Bengt Forkman

High-energy photofission in 238U, 232Th and 209Bi

Abstract The mass-yield distributions from photofission of 238 U and 232 Th at 300–1100 MeV and of 209 Bi at 700 MeV have been measured using catcher-foil techniques combined with gammaray spectroscopy. The Cross sections of symmetric and asymmetric photofission in uranium and thorium have been calculated from the peak-to-valley ratios. The experimental results have been used to work out a qualitative model for high-energy photofission in heavy elements.

222Rn and 210Pb in the Arctic summer air

Abstract As part of an extensive air chemistry programme, during summer 1980, on board the Swedish ice-breaker ‘Ymer’, levels of 222 Rn (radon) and its long-lived daughters 210 Pb and 210 Po were measured. The radon was trapped on charcoal and the long-lived daugther products sampled on filters on a daily basis. In addition, short-lived progenies were followed continuously on the filters in order to achieve a time resolution of about one hour. The concentrations of radon and 210 Pb in the Arctic summer air north of latitude 75° N averaged 75 ± 21 (1 sd) and 0·075 ± 0·028 mBqm −3 , r respectively. During a two week period of persistent polar winds, the mean radon concentration was 19 ±5 mBq m −3 . During such ‘Arctic background’ conditions, radon exhalation from the sea may contribute significantly to the measured radon-in-air concentration. It is shown that steady-state equilibrium models, applied to an air mass over the sea, overstimate the aerosol residence-time calculated from activity ratios. Time-dependent calculations indicate a mean aerosol residence time of 4 to 7 d in Arctic air. Good agreement is observed between radon levels and the time since the air mass left larger areas. Both the 222 Rn and the long-lived daughter measurements are insensitive to contamination from ship and local settlement.

Photon-induced nuclear reactions above 1 GeV: (I). Experimental

Abstract The yields of the reactions 12 C(γ, n ) 11 C , 27 Al (γ, π + ) 27 Mg , 27 Al (γ, 2 pn ) 24 Na , 127 I (γ,y p χ n ) and 197 Au (γ,χ n ) have been measured in the energy range 1–7 GeV by the activation method without chemical separation. The yields and the F/B ratios in the photo-fission of Au and Pb have been measured by glass-plate detectors. From the yields the cross sections have been deduced. Further, the mass distribution from 238U photofission was measured. The results fit well to earlier measurements below 1 GeV. The analysis and discussion of the spallation and fission results are given in a series of consecutive papers.

(γ, xn) reactions in 127I

Abstract The yields of the reactions (γ, n), (γ, 3n), (γ, 4n), (γ, 6n) and (γ, 7n) in 127 I have been measured by activation analysis from their thresholds up to 830 MeV. The cross sections and the integrated cross sections have been calculated. The experimental result has been compared with different sum rules. The (γ, n) cross-section found above the photomeson threshold is higher than expected from simple estimates of the nucleon and pion transparencies in the iodine nucleus.

PHOTODISINTEGRATION OF CALCIUM

Abstract Photoproton spectra of calcium have been recorded at bremsstrahlung maximum energies of 19.5 and 20.5 MeV. By subtraction a proton spectrum corresponding to photon absorption in the range 20±0.5 MeV was obtained. The proton spectra show well resolved peaks corresponding to transitions to the ground state and excited states at 2.9 MeV and 5.9 MeV. These states are assumed to be the single particle hole states in K39 with spin values d 3 2 , s 1 2 and d 5 2 . These assignments are supported by a comparison of experimental branching ratios and angular distributions with theoretical values calculated on the basis of the results of Brown et al.

Photoproduction of charged pions from nuclei

Abstract The yields of the reactions 41 K (γ,π + ) 41 Ar , 65 Cu (γ, π + ) 65 Ni , 88 Sr( γ , π + ) 88 Rb and 138 Ba( γ , π + ) 138 Cs have been measured by activation analysis. Data on the reactions 11 B (γ,π − ) 11 C , 27 Al (γ, π + ) 27 Mg and 51 V( γ , π + ) 51 Ti obtained earlier have been reanalysed. The experimental cross sections have been compared to predictions from simple model calculations.

PHOTOPION REACTIONS IN COMPLEX NUCLEI.

The yields of the reactions 11B(γ, π−)11C, 27Al(γ, π+)27Mg, 51V(γ, π+)51Ti and 51V(γ, π−2n)49Cr have been measured by activation analysis from threshold up to 750 MeV. The true and integrated cross sections have been calculated. The experimental cross sections have been compared with predictions from a Fermi gas model. Fairly good agreement is obtained.

Intermediate energy photofission in tantalum, gold and lead

Abstract The angular distributions of fragments in photofission of gold and lead have been measured in the bremsstrahlung energy range 250–1000 MeV with the aid of glass detectors. In the whole energy interval the angular distributions are nearly isotropic in the laboratory system with a small but significant forward peaking which increases by decreasing mass number. This anisotropy is supposed to be an effect of nuclear motion. By measuring the forward-to-backward ratios of emitted fission fragments for tantalum, gold and lead it is concluded that the anisotropy is increasing only slightly with increasing bremsstrahlung energy.

Low-energy photofission of U238

Abstract Symmetric and asymmetric photofission yields of U 238 , defined by Cd 117 3and Ba 139 , have been studied at energies near the photofission threshold. The experimental yields have been fitted to theoretical yield curves. From these fits values of the fission barrier height B f and the fission barrier form parameter R p have been obtained for both symmetric and asymmetric fission. The similarity of the fission barrier heights for the two modes of fission favours the picture that the valley-to-peak ratio in the mass distribution is determined after the nucleus has passed the saddle point. The E p value obtained is in agreement with estimates from spontaneous fission data, showing that the barrier for threshold fission and for spontaneous fission are directly related.

The photodeuteron yield from 30 MeV Bremsstrahlung irradiation

Abstract An investigation of the (γ, d) reaction in sulphur, cobalt and copper has been carried out. Bremsstrahlung of 30 MeV maximum energy was used. The emitted charged photoparticles were deflected by a magnetic field and recorded in nuclear emulsions. The range and the orientation of the tracks in the emulsions were measured and the trajectories of the photoparticles were calculated backwards for protons, deuterons and alpha particles. The intersection point of each trajectory with the target plane gave information about the kind of photo-particle which had produced the track. The ratio Rdp between the (γ, d) yield and the (γ, p) yield was determined. No strong fluctuation in the ratio between the adjacent elements cobalt and copper was observed. The ratios found were all zero for the three elements sulphur, cobalt and copper, with a standard deviation of respectively 0.009, 0.012 and 0.018. A calculation based on the statistical model of the ratio Rdp was made. The ratios obtained are, within the limits of the errors, in agreement with the experimental results. The present results also were compared with the pick-up process theory, with earlier (γ, d) experiments and with the inverse reaction. The obtained ratio between the (γ, α) yield and the (γ, p) yield was in agreement with earlier (γ, α) experiments.