Neutron Multiplicity Measurements Research Articles

Evidence of quasifission in theO16 + U238reaction at sub-barrier energies

Mass distribution of fission fragments and neutron multiplicity in the ${}^{16}\text{O}+{}^{238}$U reaction were measured at near- and below-barrier energies. A sudden change in the fragment mass width, observed in the present measurement, confirmed the transition to quasifission at below-barrier energies; the same was indicated earlier from the study of fission fragment angular anisotropy. However, the present measurement of prescission neutron multiplicity as well as the earlier measurement of evaporation residue yield did not indicate any significant departure from the respective statistical model predictions throughout the energy range. It is argued that the first two probes are more sensitive for highly asymmetric systems, whereas all probes would be useful and complimentary to each other for study of quasifission in more symmetric systems, where quasifission is more dominant.

Simulation of polyethylene-moderated plutonium neutron multiplicity measurements

Neutron multiplicity measurements are a useful technique for the characterization of special nuclear material. This technique relies on the detection of correlated neutrons from fission events. As correlated events are detected it is possible to determine the neutron multiplicity distribution for the sample. This distribution is useful for identifying the material and estimating the mass. This work focuses on the ability of the Monte Carlo code MCNP-PoliMi to simulate measured distributions. The experiment used as the basis of comparison consisted of a 4.5 kg plutonium metal sphere surrounded by up to 6 in. of polyethylene. A bank of 15 3He detectors was used to detect the correlated neutron events. MCNP-PoliMi was used to simulate the particle transport and a post-processing algorithm was developed to apply detector deadtime effects and to determine the neutron multiplicity distributions. These simulated distributions were then compared to the measured results. The simulation provided an adequate estimation of the measured data. However, we observed a systematic over-prediction in both the mean and the variance of the measured distribution.

Study of the effect of shell closure on the nuclear dissipation

Neutron multiplicity measurements are carried out for the 19 F + 194,196,198 Pt systems at excitation energy range of 48 to 67.5 MeV. Out of these three compound nuclei (CN) 217 Fr, 215 Fr and 213 Fr, one of the CN ( 213 Fr) has neutron shell closure (N C =126). Experimentally obtained neutron multiplicities are compared with the standard statistical model predictions. It has been observed that the standard statistical model under-predicts the experimental pre-scission neutron multiplicities values. Also the comparison of pre-scission neutron multiplicities for different compound nuclei (CN) indicates the signature of shell effects on nuclear dissipation, which results in the lowering of the pre-scission neutron multiplicities.

Measurement of neutron multiplicity from fission ofU228and nuclear dissipation

Pre- and post-scission neutron multiplicities are measured at different excitation energies of the compound nucleus $^{228}\mathrm{U}$ populated using the $^{19}\mathrm{F}+^{209}\mathrm{Bi}$ reaction. The measured yield of pre-scission and total neutrons are compared with the statistical model calculation for the decay of a compound nucleus. The statistical model calculations are performed using the Bohr-Wheeler transition state fission width as well as the dissipative dynamical fission width due to Kramers. Comparison between the measured and the calculated values shows that, while the Bohr-Wheeler fission width grossly underestimates the pre-scission neutron yield, a large amount of dissipation is required in the Kramers width to fit the experimental pre-scission multiplicities. Various factors contributing to the large excitation energy dependence of the fitted values of the dissipation coefficient are discussed.

Role of N/Z in Pre-scission Neutrons as an Observable of Nuclear Friction

A dynamical Langevin model is employed to evaluate the excess of the neutron emission in the fission of heavy nuclei 240Cf, 246Cf, 254Cf, 240U relative to the standard statistical-model prediction at various saddle-to-scission friction strengths. It is shown that when the neutron-to-proton ratio N/Z of the system increases, the sensitivity of the excess to the friction decreases substantially, and it almost disappears for 240 U. We suggest that using those compound systems with low N/Z favors an accurate determination for the saddle-to-scission friction strength based on the measurement of the pre-scission neutron multiplicity.

Cosmic ray events in January 2005 from the measurements of multiple neutrons in a neutron monitor

Cosmic-ray events has been analyzed by the spectrographic method using measurements of neutron multiplicities in a neutron monitor. The calculations showed an increase in the exponent of the power spectrum of variations and the geomagnetic cutoff rigidity for ground-level enhancements and their insignificant reduction for Forbush decreases.

Facility for neutron multiplicity measurements in fission

The description of an experimental set-up for neutron multiplicity measurements and its specific data processing procedures are presented together with some results for 252 Cf( sf) . With this experimental set-up the light and heavy fission fragment kinetic energies for every fission event can be registered simultaneously with the number of prompt neutrons emitted by each of the complementary fission fragments. The proposed data processing makes it possible to perform the reconstruction of pre-neutron fission fragment masses and kinetic energies as well as the initial neutron multiplicity distribution.

Neutron channel of the FOBOS spectrometer for the study of spontaneous fission

The 4π spectrometer FOBOS has been completed with the multidetector system for registration of neutrons aimed at experiments to search for collinear tripartition of heavy spontaneously fissile nuclei. A simple empirical model developed for the description of the measured neutron multiplicity is tested on the data block comprising 6×107 events.

Neutron detector array for measurement of neutron multiplicity

An array consisting of six neutron detectors has been constructed for measurement of neutron multiplicities in heavy ion reactions. The measurements are reported to demonstrate that, even with a modest array of six neutron detectors, it is possible to infer unambiguously the neutron multiplicity (upto M=4) of the reaction channel from the neutron fold distribution obtained in coincidence with γ-rays, if the array is calibrated with known reaction channels. The neutron multiplicity for the 991 keV γ-ray produced in the two heavy ion reactions 133 Cs( 12,13 C,xn) has been determined from this set-up-indicating that it deexcites levels in 142 Pm . This is in agreement with the excitation function measurements for the same reactions.

A monte-carlo code for neutron efficiency calculations for large volume Gd-loaded liquid scintillation detectors

This paper reports on a Monte-Carlo program, MSX, developed to evaluate the performance of large-volume, Gd-loaded liquid scintillation detectors used in neutron multiplicity measurements. The results of simulations are presented for the detector int

Neutron multiplicity measurements on 220 l waste drums containing Pu in the range 0.1–1 g 240Pu eff with the Time Interval Analysis method

Measurement results are presented for the assay of plutonium in 220 l waste drums containing Pu-masses in the range 0.1–1g 240Pu eff obtained with the Time Interval Analysis (TIA) method. TIA is a neutron multiplicity method based on the concept of one- and two-dimensional Rossi-alpha distributions. The main source of measurement bias in neutron multiplicity measurements at low count-rates is the impredictable variation of the high-multiplicity neutron background of spallation neutrons induced by cosmic rays. The TIA-method was therefore equipped with a special background filter, which is designed and optimized to reduce the influence of these spallation neutrons by rejecting the high-multiplicity events. The measurement results, obtained with the background correction filter outlined in this paper, prove the repeatability and validity of the TIA-method and show that multiplicity counting with the TIA-technique is applicable for masses as low as 0.1 g 240Pu eff even at a detection efficiency of 12%.

The dependence of fission time-scales on fragment mass asymmetry in the 20Ne+ 165Ho reaction at 50 A MeV

The pre-scission and post-scission multiplicities of alpha particles were determined in coincidence with fission fragments for four windows of exit channel mass asymmetry in the reaction 20Ne+ 165Ho at 50 A MeV. The technique used employed a kinematic analysis of the energy spectra of alpha particles as a function of the emission angle with respect to the fission axis. Using a procedure of fitting the experimental energy spectra with those resulting from a Monte Carlo simulation program, the total multiplicity of alpha particles was separated into pre-scission and post-scission components. Using the pre-scission and post-scission multiplicities, neutron multiplicity measurements and other empirical observations, both the initial excitation energy and the excitation energy at scission of the compound nucleus were determined. The pre-scission times in the de-excitation of a highly excited 178W compound nucleus with initial excition energy of 570 MeV were evaluated using statistical model calculations. Only a small decrease in scission time was derived for the most asymmetric events studied. The possible contribution of very damped deep inelastic processes is discussed.

Azimuthal 2α correlations and projectile-residue distributions selected by neutron and charged-particle multiplicity measurements

The resolution of reaction filters for intermediate-energy heavy-ion collisions based upon charged-particle and neutron multiplicity measurements with 4\ensuremath{\pi} coverage is compared for ${}^{112}\mathrm{Sn}{+}^{112}\mathrm{Sn}$ and ${}^{124}{\mathrm{S}\mathrm{n}+}^{124}\mathrm{Sn}$ collisions at $E/A=40$ MeV. The quality of event characterization is inferred from the azimuthal correlations of \ensuremath{\alpha} particles emitted at intermediate rapidity and from the Z distributions of projectilelike fragments emitted at forward angles. For central and midimpact parameter collisions, the associated charged-particle multiplicity appears to provide better impact-parameter selection than the associated neutron multiplicity.

New Spontaneous Fission Mode for 252Cf: Indication of Hyperdeformed 144,145,146Ba at Scission.

From $\ensuremath{\gamma}$-ray coincidence studies following spontaneous fission of ${}^{252}\mathrm{Cf}$, direct measurements of yields and neutron multiplicities were made for Sr-Nd, Zr-Ce, Mo-Ba, Ru-Xe, and Pd-Te correlated pairs. A strong enhancement of the 7-10 neutron channels for Mo-Ba pairs is observed. A new fission mode associated with the enhanced neutron yields is identified. These data can be interpreted in terms of one or more of ${}^{144,145,146}\mathrm{Ba}$ being hyperdeformed at scission. Mean field calculations predict a hyperdeformed third minimum in ${}^{252}\mathrm{Cf}$ and an extremely deformed ${}^{146}\mathrm{Ba}$ fragment at scission.

Decay of the Isovector Giant Resonances excited in Charge Exchange Reactions

Isovector Resonances are studied through the ( 13C, 13N-n) reaction at 60A.MeV incident energy on 208Pb and 58Ni targets. Neutron decay of the resonances observed in these reactions has been measured in coincidence with the 13N ejectile. The measured neutron multiplicity has been used to deduce the contribution of fast processes like Pick-Up Break-Up and Quasi-Free Charge Exchange to the singles spectrum. The analysis of the neutron decay of the observed resonances revealed a direct branch to well defined final states in the residual nuclei, 207Tl and 57Co. In the case of isovector resonances in 58Co, an angular distribution of this direct branch has been extracted. The strong anisotropy observed for this distribution does not appear compatible with the decay of a pure monopole state.

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 was carried out with the model proposed by Brosa et al. who assumed multichannel fission paths and a random neck rupture. It is seen that the calculated results of both fragment mass dependent kinetic energy and neutron multiplicity represent experimental data well.

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 was carried out with the model proposed by Brosa et al. who assumed multichannel fission paths and a random neck rupture. It is seen that the calculated results of both fragment mass dependent kinetic energy and neutron multiplicity represent experimental data well.

New insights from studies of spontaneous fission with large detector arrays

The new insights into a) the rapidly varying structures of neutron-rich nuclei up to spins as high as 20 + and 19 −, b) the spontaneous fission process itself, and c) modes of cluster radioactivity that have come out of studies of spontaneous fission of 242Pu, 248Cm and 252Cf with large detector arrays are presented. The studies include γ-γ-γ coincidences with the second generation (14–20 Compton-suppressed Ge arrays) and new, third generation (35–45 Compton-suppressed Ge arrays, Gammasphere and Eurogam) gamma-ray detector arrays. The neutron-rich nuclei observed span the full range from superdeformed ( β 2 ≥ 0.4) double-magic ground states to spherical double-magic nuclei and competing shapes in between. New structure insights include the following: New regions of identical ground state bands to spins of 10 + to 16 + were discovered; one at the sudden onset of large deformation at N=60,62 in both even- and odd-A 98–101Sr, one at midshell N = 64,66 in 108,110Ru, one for N = 88–90 144,146Ba, one for N = 92–94 152,154Nd, and at high spin in 156Nd and also in excited bands in these regions. New shape coexisting structures in both even- and odd-A for A = 96–102 Sr and Zr nuclei are found. Evidence is found in both even-even and odd-A nuclei for a new region of octupole deformation around Z = 56 and N = 86–88. Rapid changes in moments of inertia are observed with changes in Z and N of two units with sudden changes in the moments of inertia associated with band crossings. The levels in many neutron-rich nuclei are observed for the first time. Earlier, plunger studies yielded lifetimes of low spin states and recently the Doppler Shift Line Shape Analysis yielded lifetimes of high spin states. For the first time, direct measurements of yields and neutron multiplicities have been made for five correlated pairs of fission fragments of SrNd, ZrCe, MoBa, RuXe and PdTe nuclei. Neutron multiplicities from 0–10 v emission (10v for the first time) were observed in MoBa correlated pairs with the 0 and 7–10 neutron-emission yields enhanced compared to gross yields for all fragments. The MoBa data provide evidence for two different fission modes. By unfolding the observed MoBa yields, the masses and excitation energies and mass distributions at scission were extracted. These data revealed a new mode involving the high neutron multiplicities that occurs essentially through one pair, 108Mo 144Ba, 107Mo 145Ba, or 106Mo 146Ba or some combination where the 144Ba, 145Ba and/or 146Ba at scission are hyperdeformed with a long-to-short axis ratio of 3:1. The zero-neutron emission channels provide new examples of the cold rearrangements of nucleons in a new type of cluster radioactivity. The cluster radioactivity model predicted the observed enhancements of the zero-neutron channels for odd-A-odd-A nuclei as well as the observed fine structure, that is, the spin distributions of the two fragments. The measurements of intensities and γ-γ-γ coincidences in SF with large detector arrays open a new era in the determination of previously inaccessible properties of neutron-rich nuclei and the fission process.

Excitation energy deposition in central collisions of 40AMeVAr40withTh232

Excitation energy depositions in the reactions of 40A MeV $^{40}\mathrm{Ar}$ with $^{232}\mathrm{Th}$ have been determined from measurements of neutron multiplicities in coincidence with mass identified heavy reaction products. For the most central collisions the derived excitation energies of 880 MeV are 200--300 MeV above previous estimates and in excellent agreement with momentum transfer systematics. Heavy evaporation residues are observed for these collisions indicating apparent dynamic delays in the fission channel of (1--5)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}20}$ s. While a massive-transfer simulation incorporating preequilibrium emission is in generally good agreement with the experimental results evidence is also found for strongly damped collisions.

Mass asymmetry dependence of fusion time-scales in11B+237Np and12C,16O,19F+232Th reactions in a dynamical trajectory model

Dynamical trajectory calculations were carried out for the reactions of11B+237Np and12C,16O and19F+232Th, having mass asymmetries on either side of the Businaro-Gallone critical mass asymmetry αBG, in order to examine the mass asymmetry dependence of fusion reactions in these systems. The compound nucleus formation times were calculated as a function of the partial wave of the reaction for all the systems. This study brings out that for systems with α αBG, which is caused by the dynamical effects involved in the large scale shape changes taking place in the fusion process as well as due to the interplay between the thermal and the collective motion during the collision process. The calculated time scales are comparable to the experimental values derived from the pre-fission neutron multiplicity measurements.