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Photofission and Photoneutron Measurements on 24 Am Between 5 and 10 Mev

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Photofission and Photoneutron Measurements on 24 Am Between 5 and 10 Mev Nuclear Physics A~ (1992) 365-373 NUCLEAR North-Ho|laad PHYSICS A Photofission and photoneutron measurements on 24 Am between 5 and 10 MeV S.J. Watson ~, DJ.S. Findlay and M.R. Sen6 Hat.cell Laborato~, UKAEA, Oxfordshire OXI! ORA, UK Received 8 April 1992 Abreact: Measurements are presented of the photofission and photoneutron cross sections of 241Am and of the mean number ~ of neutrons per fission between 5 and 10 MeV. The measurements were carded out using a bremsstrahlung beam generated by the Harwell electron linear accelerator HELIOS and a 1.5 g metallic 24~Am target. Neutron multiplicity distributions were recorded using a high-efficiency neutron detector. Yield measurements made in steps of 50 keV show possible evidence for a resonance at 6.! MeV in the photofission cross section, but the statistical significance is not high. NUCLEAR REACTIONS 24~Am(-y,F), 24~Am(y, n), E = 5.6-10 MeV; measured photon E induced ~r( E ~; deduced photofission, photoneutron ~r, mean number of neutrons per fission. '4~Am deduced fission characteristics. |. Introduction Photofission has been used extensively in the study of actinide fission near threshold due to the relatively small number of low-lying states excited by the predominantly E1 interaction. Near threshold most excitation energy is converted into nuclear deformation energy, and so fission proceeds through only a few low-lying fission channels. This makes interpretation of experimentally measured cross sections easier for fission induced by photons than fission induced by other particles (e.g. neutrons) ~). |n photofission experiments there has tended to be an emphasis on actinide nuclei such as even-even -'3~'Th and -'38U for which suitable targets are readily obtainable. A comprehensive reference list for such experiments is included in ref. 2). There have been fewer experimental photofission studies of heavier nuclei, and in particular of the heavier odd-A nuclei 3_7). Photofission of the odd-A nucleus 24tAm has been studied only by Zhuchko et a]. 3) and Koretskaya et al. 4). Zhuchko et alL. used bremsstrahlung to study the photofission cross section between 5 and 7 MeV with an energy resolution of Correspondence to: Dr. DA.S. Findlay, Accelerator Applications Department, AEA Technology Building 418, Harwell Laboratory, Oxfordshire OXI ! 0RA, UK. Energy Research Unit, Rutherford Appleton Laboratory, Oxfordshire OXI 1 0QX, UK. 0375-9474/92/S05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved October ~g92 366 s.J. Watson et al. / Phmofission and photoneutron ---200-300 keV. Koretskaya et al. made measurements at 1 MeV intervals between 7 and 26 MeV. Measurements of the photoneutron cross section between 11 and .15 MeV have been made by Batij et al. ~). The measurements for 24~Am in the photon energy range 5.6-10 MeV presented in this paper are intended to supplement existing data on 2~Am, and are of the photofission and photoneutron cross sections and the mean number ~ of prompt fission neutrons per fission. 2. The experiment Measurements for this experiment were made on the electron linear accelerator HELIOS 9) at the Harwell Laboratory of the UK Atomic Energy Authority. Only brief details of the experimental apparatus and procedures will be given here, since full details are given in ref. ~o). After the second section of the accelerator, the electron beam was energy analysed by a magnetic analysis system incorporating a computer-controlled power supply locked to a Rawson-Lush rotating coil gaussmeter. Bremsstrahlung from a gold radiator was collimated and intercepted by the 24~Am target. Neutrons photo- produced from the target were detected in a large high-efficiency neutron detector consisting of an oil-moderated tank containing fifty-six ~°BF3 proportional counters arranged in five concentric rings (the neutron detection efficiency varied between 40% and 48% for neutron energies between 0.5 and 5 MeV). The bremsstrahlung flux was measured absolutely using a type P2 ionisation chamber ~) connected to a Brookhaven model 1000A current integrator. The :~Am target itself was supplied by the Oak Ridge National Laboratory (USA) in the form of a square 2 x 2 cm metallic foil of mass 1.533 g and thickness 0.38 gcm -2 mounted between two thin (0.01 mm) nickel foils in an evacuated aluminium can 89 mm in diameter and 569 mm long with a 0.5 mm thick aluminium entrance window. Around the can was placed 2 mm of lead to attenuate the 60 keV y-rays produced by the ~5 Ci target. The position of the target within the can was confirmed by autoradiography. The can had been evacuated to prevent oxidation of the high!y a-active americium and hence avoid production of neutrons through the t60(a, n) reaction. Unfortunately, upon receipt of the target it was found that the target was already emitting ---7500 neutrons/s. From y-ray spectroscopy measurements, these neutrons were found to be due to the ~gF(a, n) reaction implying that the target had been contaminated by fluorine which could have been a by-product of the original metal extraction process. The presence of the fluorine impurity had not been declared in the chemical analysis of the target. In the present experiment this large neutron background limited the minimum energy which could be reached. Yields were measured at regularly spaced bremsstrahlung endpoint energies over the range of interest. Two sets of measurements were made at 100 keV intervals between 5.6 and 8.0 MeV, one set of measurements was made at 200 keV intervals between 8.0 and 10.0 MeV, and one set of measurements was made at 50 keV intervals S.Z Watson et aL / Photofission and photoneutron 367 between 5.7 and 6.4 MeV. To minimise the effects of small systematic drifts in energy, yields were measured at two neighbouring energy points during each run and the energy was cycled between these points so that data were taken for 1 minute at each endpoint energy. The difference of the pair of yields from each run was then used as the input to the unfolding procedure described in sect. 3. To eliminate effects of drift in the zero of the P2 ionisation chamber and current integration system, especially at higher energies where the electron beam current had to be greatly reduced to keep the detected event rate at or below 0.1 events per beam burst, the integrator was operated with a permanently positive offset. Therefore between each minute's running, the electron beam was switched off for 10 seconds and the integrator output in the absence of beam was recorded. 3. Analysis The raw data are sets of pairs of neutron multiplicity distributions, each pair corresponding to the two adjacent endpoint energies in each run. After correcting for background, dead time and overlap as described in ref. 12), the sum and difference of each pair of multiplicity distributions was formed. The "sum" multiplicity distribution was fitted by the TRDG ~3) multiplicity-distribution shape to extract the parameters ~ and o.2, the mean and variance respectively of the distribution. Assuming that changes in P and o .2 between adjacent end point energies are relatively small, these values of ~ and o.2 were then used for the "difference" multiplicity distribution to extract the numbers of photofission and photoneutron events corres- ponding to the difference multiplicity distribution. These "difference" yields were then taken as input to a modified Penfold-Leiss unfolding procedure ~4) re-cast in terms of yield differences ~5). Between 5.6 and 8.0 MeV, two-fold interlacing* was used to reduce the statistical errors on the unfolded cross-section data points at the expense of energy resolution. Above 8 MeV, multiplicity-distribution shape parameters were measured every 500 keV, and these parameter values were used to extract photofission and photo- 11eutron cross sections from the set of yield data points measured every 200 keV above 8 MeV. 4. Results Fig. 1 shows the photofission cross sections from the present experiment. Also shown are the results of Zhuchko et al. 3) and Koretskaya et al. 4). Below 8 MeV the energy resolution of the present data is 220 keV, the 100 keV data being two-fold * The set of yields at 100 keV intervals between 5.6 and 8.0 MeV was considered to be two sets of yields, one set at 200 keV intervals between 5.6 and 8.0 MeV, the other set at 200 keV intervals between 5.7 and 7.9 MeV. The two sets of yields were unfolded separately and the two sets of unfolded cross-section values were then combined to form one composite set. 368 SJ. Watson et at / Photofission and photoneutron L~IAm photoflssion ® 100.0 • + • * 0 1,Q * * • , ! ' W~d d~re L ,' t[ lO,~L. ..... |, 11 s6 s8 6° s, e e 0.1b', ,/d . .., , , ±~, ...... '~ t , , ~ ......... 5 6 7 8 9 10 Photon energy (MeV) Fig. !. Photofission cross section for :4'Am. Data: Solid circles, present measurements; crosses, Zhuchko et at. s); triangles, Koretskaya et al. 4). Curves (as discussed in the text): solid line, no damping in second well; long dashes, as solid curve but including effects of 220 keV experimental energy resolution; short dashes, 300 keV damping in second well and 220 keV resolution. The inset shows the yield differences (counts IAC-' ). interlaced, and above 8 MeV the energy resolution is 500 keV. The Zhuchko et al. data has a resolution of between 209 and 300 keV. The present dala agree well with those of Zhuchko et al. In the ir~et to fig. 1 are shown yield differences from the 50 keV measurements between Y.7 and 6.4 MeV where the energy "resolution" is -50 keV.
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