Yoshihiro Nakagome

MULTIPLICITY AND ENERGY OF NEUTRONS FROM 235U(NTH, F) FISSION FRAGMENTS

Abstract The correlation between fission fragments and prompt neutrons from the reaction 235U(nth,f) was measured with improved accuracy. The results determined the neutron multiplicity and emission energy as a function of fragment mass and/or total kinetic energy. The average multiplicity as a function of fragment mass followed a saw-tooth distribution and the distribution of average energy with mass had a bell-shaped distribution centered about the equal-mass split. The slope of the neutron multiplicity with kinetic energy had a saw-tooth distribution with a minimum at m ∗ ≈ 130 . The total excitation energy implied by the measurements was consistent with energy conservation and had a concave distribution with heavy-fragment mass, with a flat minimum between m ∗ ≈ 132 and 152 , suggesting that fragment pairs are preferentially formed in a compact configuration at the scission point.

Measurement of Fragment Mass Dependent Kinetic Energy and Neutron Multiplicity for Thermal Neutron Induced Fission of Plutonium-239

Kinetic energy and neutron multiplicity as a function of fragment mass were measured for the thermal neutron induced fission of 239Pu. By measuring the velocities and energies of two fission fragments simultaneously, both of the pre-neutron emission fragment mass m* and the post-neutron mass m were obtained. The fragment mass dependent neutron multiplicity ν(m*) was deduced by subtracting m from m*. The fragment mass dependent total kinetic energy TKE(m*) was also obtained from this data. The fragment velocity was measured by time-of-flight (TOF) method, for which the start signal was triggered by a very thin plastic scintillation film detector (TFD) and the stop signal was obtained by a silicon surface barrier detector (SSBD) which was also used for the measurement of [he fragment kinetic energy. The present result of ν(m*) is in good agreement with that of Apalin et al. and that in the light fragment region of Fraser et al. The obtained TKE(m*) agrees well with the data of Wagemans et al. A calculation ...

Simultaneous measurement of prompt neutrons and fission fragments for 239Pu(nth, f)

The multiplicity and energy of the prompt neutrons emitted from the fission fragments for 239Pu(nth, f) were measured as functions of the fragment mass and total kinetic energy. The results were compared with those for 233U(nth, f) and with the predicted values by the multi-channel fission theory with the random neck rupture model. The measured and predicted values of the neutron multiplicity, ⟨V⟩(m*), show the saw-tooth trend and agree with each other. The total neutron multiplicity decreases linearly with increasing total kinetic energy resulting in —d⟨TKE⟩/d⟨Vtot⟩=16.5±0.4MeV/neutron. The slope of the neutron multiplicity vs. the total kinetic energy, —d⟨V⟩/d⟨TKE⟩, was plotted against the fragment mass. Its shape agrees with that for 233U(nth, f). The average neutron emission energy, ⟨η⟩(m*), follows a bell shape about the symmetric fission accompanying higher values for very asymmetric fissions and agrees with that for 233U (nth, f). The total excitation energy (TXE)(m*) was determined by two manners: (1) neutron data and (2) Qmax—⟨TKE⟩. Both results satisfactorily agree with each other as the case of 233U( nth, f), and thereby the present derivation of ⟨TKE⟩ from the neutron data is confirned.

Characteristics of the Kyoto University Lead Slowing-down Spectrometer (KULS) coupled to an electron linac

A lead slowing-down spectrometer coupled to a 46 MeV electron linear accelerator (linac) was installed at Research Reactor Institute, Kyoto University (KURRI). The size of this Kyoto University Lead Slowing-down Spectrometer (KULS) is 1.5 × 1.5 × 1.5 m3, and it is covered with Cd sheets 0.5 mm thick. One of the eleven experimental holes in the KULS is covered with 10 to 15 cm thick bismuth layers to suppress high energy capture gamma-rays from lead. The characteristics of this KULS have been experimentally obtained and the results are compared with the predicted values by Monte Carlo calculations using the MCNP code. 1) The slowing-down constant K in the relation E = Kt2 between the neutron slowing-down time t and energy E is 190±2 (keV μs2) for the bismuth hole and 156±2 (keV μs2) for an ordinary lead hole, respectively. The K values agree with the calculated ones. 2) The measured energy resolution ΔEE at full-width-at-half-maximum (FWHM) was about 40% for both holes, while the calculated values were lower by about 10% than the measured ones in the relevant energy region. 3) The neutron energy spectrum from 0.01 eV to 20 MeV and the spatial distribution of neutrons in the KULS were measured by the foil activation method. The angular neutron spectrum perpendicular to the linac electron beam was also obtained experimentally in the energy range from a few eV to about 10 MeV by the neutron time-of-flight (TOF) method. The measured results are compared with the calculated ones in which we have used the three evaluated nuclear data JENDL-3, ENDL-85 and ENDF/B-IV for lead. Through the comparison a check on the nuclear data has been performed.

A system for correlation measurement of fission fragments and prompt neutrons for thermal neutron induced fission

For the purpose of the correlation measurement of fission fragments and prompt neutrons for thermal neutron induced fission, a system consisting of two fragment detectors and a neutron detector was developed. In this system, one fission fragment was detected with a silicon surface barrier detector (SSBD) and the other with a specially designed parallel plate avalanche counter (PPAC). The PPAC had double structures, one for fast timing and the other for position detection. A prompt neutron emitted from a fragment with a specified mass was detected with an organic liquid scintillator (NE213) in this system. The SSBD supplied not only the energy signal of one fragment but also the start timing signal to the time-of-flight (TOF) measurements of the other fragment and the prompt neutron. We tried to carry out the fragment-neutron correlation measurement for 235U(nth, f) with this system, and succeeded in obtaining the neutron energy spectrum in the center-of-mass system and the neutron multiplicity depending on the fragment mass.

Response characteristics of thin film detectors to fission fragments

Abstract The dependence of the pulse height spectrum of a thin film detector (TFD) for fission fragments of 252 Cf spontaneous fission on the thickness of scintillator film of the TFD was studied experimentally. The scintillator films used in the experiments were about 50, 100, 200 and 300 μg/cm 2 in thickness. The ratio of the yield of the heavy fragment peak to that of the light fragment peak became smaller as the film became thinner. The experimental result of the pulse height spectrum was analyzed by a luminescence production model which the present authors recently reported. The pulse height spectrum was represented satisfactorily by this model. The dependence of the pulse height spectrum on the incident beam position in the scintillator film was also investigated.

A new model of luminescence production in a very thin plastic scintillator film

Abstract A new model of luminescence production in a very thin (less than 100 μg/cm 2 ) plastic scintillator film is described. This model contains the thickness of the scintillator film and needs only one parameter to fit to the experimental data. Applications of this model are presented for the response of heavy ions as a function of the energy and for the double-valuedness of the luminescence production on the stopping power of the 16 O ion. The calculated results show good agreement with the experiments reported by other authors. This model is useful for heavy ions with low energy.

Contingent Valuation Method Applied to Survey on Personal Preferences on Choice of Electric Power Source

A Survey was conducted on personal preferences regarding their choice of electric power source to verify the applicability of Contingent Valuation Method (CVM) to such analysis. The survey was carried out on local and urban inhabitants in two steps, first by mail and thereafter by direct interview. A choice of four typical forms of power source was presented: nuclear, coal, hydro and green power; and the question was asked whether the respondent would be willing to pay additional charge for specifying their preferable power source. The mail survey indicated more than half of the respondents hold some willingness to pay either for disuse of nuclear power or expansion of green power. The interview survey revealed various complex motives lying behind their answers. Consequently, it was found that their preference is significantly correlated to their personal image or knowledge of power sources, their thinking or attitude toward energy conservation, their sense of consumption and their private view of life. It is concluded that CVM is pertinently applicable to quantitative analysis of individual opinions, particularly in terms of their motivation to participate in national energy issues. A number of modifications, however, should be required to be brought to the survey design in order to ensure smooth application in practice.

Fission fragment configurations at the scission point of233U,235U and239Pu(nth,f)

In order to estimate the deformation rate of fission fragment at the scission point for thermal neutron-induced fission of233,235U and239Pu, double-velocity and double-energy measurements were carried out. As the result of the estimation of the deformation rate, two types of scission point configurations were found. One type is composed of deformed light and heavy fragments, and the other type is a combination of deformed light and spherical heavy fragments. Mass and total kinetic energy distributions were sorted in two distributions by means of the type of configuration.

Resistivity estimation of irradiated silicon surface barrier detector

Abstract Energy spectra of 235 U(n, f) fission fragments were measured with a silicon surface barrier detector (SSBD) being irradiated by neutrons up to a fluence of 2 × 10 12 n/cm 2 . Pulse height decreases of light and heavy fragment peaks were obtained; however, no significant change of peak width was observed. The authors tried to explain this pulse height decrease by an increased recombination effect due to the enhanced resistivity of silicon. The resistivity was reflected in the recombination effect through the charge collection time. For the resistivity estimation, a method of calculating the recombination effect was proposed. This method predicted the pulse height of α particles fairly well. The estimated resistivity showed excellent agreement with the one calculated using the donor concentration of silicon. Applying this method, the increases of the recombination effect were calculated for fission fragments. The calculated increases of recombination were much smaller than the experimentally obtained ones. A possible increase of recombination center concentration during the irradiation was suggested.