Mass-yield Distribution Of Fission Products Research Articles

Mass-yield distributions of fission products in bremsstrahlung-induced fission of Th232

The cumulative yields of various fission products within the 77--153 mass regions in the 2.5-GeV bremsstrahlung-induced fission of $^{232}\mathrm{Th}$ have been determined by using the recoil catcher and an off-line \ensuremath{\gamma}-ray spectrometric technique at the Pohang Accelerator Laboratory, Korea. The mass-yield distributions were obtained from the cumulative yields after charge-distribution corrections. The peak-to-valley ($\mathrm{P}/\mathrm{V}$) ratio, the average value of light mass ($\ensuremath{\langle}{A}_{L}\ensuremath{\rangle}$) and heavy mass ($\ensuremath{\langle}{A}_{H}\ensuremath{\rangle}$), and the average postfission number of neutrons (${\ensuremath{\langle}v\ensuremath{\rangle}}_{\mathrm{expt}}$) were obtained from the mass yield of the $^{232}\mathrm{Th}(\ensuremath{\gamma},\phantom{\rule{0.16em}{0ex}}f)$ reaction. The present and literature data in the $^{232}\mathrm{Th}(\ensuremath{\gamma},\phantom{\rule{0.16em}{0ex}}f)$ reaction were compared with the similar data in the $^{238}\mathrm{U}(\ensuremath{\gamma},\phantom{\rule{0.16em}{0ex}}f)$ reaction at various excitation energies to examine the role of potential energy surface and the effect of standard I and standard II asymmetric modes of fission. It was found that (i) even at the bremsstrahlung end-point energy of 2.5 GeV, the mass-yield distribution in the $^{232}\mathrm{Th}(\ensuremath{\gamma},\phantom{\rule{0.16em}{0ex}}f)$ reaction is triple humped, unlike $^{238}\mathrm{U}(\ensuremath{\gamma},\phantom{\rule{0.16em}{0ex}}f)$ reaction, where it is double humped. (ii) The peak-to-valley ($\mathrm{P}/\mathrm{V}$) ratio decreases with the increase of excitation energies. However, the $\mathrm{P}/\mathrm{V}$ ratio of the $^{232}\mathrm{Th}(\ensuremath{\gamma},\phantom{\rule{0.16em}{0ex}}f)$ reaction is always lower than that of the $^{238}\mathrm{U}(\ensuremath{\gamma},\phantom{\rule{0.16em}{0ex}}f)$ reaction due to the presence of a third peak in the former. (iii) In both the $^{232}\mathrm{Th}(\ensuremath{\gamma},\phantom{\rule{0.16em}{0ex}}f)$ and $^{238}\mathrm{U}(\ensuremath{\gamma},\phantom{\rule{0.16em}{0ex}}f)$ reactions, the nuclear structure effect almost vanishes at the bremsstrahlung end-point energies of 2.5--3.5 GeV.

Mass-yield distributions of fission products from 20, 32, and 45 MeV proton-induced fission of 232Th

The yields of various fission products in the 19.6, 32.2, and 44.8 MeV proton-induced fission of 232Th have been determined by recoil catcher and an off-line γ-ray spectrometric technique using the BARC-TIFR Pelletron in India and MC-50 cyclotron in Korea. The mass-yield distributions were obtained from the fission product yield using the charge distribution corrections. The peak-to-valley (P/V) ratio of the present work and that of literature data for 232Th(p,f) and 238U(p,f) were obtained from the mass yield distribution. The present and the existing literature data for 232Th(p,f), 232Th(n,f), and 232Th(γ,f) at various energies were compared with those for 238U(p,f), 238U(n,f), and 238U(γ,f) to examine the probable nuclear structure effect. The role of Th-anomaly on the peak-to-valley ratio in proton-, neutron-, and photon-induced fission of 232Th was discussed with the similar data in 238U. On the other hand, the fine structure in the mass yield distributions of the fissioning systems at various excitation energies has been explained from the point of standard I and II asymmetric mode of fission besides the probable role of even-odd effect, A/Z ratio, and fissility parameter.

Mass-yield distributions of fission products from photofission of232Th induced by 45- and 80-MeV bremsstrahlung

Mass-yield distributions of fission products from photofission of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mrow /><mml:mn>232</mml:mn></mml:msup></mml:math>Th induced by 45- and 80-MeV bremsstrahlung

Mass-yield distribution of fission products from photo-fission of natPb induced by 2.5 GeV bremsstrahlung

The mass-yield distribution of fission products in the 2.5GeV bremsstrahlung-induced fission of natPb has been measured by using a recoil catcher and an off-line $ \gamma$ -ray spectrometric technique in the 2.5GeV electron linac at the Pohang Accelerator Laboratory. The determination of mass-yield distribution involves measurement of cumulative yields of 27 fission products and independent yields of 15 fission products in the photo-fission of natPb . It was found that the mass-yield distribution of fission products in natPb is symmetric with an average mass of 94±0.5 and a full-width at half-maximum of 55±2.0 mass units. The present data along with the literature data in the 50-85MeV bremsstrahlung-induced fission of natPb and 209Bi as well as the data at 2.5GeV for 209Bi show that the average mass of the mass-yield distributions in the bremsstrahlung-induced fission of natPb and 209Bi decreases from 102.63±0.51 at 50-85MeV to 94.5±0.5 at 2.5GeV. The full-width at half-maximum of the mass-yield distribution increases from about 21.5±1.44 mass units at 50-85MeV to 53±2 mass units at 2.5GeV. These observations in the 50MeV to 2.5GeV bremsstrahlung-induced fission of natPb and 209Bi have been explained from the point of view of increase in multi-chance fission probability with increasing excitation energy. The role of the fissility parameter (Z F 2/A F) on the full-width at half-maximum is also discussed.

Measurement of photo-fission yields and photo-neutron cross-sections in 209Bi with 50 and 65 MeV bremsstrahlung

The photo-fission yields and photo-neutron cross-sections of ( $ \gamma$ , 3n) and ( $ \gamma$ , 4n) on 209Bi induced by 50 and 65MeV bremsstrahlung have been measured by using a recoil catcher and an off-line $ \gamma$ -ray spectrometric technique. The mass-yield distribution of fission products in 209Bi induced by bremsstrahlung photons from the present work and literature data in the energy range 28-85MeV is symmetric around 103 mass units. However, the full width at half maximum of the yields distribution increases from 19 mass units at 28-40MeV to 23 mass units at 85MeV. The ( $ \gamma$ , 3n) reaction cross-section in the 50MeV and the ( $ \gamma$ , 4n) reaction cross-section in the 50 and 65MeV bremsstrahlung-induced reaction of 209Bi were determined for the first time.

Product yields for the photo-fission of 209Bi with 2.5 GeV bremsstrahlung

The mass-yield distribution of fission products in the 2.5GeV bremsstrahlung-induced fission of 209Bi have been determined by using the recoil catcher and the off-line γ-spectrometry technique in the high energy electron linac at the Pohang Accelerator Laboratory. The mass-yield determination involves the measurements of cumulative yields for 32 fission products and independent yields of 17 fission products in the photo-fission of 209Bi nuclei. It was found that the mass-yield distribution of fission products in 209Bi is symmetric with an average mass of 95±0.5 and a FWHM of 51±2.0 mass units. Present data at 2.5GeV along with the literature data at 1GeV, 700–600MeV, and 85–28MeV were interpreted from the point of increase of multi-chance fission and multi-nucleon emission probabilities with an increase in excitation energy. It was found that the average mass of the mass-yield distribution of the fission products decreases from 103±0.5 at 28–85MeV to 95±0.5 at 2.5GeV. On the other hand, the FWHM of the mass-yield distribution increases from 19 mass units at 28–40MeV to 51 mass units at 2.5GeV. It was also found that the nuclear structure effect observed at the photo-fission of 209Bi with 28–85MeV bremsstrahlung is washed out at higher energy.

Mass Distribution in the Fission of232Th by Degraded-Fission-Spectrum Neutrons

The mass-yield distribution of fission products following degraded-fission-spectrum neutron-induced fission of /sup 232/Th was measured by gamma spectrometry for 25 mass chains including mass 138 for the first time for fast fission. Cumulative yields for /sup 83g/Se and /sup 130g/Sb were observed, the latter also for the first time for fast fission. In general, the yields for degraded-fission-spectrum neutron-induced fission of /sup 232/Th were slightly higher in the inner portions of both the heavy and light mass wings than for those from reactor-neutron-induced fission of /sup 232/Th. This was expected, since the average energy of degraded-fission-spectrum neutrons is slightly above that of reactor neutrons.

Photofission ofTh232with 9, 15, and 38 MeV peak bremsstrahlung

The mass-yield distribution of fission products following photofission of $^{232}\mathrm{Th}$ using peak bremsstrahlung energies of 9, 15, and 38 MeV were measured by $\ensuremath{\gamma}$ spectrometry for 25 mass chains in the light and heavy mass wings. Fission yields for mass chains 85, 87, 88, 138, and 146 were measured for the first time for the photofission of $^{232}\mathrm{Th}$ in this work. Several fractional chain yields were measured and various postulates of charge distribution were tested to correlate the experimental data. The Nethaway modified empirical Coryell method showed the best correlation with the observed charge distributions. Evidence for fine structure was observed from the 9 and 15 MeV irradiations in both the heavy and light mass wings with peaks occurring at $A=134 \mathrm{and} 93$, respectively. The inner portions of the mass wings both exhibited increasing splay as the irradiation energy was increased, indicating that symmetric fission was enhanced with increased energy.NUCLEAR REACTIONS, FISSION $^{232}\mathrm{Th}(\ensuremath{\gamma},f)$, ${E}_{(\mathrm{peak} \mathrm{bremsstrahlung})}=9, 15, \mathrm{and} 38$ MeV; measured mass-yield distribution, 25 mass chains, heavy, light mass wings by $\ensuremath{\gamma}$ spectrometry; measured several fractional yields, tested several charge distribution postulates, Ge (Li) detectors, 3.34 keV at 1.33 MeV, 2.3 keV at 1.33 MeV.

Mass Distribution of Fission Products Following Photofission of Uranium-238

The mass-yield distribution of fission products following photofission of /sup 238/U using bremsstrahlung energies of 22, 24, and 28 MeV were measured by radiochemically isolating the fission products belonging to 24 mass chains. The absolute activities of these nuclides were determined by BETA - and gamma counting techniques, and the cumulative fission yields were calculated relative to /sup 140/Ba. The peak-to-valley ratio was found to be effectively constant; thus, the average photon-energy-inducing fission is essentially equal for all three energies studied. The fission yields in the mass region from 130 to 140 were examined for evidence of fine structure around A = 132 or A = 133. Although there were some deviations in this region from a smooth curve,'' it did not yield conclusive evidence for fine structure. Finally, to set the slope of the heavy wing of the mass yield curves, cumulative fission yields of mass chains in the rare-earth region (in addition to others) were measured. (5 figures, 2 tables) (auth)