Characteristic Γ-rays Research Articles

Cross-section measurements for 7Li + 11B AND 7Li + 13C reactions at low energies

The 7Li + 11B and 7Li + 13C reactions have been studied at incident energies below and around the Coulomb barrier, by measurement of the cross sections for the characteristic γ-rays emitted by the residual nuclei. The nα, αα and np emission channels are found to be enhanced with respect to the statistical compound nucleus predictions, while the nn and single-particle emission channels are not enhanced. An unusual energy dependence is observed for the 7 Li + 11 B → nn 16O 7.12 MeV channel cross section, suggesting the existence of a broad resonance.

Depth profiling by ion-beam analysis

In an ion-beam analysis, a sample is bombarded with charged-particle beams and characteristic X-rays (PIXE), nuclear γ-rays (PIGE) or charged particles produced in the target material are measured. A method for deducing a concentration depth profile of an element in the sample from a yield curve of these reaction products as a function of the projectile energy is derived on the basis of the least-squares method. Validity of this method is demonstrated taking an example of PIXE.

Measurements of anomalous dispersion in rayleigh scattering of characteristic X-ray fluorescence

When using characteristic fluorescence X-rays or γ-rays of energy under 100 keV to study the Rayleigh scattering of photons from atoms the effect of anomalous dispersion is not in general negligible. On the contrary, it may cause the atomic cross-section values to vary with respect to the tabulated values—usually calculated in nonresonant approximation—of an order of magnitude in the most critical cases. In this work we present experimental results of Rayleigh scattering where this effect is well evident. We used as sources of excitation a radioisotope (241Am,Eγ 59.6 keV) and a X-ray tube with a series of different secondary anodes and solid-state detectors to detect the scattered radiation. We reviewed the atomic form factor formalism which describes the phenomenon in a nonrelativistic treatment and we show its strict analogy with QEDS-matrix theory. We obtained numerical values for the cross-sections by using the tabulated values of the photoelectric cross-sections and we made a comparison with the experimental values. The error of the measurements (∼3%) derives from the superposition of the Rayleigh scattering with the characteristic X-ray fluorescence lines. Within the error limits the agreement with the values from the form factor approximation is very satisfactory.

Neutron capture cross section measurement of 232Th in the energy range of 400–950 keV

Neutron capture cross sections of 232Th have been measured relative to 197Au in the neutron energy range of 425–950 keV. The monoenergetic neutrons were produced by a 7Li (p, n) reaction. The activation analysis technique was employed where γ-rays emitted by the product nucleus 233Th, were measured using a large volume high resolution Ge(Li) detector. A computer code “SAMPO” was used to extract the photopeak areas of characteristic γ-rays from the measured complex γ spectrum. The measured cross sections, after applying the various corrections are found to be in good agreement with the ones reported in the literature.

Reactions with 9Be at subbarrier energies

The 9Be+ 9Be, 9Be+ 10B, 9Be+ 11B and 9Be+ 13C reactions have been studied at subbarrier energies measuring the cross sections for the characteristic γ-rays emitted by the residual nuclei and comparing them with the statistical evaporation prediction. A substantial enhancement in the αn channel, which contains the the α-transfer reaction from 9Be, has been observed in all the systems studied. Evidence suggesting that the α-transfer reaction is not exclusively a direct transfer process is being presented.

Background events in the energy region below 4 MeV detected with a 208 cm 3 intrinsic germanium detector : I. Characteristic γ-radiation

A detailed study of the events recorded with a 208 cm 3 intrinsic germanium γ-ray detector produced by interactions of natural environmental radiation was undertaken. In the first part of this work, the data on events from characteristic radioactivity are analysed to optimise the shielding requirements for reducing the γ-ray background as seen by the detector. A thickness of 15 cm of Pb all around the detector in the laboratory location, or only 5 cm in the underground location at the mine, is found to be sufficient to attenuate the event rates from environmental characteristic γ-rays to less than the event rates from intrinsic contamination of the detector components in the detector assembly. The salt mine is found to be a natural environment low in γ-ray background.

Total reaction and transfer cross sections for 11B + 9Be and 13C + 9Be reactions at low energies

The total reaction cross sections for 11B + 9Be and 13C + 9Be have been measured by the total γ-ray yield method over the energy intervals E c.m. = 1.4–4.4 MeV and E c.m. = 2.0–5.2 MeV, respectively. The cross sections for the neutron transfer reactions 11B( 9Be, 8Be) 12B, leading to the 12B 0.953 and 1.674 MeV states, and 13C( 9Be, 8Be) 14C, leading to the 14C 6.094, 6.728 and 6.902 MeV states, have been obtained from the yields of the characteristic γ-rays. The α-transfer reaction 11B( 9Be, 5He) 15N, leading to many unresolved 15N states, has been observed with large cross section. There is, however, no evidence for the 13C( 9Be, 5He) 17O transfer process in the 17O + nα channels. This different behaviour of the 11B + 9Be and 13C + 9Be systems seems to indicate that the α-transfer reaction at sub-barrier energies is not a direct transfer process, and that it probably occurs via molecular state formation.

Fusion and neutron-transfer cross sections for 13C + 10B and 13C + 11B reactions at subbarrier energies

The cross sections for the 10B( 13C, 12C) 11B neutron-transfer reaction, leading to the 11B 4.45 and 6.74 MeV and 12C 4.44 MeV excited states, and for 13C + 10B fusion have been measured by the characteristic and total γ-ray yield methods, respectively, over the energy (c.m.) interval 2.4–5.8 MeV. For 13C + 11B, with no transfer reactions present, the fusion cross sections have been measured between E c.m. = 2.3 and 6.4 MeV. The fusion cross sections for 13C + 10B and 13C + 11B are found to be almost equal and slightly enhanced with respect to those for 12C + 10B and 12C + 11B.

Cross sections for 18O+ 9Be induced reactions at sub-coulomb energies

Cross sections for reactions induced by 18O on 9Be have been measured in the energy range E c. m. = 1.8 MeV to 6.7 MeV. The yields of characteristic γ-rays were observed with a Ge(Li) detector and absolute cross sections were deduced for formation of 19O, 21, 22, 23Ne, 24, 25Na and 25Mg. The 9Be( 18O, 19O) 8Be transfer cross section was found to dominate over the total fusion cross section below E c. m. = 4.5 MeV. The energy dependence of the measured transfer cross section agrees well with what is expected for a reaction of optimum Q-value at low incident energies.

Nuclear reaction cross sections of 16O + 16O obtained by γ-ray detection

The partial production cross sections for reaction residues produced by the fusion of 16O with 16O have been measured at E c. m = 9–30 MeV by detecting the characteristic γ-rays with a Ge(Li) detector. The dominant products are 24Mg and 27A1 corresponding to 2α and αp emission from the compound nucleus, respectively. The total γ-producing cross sections σ R were also derived by summing the partial cross sections after correction for the observed (average) γ-ray angular distributions. The trend in the total cross sections is very similar to the trends derived from an optical model or a statistical-evaporation model calculation. The partial production cross sections were compared with other experimental results at 11.9 MeV and 30 MeV and with the results of the statistical-model calculation. It is concluded that the treatment of angular momentum in the calculation is inadequate for describing the partial cross sections. Structure in the partial and total cross section excitation functions is observed with minima occurring at E c.m. = 27, 24, 20, 17.5, and possibly 15 MeV. Some of this structure is well established by the statistical accuracy of the data and most, but perhaps not all of it, is correlated in the various channels. This structure is compared with that observed in another experiment and some of its implications are discussed.

The half-life of 207Bi and decays of 211At and 211Po

The half-life of 207Bi was obtained from the genetic relation between 207Po and 207Bi, and between 211At and 207Bi. The half-life was found to be 33.4 ± 0.8 y. The half-life of 207Po was determined to be 5.81 ± 0.04 h by following the decay of the characteristic γ-rays from 207Po. The half-life of 211 At was determined tobe 7.23 ± 0.02 h by following the decay of γ-rays and α-particles from 211At and 211Po. The half-lives determined in the present work for 207Po and 211At agree with the literature although the half-life of 207Bi differs considerably from the currently accepted value of 38 y. The branching ratio of 211 At decaying through EC and α-decay modes was determined together with the branching ratios of the three α-particles emitted from 211Po.

Fusion and neutron transfer cross sections for 9Be + 12C at sub-Coulomb energies

The energy dependence of the fusion cross section for 9Be + 12C has been measured over the range E c.m. = 2.8–6.3 MeV by detecting γ-rays from the various possible residual nuclei with two large NaI(Tl) detectors placed close to the target. The single-neutron transfer reactions 9Be( 12C, 13C) 8Be leading to the 3.088 MeV ( 1 2 + ) and 3.854 MeV ( 5 2 + ) states of 13C were measured over the range E c.m. = 1.0–5.8 MeV and 2.1–5.8 MeV, respectively by detecting the characteristic γ-rays with a Ge(Li) detector. The energy dependence of the 9Be( 12C, 13C 1) 8Be transfer cross section was found, in the sub-Coulomb energy region, to be in good agreement with the simple calculation proposed by Switkowski et al. The fusion cross sections for 9Be + 12C are compared with the optical model calculations, using various parameter sets, and with experimental results for other light-nuclei systems. The fusion cross sections for 9Be + 12C, 12C + 13C and 9Be + 16O are found to be enhanced with respect to other systems at incident energies below the Coulomb barrier. It is suggested that a loosely bound neutron fosters, via polarisation, the fusion of the two nuclei at energies just below the Coulomb barrier.

Quantitative analysis of phosphorus by (p, γ) reactions

Measured values of the intensities of characteristic γ-rays emitted under the bombardment of protons on a thick target of phosphorus are tabulated. The intensity of each characteristic γ-ray is measured and used for quantitative analysis of phosphorus in any sample with a sensitivity of 50 to 1000 ppm.

Isomeric yield ratios of 44Sc produced by the 14N bombardment of 32S and 31P

Thick targets of red phosphorus and zinc sulphide were bombarded with 27·5 MeV 14N ions accelerated in the Oak Ridge 63 in. Cyclotron. Cross sections, as a function of the bombarding energy, were measured for the production of 43Sc, 44 m Sc, and 44 g Sc from 31P and 32S. The irradiated materials were γ-counted, and the radioactive products were identified by their characteristic γ-rays and half-lives. From the cross sections, the isomeric yield ratios of 44m Sc 44g Sc were found. For 31P the isomer ratio varied from 0·68 at 23·5 MeV to 1·68 at 27·0 MeV, while for 32S it varied from 0·12 at 20·5 MeV to 1·15 at 27·0 MeV. This increase in the ratio for both reactions is consistent with a compound nucleus formation mechanism for 44Sc at all energies considered. Accordingly, the higher ratio in the phosphorus reaction than in the sulphur reaction may be attributed to the smaller amount of angular momentum taken off by the single proton emitted from the 31P reaction than by the two protons emitted in the 32S case.

The relative intensities of the groups in the magnetic β-ray spectra of radium B and radium C

It is well known that the study of the energy of the groups of homogeneous β-particles emitted by radio-active bodies has led to many important results, of these the chief is that these groups are due to the conversion of the characteristic γ-rays emitted by the body during the disintegration. From the measurement of the energy of these groups, it has been possible to deduce the energy or frequency of the γ-rays, and there appears to be evidence indicating that the γ-rays are associated with a nuclear level system in the manner familiar from X-ray and optical spectra. This opens up a promising field of work, since a knowledge of these level systems, and the way they varied from body to body would be an important addition to our all too scanty knowledge of the nucleus. Unfortunately, it must be admitted that the existing measurements of the β-ray groups are not of sufficient accuracy to provide a unique determination of the level system, they indicate strongly that the level systems exist, but it is difficult to deduce the actual arrangement with any certainty. An increase in the accuracy of the β-ray measurement would undoubtedly lead to interesting conclusions, but no simple way has as yet been proposed by which this can be achieved. Instead of attempting this method we have attacked the β- and γ-ray problem from another side. Previous investigations have been concerned chiefly with the energy of the individual electrons comprising the β-ray groups and of the γ-rays which can be deduced from them, but it is clear that any theory of their role in the disintegration must also take into consideration their intensities. This seems to be a more promising method of advance than that of trying to increase the accuracy of measurements already carried to a fraction of a per cent., since even approximate values of the intensities would be valuable. There are three main lines along which intensity measurements can usefully be made, one of which is the subject of the present paper. Considered by itself, each is interesting, but since it appears likely that the most valuable results will be obtained by comparing all three, it is permissible to refer briefly to them here.