Radioactive beams — A new application of radioanalytical chemistry

An emerging, exciting area of nuclear science is the use of radioactive nuclear beams to study chemical and physical phenomena. One uses the radioactive products (ms<t_1/2<y) of nuclear reactions induced by charged particles or nuetrons to induce secondary reactions or to act as radiotracers. The use of such beams enhances the tools of the radioanalytical chemist. Applications of such beams in materials science (where the beams act as tracers), and the synthesis of new nuclei (especially with Z104) are described.     

Measurements of fusion reactions induced by radioactive 132Sn on 64Ni

Evaporation residue and fission cross sections of radioactive ¹³²Sn on ⁶⁴Ni were measured. Statistical model calculations using parameters simultaneously fitting stable Sn+⁶⁴Ni data reproduce the ¹³²Sn induced reaction very well. A large sub-barrier fusion enhancement was observed. The enhancement can be accounted for by coupled-channels calculations including inelastic excitation of the projectile and target, and neutron transfer.     

Photoconductivity and absorption in amorphous Si

The paper deals with photoconductivity and absorption in aSi specimens, prepared mainly by the decomposition of silane in a glow discharge. Substrate temperatures, T_d, between 300 K and 650 K were used during deposition. The normalised photoresponse was measured at room temperature as a function of photon energy and T_d in a spectral range from 0.5 eV to 3 eV. The absorption coefficient was determined for evaporated, sputtered and glow discharge specimens.The main features of the results are in agreement with conclusions drawn from previous electrical transport and field effect measurements and can be interpreted on the basis of the proposed model for the localised state distribution. It is confirmed that ?_c??_v is 1.5 to 1.6 eV, and that there is a local density of state maximum at about 1.2 eV below ?_c. At room temperature the steady photocurrent is carried predominantly by electrons in states above ?_c, whether excitation is from localized or extended states. Specimens prepared at T_d > 500 K are highly photosensitive, with electron recombination lifetimes, τ, of up to 10^?5s. Rise and decay times of the signal lie in the millisecond range. For T_d < 500 K there is a drastic decrease in τ, which falls to 10^?11 s at $\text{T}{}_{\mathrm{<mtext>d</mtext>}}\text{? 300}\text{K}$ and is even less for evaporated specimens. These results are discussed in some detail.     

Large contribution of deep inelastic processes to reactions of40Ar and48Ca with238U

Differential recoil range distributions have been measured for heavy-reaction products ranging from Te(Z=52) to quasielastic transfer products near the charge and mass of the targets for the reactions of 276 MeV⁴⁸Ca+²³⁸U, 237MeV and 250 MeV⁴⁰Ar+²³⁸U, and 259 MeV⁴⁰Ar+¹⁹⁷Au. The measured recoil range distributions for the⁴⁰Ar+¹⁹⁷Au reaction agree with range distributions calculated from the known projectile-like fragment angular distributions for this reaction. The angular distributions of recoil products formed in the uranium target reactions are deduced and show that the products in the₇₅Re to₈₃Bi region have backward peaked angular distributions characteristic of deep inelastic reactions. The heavy product angular distributions smoothly vary from a (1/sinθ) shape to an exponential shaped backward peak as the atomic number of the product increases from 52 to 83. The trend in the deduced angular distributions for those elements for which recoil range distributions were determined in the⁴⁰Ar+¹⁹⁷Au reaction and the 250 MeV⁴⁰Ar+²³⁸U reaction is similar, suggesting that just as for the Ar+Au system the composite system for the uranium target reaction is also not fully equilibrated along the mass asymmetry coordinate. These conclusions show that the fraction of the total reaction cross section resulting in complete fusion must be re-evaluated for the⁴⁰Ar+²³⁸U reaction and similar heavy-target reactions.     

The temperature dependence f photoconductivity in a-Si

The photoconductivity and its dependence on light intensity have been investigated in a-Si as a function of temperature between 100 and 500 K. In most experiments a photon energy of 2 eV was used. Specimens were deposited on to a substrate held at a temperature between 300 and 600 K by the r.f. decomposition of silane. Graphs of the photocurrent versus 1/T show a photoconductive maximum and the general features of the curves are similar to those found for the chalcogenide glasses. The main emphasis of the paper lies in the interpretation of the results in the light of the information on transport properties and the density of state distribution obtained from drift mobility and field effect experiments. It is shown that recombination takes place predominantly between two groups of localized states, which have been identified in the previous work. The initial state at ?_A is situated about 0.18 eV below ge_C in the electron tail states, the final state lies in a density of state maximum, 0.4 eV above ?_V. Above about 250 K, the photocurrent is carried by electrons in extended states, but below this temperature transport is by phonon assisted hopping through states near ?_A. A recombination process involving two states of a structural defect centre is discussed on the basis of the results and appears to be a feasible interpretation.     

Recent Advances in Understanding Nuclear Reactions

A brief overview of nuclear reactions is given. Recent advances in our understanding of sub-barrier fusion with stable and radioactive nuclei, the synthesis of new heavy nuclei, the synthesis of new n-rich nuclei, the giant dipole resonance and multifragmentation are discussed.     

Target preparation for the fission TPC

A project is underway to build and use a fission time projection chamber (TPC) to make high precision/accuracy measurements of neutron-induced fission cross sections to address issues in fast reactor design. A critical aspect of this program is to have thin TPC targets on thin backings with uniform deposits of the actinides. We have prepared, using vacuum deposition, 100–200 μg/cm² deposits of ²³²Th, ²³⁵U and ²³⁸U on 30–100 μg/cm² C backings with a measured thickness variation of <1.5%. To facilitate measurement of cross section ratios, we are preparing targets with an n-leaf clover design where each petal is a different nuclide.     

The Living Textbook of Nuclear Chemistry

Summary A Summer School in Nuclear Chemistry sponsored by the U. S. Department of Energy and the American Chemical Society has been held at San Jos&eacute; State University for the past 20 years. The intent of the program is to introduce outstanding college students to the field of nuclear and radiochemistry with the goal that some of these students will consider careers on nuclear science. The program features radiochemistry experiments along with radiation safety training, guest lectures by well known nuclear scientists and field trips to nuclear chemistry facilities in the San Francisco area.