characteristic of induced γ emission by low-energy x rays has been
a focus of attention for many scientists and researchers in recent
decades. In this paper, an experiment regarding triggering
178Hfm2 emission decay is conducted at Shanghai
Synchrotron Radiation Facility. An x-ray beam with 20.825-keV energy
irradiates a rectangular sample containing about
1011178Hfm2 nuclides. By comparing the
net gamma count rates during the irradiation with those after the
irradiation, the data show that at the 426-, 495-, and 574-keV gamma
lines, there is no significant enhancement, indicating that the induced
γ decay of 178Hfm2 has not been observed.
It is widely acknowledged that the 178m2Hf nuclide is the most suitable substance to study the decay characteristic of the isomer induced by low-energy X-ray. In order to conduct the experiment on the induced gamma emission, the research group has started producing the 178m2Hf nuclide based on the 176Yb(α, 2n)178m2Hf reaction. After the chemical purification is conducted, the sample is prepared and used in Shanghai Synchrotron Radiation Facility. During the production of isomer, the natural metal Yb target is got through magnetron sputtering. Bombarded by α particles about 27 MeV, the 178m2Hf nuclide reaches about 1012. Yb target prepared in this way is most suitable for the production of 178m2Hf nuclide in the CS30 cyclotron. There are various nuclides in the irradiated target and the main long-lived nuclides are 173Lu, 172Lu, 175Hf, 172Hf and 65Zn. The chemical separation of 178m2Hf is studied and its process is monitored by radioactive tracer. The above result shows that decontamination factors of Zn and Lu are 105 and 103, respectively, and the yield of hafnium is 69%. Under the protection of vacuum filtration technology, the purified 178m2Hf isomers are entirely transferred to the surface of filter paper, in order to form the sample which satisfies requirements of X-ray triggering the 178m2Hf isomer decay experiment in Shanghai Synchrotron Radiation Facility in the future.
The production of several grams or more of the isomer 178m2Hf is an extremely difficult task. So far, no effective processe for such production has been described in the literature. There are some reactions which could be used for the purpose, but these methods are very costly and require large investments. Theoretical calculations and the analysis of the existing experimental data suggst that the hafnium problem does not exist. by.
This article reviews experimental results obtained recently on the x-ray-induced acceleration of the decay of the long-lived isomer 178Hfm2. Two basic mechanisms for the induced decay are considered: (1) direct interaction of the incident x rays with the nucleus and (2) the nucleus-ray interaction proceeding via atomic shells. We establish that the absence of K forbiddenness for all transitions to a hypothetical “mixed K“ level cannot explain the measured cross sections even if collective nuclear matrix elements, resonant conditions, and so on, are assumed. We also tested, and rejected, the hypothesis that the enhancement is due to normal nuclear transitions in the inverse nuclear excitation by electron transition process. The possibility to make measurements with intense laser radiation is considered too. Thus, there appears to be no explanation of these experimental results within quantum electrodynamics and the contemporary concepts of atomic nuclei.
The spontaneous decay of the Kπ=16+, 31-yr 178Hfm2 isomer has been investigated with a 15-kBq source placed at the center of a 20-element γ-ray spectrometer. High-multipolarity M4 and E5 transitions, which represent the first definitive observation of direct γ-ray emission from the isomer, have been identified, together with other low-intensity transitions. Branching ratios for these other transitions have elucidated the spin dependence of the mixing between the two known Kπ=8- bands. The M4 and E5 γ-ray decays are the first strongly K-forbidden transitions to be identified with such high multipolarities, and demonstrate a consistent extension of K-hindrance systematics, with an inhibition factor of approximately 100 per degree of K forbiddenness. Some unplaced transitions are also reported.
Upper limits for the energy-integrated cross section for the decay of the 31-yr 178Hf isomer, induced by x-ray irradiation, are extended to lower x-ray energies than we reported previously. Over the range of incident x-ray energies 6<Ex (keV)<20, we report the energy-integrated cross section as less than 1×10-26cm2keV, with less stringent limits set down to 4 keV. These limits are at least 3 orders of magnitude below values for which enhancements to the isomer decay rate have been reported by other workers.
Experiments on photo-induced de-excitation triggering of 178m2Hf are
revisited. We present an alternative and more effective way of
triggering by exploiting the resonance internal conversion. Full
theoretical description of a possible channel is presented. It is
therefore revisited the impact itself produced by those experiments.
The study of triggered depopulation of nuclear isomers, with accompanying gamma emission, is a rapidlychanging field that is only now attaining some degree of maturity. Because isomer decays via electromagnetic transitions are strongly inhibited, the interaction of these levels with externallyproduced photons provides an important probe of nuclear structure. Also, since some isomers may store large amounts of energy for long times, a number of applications have been proposed, including the creation of a gamma-ray laser. Early experiments conclusively demonstrated triggering of the 1015-year isomer of 180Ta, but even a partial correlation of the effect with known levels could only be obtained recently. Investigations of triggering for the 31-year isomer of 178Hf were initially guided just by systematics and experiments are characterized by considerable controversy. Against this background, the field of triggered gamma emission is entering a new phase in which improved level data allow targeting of specific potentially-useful transitions. This paper summarizes the current state-of-the-art and discusses the changing nature of the field. (© 2004 by ASTRO Ltd., Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA)
Induced release of the high energy densities stored in isomeric nuclear states may be important in the development of ultrashort wavelength lasers. Such a release could compensate the spontaneous power density radiated from the laser medium at threshold. The most promising candidate for such a role seems to be the 31-yr isomeric nucleus of Hf-178 that stores 1.3 GJ/g in the electromagnetic excitation of its constituent protons and neutrons. Successful studies of the induced release of energies from such isomeric states have required an extension of techniques for nuclear resonance spectroscopy using synchrotron radiation (SR) that had previously been applied only to ground state materials. In 2004, monochromatic X-rays from the SPring-8 SR source were used to identify one of the excited nuclear states that mediates the induced decay of the 31-yr isomer of Hf-178. That trigger level was found to lie at 2457.20(22) keV. It was excited when an isomeric nucleus absorbed an incident X-ray photon. We found that one branch of its subsequent decay consisted of a strong electromagnetic transition to the ground state of the nucleus. The energy of the γ-photon emitted was equal to the energy of the trigger level. Proximity in energy of that level to the energy of 2446.06 keV stored by the isomer makes it easy to induce a release of the stored energy and encourages prospects for the development of a gamma ray laser. (© 2005 by Astro, Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA)
Induced gamma emission with the potential for substantial energy gain is an exciting area of research. This paper reviews
related work done internationally, including some that has potential for a gamma-ray laser.
Experiments on photoinduced deexcitation of 178m2Hf are reanalyzed. A mechanismof triggering the 12.7-keV E3 transition, based on resonance conversion, is considered. That rises the efficiency by an order of magnitude. The main obstacle
for enhancing the efficiency is a high internal conversion rate. For this reason, isomers with low multipole order (E1, M1) with high enough energy of the triggering transition are more suitable for triggering. Another way is offered by partial
ionization of the outer electrons for decreasing the conversion rate. The results are applied to experiments in laser-produced
plasma.
Experiments with the long-lived, high-K isomer 178Hfm2 have been recognized as intriguing tests of multi-quasiparticle state structures and their interactions with external radiation.
A triggered release of the energy stored by this isomer, 2.5 MeV per nucleus or 1.2 GJ/gram, in the form of a gamma-ray burst
might prove valuable for numerous applications. The observation of “accelerated” decay of 178Hfm2 during irradiation by 90-keV bremsstrahlung has already been reported, but with poor statistical accuracy due to the experimental
approach. That approach employed single Ge detectors to seek increases in the areas of peaks at energies corresponding to
transitions in the spontaneous decay of the isomer. The need for better quality data to confirm those results has motivated
the development of improved detection concepts. One such concept was utilized here to perform an initial search for low-energy
(<20 keV) triggered gamma emission from 178Hfm2 using the YSU miniball detector array.
Nuclear excitation by an electron transition (NEET) may be used for triggering the decay of nuclear isomers only when there
are compensations between energies (ΔE) and multipolarities (ΔL) of the nuclear transition and the transition in an electron shell. It is shown that using the autoionization states (AS)
allows one to compensate for the ΔE and ΔL differences. Laser radiation may be used for the excitation of AS with energies up to 10–15 eV and 229m
Th (3.5 eV) nuclear isomer excitation by NEET via AS decay. Ion beams, electron beams, and X rays may be used for the excitation
of the trigger nuclear levels with energies up to 150 keV by NEET via AS and for the triggering of the nuclear isomer decay.
For excitation of AS with the energies up to 150 keV, two or more hole states in deep inner electron shells must be excited.
The cross section for such two-hole state excitation in electron shells by ion beams may be sufficiently high. The possibilities
of NEET via AS for the triggering of nuclear isomer decay are discussed.
Electromagnetic transitions within nuclei reflect specific aspects of nuclear structure. This is particularly true for metastable excited states, or isomers, like 178Hfm2 (T1/2=31 years, excitation energy 2446 keV). The interaction of external radiation with isomers can be used to study atomic and nuclear properties and, perhaps, to induce a release of the stored energy. Some experiments indicated that low-energy photons near the L3 edge (9.561 keV) of hafnium could cause this to occur for 178Hfm2, but the lack of a viable physical model and null experiments by other groups have left these claims in doubt. The present work describes a new experiment to examine this process by closely duplicating the irradiation conditions in positive studies, but using a more advanced multi-detector γ array. No support for an induced depletion of 178Hfm2 by low-energy photons was obtained, with an upper limit for the integral cross section that is eight orders-of-magnitude below the reported value.
AFIT/GA/ENY/04-M01. "March 2004." Thesis (M.S.)--Air Force Institute of Technology, 2004. Includes bibliographical references (leaves 113-117).
Enhanced decay of the 31-yr isomer of (178)Hf induced by x-ray irradiation has been reported previously. Here we describe an attempt to reproduce this result with an intense "white" x-ray beam from the Advanced Photon Source. No induced decay was observed. The upper limits for the energy-integrated cross sections for such a process, over the range of energies of 20--60 keV x rays, are less than 2 x 10(-27) cm(2) keV, below the previously reported values by more than 5 orders of magnitude; at 8 keV the limit is 5 x 10(-26) cm(2) keV.
A sample containing 6.3x10(14) nuclei of the 16(+) isomer of Hf-178 having a half life of 31 years and excitation energy of 2.446 MeV was irradiated with x-ray pulses derived from a device operated at 15 mA to produce bremsstrahlung radiation with an end point energy set to be 63 keV. Gamma spectra of the isomeric target were taken with two independent Ge detectors. Intensities of the 213.4 keV (4(+)-->2(+)) and 325.5 keV (6(+)-->4(+)) transitions in the ground state band of Hf-178 were found to increase when irradiated. The largest enhancement was 1.6+/-0.3% measured in the 213.4 keV transition. Such an accelerated decay of the Hf-178 isomer is consistent with an integrated cross section exceeding 2.2x10(-22) cm(2) keV if the resonant absorption takes place below 20 keV as indicated by the use of selective absorption filters in the irradiating beam.
Photoexcitations of the short-lived isomers 167Erm, T1/2=2.28 s, 179Hfm, T1/2=18.68 s, 191Irm, T1/2=4.94 s, and 197Aum, T1/2=7.8 s, were produced with bremsstrahlung from the superconducting Darmstadt linear accelerator. Excitation functions were measured for the population of these isomers by (γ,γ’) reactions between 2 and 7 MeV. They indicated that the isomers were excited by resonant absorption through isolated intermediate states having integrated cross sections in excess of 10-26 cm2 keV, i.e., values about 1000 times larger than most (γ,γ’) activation reactions reported previously although they were comparable to those reported earlier for the depopulating reaction 180Tam(γ,γ’)180Ta. In all four nuclei a common onset was observed near 2.5 MeV for intermediate states with strengths much larger than those occurring at lower energies. The summed cross sections exhibit a clear correlation with the ground state deformations.
Photoexcitation of the long-lived isomers 123Tem, T1/2=119.7 d, and 125Tem, T1/2=58 d, was produced with bremsstrahlung from the superconducting Darmstadt linear accelerator. The excitation function for the reaction 123Te(γ,γ’)123Tem was measured between 2 and 6 MeV. It indicated that the isomer was populated by resonant absorption through isolated intermediate states having integrated cross sections in excess of 10-26 cm2 keV, i.e., values about 100 times larger than most (γ,γ’) activation reactions reported previously. An excitation function was also obtained for the reaction 125Te(γ,γ’)125Tem in this energy range.
The most recent investigation of the photoactivation of isomeric nuclei reported by Krčmar et al. continues to contain a strong contribution from nonresonant channels. In this Comment we report that integrated cross sections for resonant photoactivation of isomeric levels in 87Sr, 111Cd, and 113,115In have been calculated from recent nuclear structure data and compared to those experimental results which assert that nonresonant contributions are important. The latter have shown systematically smaller values and the amount of missing resonant strength can be correlated to the magnitude of nonresonant cross section found in these investigations. Monte Carlo simulations of realistic experimental geometries display important components in the photon fields, resulting from environmental Compton scattering, which have been omitted in previous analyses of experimental data. The strength and shape of this contribution as a function of the experimentally important parameters suggest that the data can be explained entirely on the basis of a resonant excitation mechanism without any need to introduce a nonresonant contribution.
Irradiation of Ta with a superconducting linear accelerator has provided data for the characterization of the reaction 180Tam(gamma,gamma')180Ta. The depopulation of the isomer 180Tam via an intermediate state or narrow band of states near 2.8 MeV has been found with an integrated cross section of 1.2×10-25 cm2 keV. This large value exceeds, by nearly an order of magnitude, known cross sections for (gamma,gamma') reactions producing isomers of other species. Another intermediate state or narrow band is also indicated by the data at an energy 0.6 MeV higher.
A sensitive study of the decay of the deformation-aligned K=14, 4-mus isomer in 174Hf has revealed a multitude of the K-forbidden branches to the ground-state rotational band and other low-K bands, in competition with the known decays to high-K bands. The isomeric transitions have consistently low hindrance factors. These anomalous findings in an axially symmetric deformed nucleus severely test our understanding of the K-selection rule. The isomeric decay to an I=12 rotation-aligned state, and its mixing with the I=12 yrast state, provide a partial explanation.
A sample containing 0.63 x 10 15 nuclei of the 4 quasiparticle isomer of 178Hf having a half life of 31 years and excitation energy of 2.45 MeV was irradiated with X-ray pulses derived from a commercial X-ray unit typically used in dental medicine. It was operated at 15 mA to produce Bremsstrahlung radiation with an endpoint energy of 90 keV. With a high purity Ge detector the number of gamma photons collected from transitions between the lower members of the ground state band was found to increase by about 2% above that emitted by spontaneous decay of the isomer. Such an enhancement is consistent with an integrated cross section of 2 x 10 -21 cm 2 keV, a value in agreement with results scaled from comparable measurements for 180Ta. However, this is believed to be the first observation of photon initiated gamma emission, with a substantial gain in energy. This result has considerable significance to the development of a gamma-ray laser.
A sample of 6.3Ã10¹ⴠnuclei of the 4-quasiparticle isomer of ¹â·â¸Hf having a half-life of 31thinspthinspyr and excitation energy of 2.446thinspthinspMeV was irradiated with x-ray pulses from a device typically used in dental medicine. It was operated at 15thinspthinspmA to produce bremsstrahlung radiation with an end point energy set to be 70 or 90thinspthinspkeV. Spectra of the isomeric target were taken with a high purity Ge detector. Intensities of selected transitions in the normal decay cascade of the ¹â·â¸Hf isomer were found to increase by about 4% . Such an accelerated decay is consistent with an integrated cross section of 1Ã10â»Â²Â¹thinspcm²thinspkeV for the resonant absorption of x rays to induce gamma decay. {copyright} {ital 1999} {ital The American Physical Society }
The EGS (Electron Gamma Shower) system of computer codes is a general purpose package for the Monte Carlo simulation of the coupled transport of electrons and photons in an arbitrary geometry for particles with energies above a few keV up to several TeV. This report introduces a new, enhanced version called EGS4. In addition to explaining and documenting the various enhancements and charges to the previous version (EGS3), this document includes several introductory and advanced tutorials on the use of EGS4 and also contains the EGS4 User Manual, the PEGS4 User Manual and an EGS4 User Guide to Mortran 3. 81 refs., 40 figs., 9 tabs. (WRF)
The irradiation of an enriched sample of 180Tam with bremsstrahlung from a linear accelerator having an end-point energy of 6 MeV has excited a strong channel for the reaction 180Tam(γ,γ′)180Ta, which requires a total spin change of 8ℏ. An integrated cross section of 4.8×10-25 cm2keV has been found. This is a large value exceeding by two orders of magnitude known cross sections for (γ,γ′) reactions producing isomers of other species.
A new technique has been recently described for the absolute calibration of intense sources of pulsed radiation in the 0.2–1.5 MeV range of photon energies. An activation technique, it depended upon the storage of samples of the irradiating spectrum in the form of populations of nuclei excited to isomeric states with lifetimes of seconds to hours. Described here is the use of such a calibrated source to resolve severe conflicts in previous studies of the reaction 115In(γ,γ’)115Inm through the 1078 keV Jπ=(5/2+ level; this mode has traditionally served as the archetype for (γ,γ’) reactions. We report an integrated cross section of (18.7±2.7)×10-2 cm2 keV with no evidence of any importance of nonresonant channels of excitation.
It has been suggested that the long-lived nuclei Ta-180(m) and Lu-176
may be used as stellar s-process chronometers. However, the recent
reports of large cross sections for the (gamma, gamma-prime) reactions
raise the concern that the evolution of these species may have been
affected by the bath of thermal photons present at the s-process
temperatures. The recent experimental results did not identify the
energies at which the gamma, gamma-prime reactions occurred, so in this
work the effective half-lives of Ta-180(m) and Lu-176 in a stellar
environment are calculated as a function of the possible energies of
states mediating the resonance reactions. The time scales indicate that,
unless these large gateway states lie at relatively high energies, the
s-process cannot be effectively dated from the observed abundance of
these nuclei.
A new technique has recently been described for the absolute calibration of intense sources of pulsed radiation in the 0.2–1‐MeV range of photon energies. An x‐ray activation technique, it depended upon the storage of samples of the irradiating spectrum in the form of populations of nuclei excited to isomeric states with lifetimes of seconds to hours. Accuracy was dependent upon the precision assumed for tabulated values of nuclear parameters. Described here is an extension of this technique to the larger range of photon energies, 0.2–1.5 MeV and to intensities from 10<sup>1</sup><sup>2</sup> keV/keV to 10<sup>1</sup><sup>6</sup> keV/keV using the target nuclei <sup>7</sup><sup>9</sup>Br and <sup>7</sup><sup>7</sup>Se. In this work self‐consistency of the nuclear parameters was directly determined. Important changes were found to be necessary for improved accuracy, particularly over the larger ranges of experimental variables being considered. Values of the now‐consistent set of nuclear parameters are reported.
A method of obtaining absolute, direct measurements of the spatial and spectral characteristics of bremsstrahlung is discussed. This technique, called x‐ray activation of nuclei (XAN) is based on the use of well‐known photonuclear reactions which populate long‐lived nuclear isomers. These populations sample incident photon continua at discrete excitation energies and effectively store this information for convenient retrieval following the irradiation of gram‐sized targets. Recently a series of experiments has been conducted which has significantly expanded the available data for the photoexcitation of a wide range of isomers at higher energies. Thus it has become feasible to extend the use of XAN to energies approaching 4 MeV. The utility of this technique is demonstrated by the characterization of bremsstrahlung from the newly installed research linac at the University of Texas at Dallas.
A new method is reported for calibrating bremsstrahlung spectra in the 200–1000‐keV range with x‐ray activation techniques. This approach is particularly significant for pulsed sources that cannot be characterized with conventional spectroscopy methods.
Long-lived isomeric states in stable nuclei 87Sr, 111Cd, 113In and 199Hg are excited by photons and electrons at energies below 3 MeV. Excitation functions and absorption measurements give new information on energy levels, transition probabilities and multipolarities of excited states which decay to the isomeric states.
Described here is the use of a calibrated bremsstrahlung source to resolve conflicts in previous studies of the reaction 111Cd(γ,γ’)111Cdm through a level located near 1200 keV. We report an integrated cross section of (9.8±2.5)×10-29 cm2 keV with no evidence of any importance of nonresonant channels of excitation.
Photoexcitation of the isomers of 19 nuclides was examined in this work. Four accelerators were used as sources of bremsstrahlung to expose the samples and end-point energies covered the range from 0.5 to 11 MeV. No evidence was found for nonresonant processes of excitation. However, more than half the cases showed enhanced channels for the resonant photoexcitation of isomers with integrated cross sections approaching 10-21 cm2 keV. These results are three to four orders of magnitude larger than values usually characterizing (gamma,gamma') reactions.
Further exploration of these nuclear analogies is available at the URL
- C B Collins