Study On A New Experimental Method For The Reaction Of²⁴¹Am（n,γ）^242m,gAm

The neutron capture cross section data with higher precision are very important for nuclearreaction theories, nuclear astrophysics and nuclear reactor design. However, with the studyfurther on nuclear physics experiments, it is becoming more complex and difficult to measurethe neutron cross section at higher precision because of the little reaction rate, highbackground, large multiplicities, and so on.Minor actinides like Am, Cm are always present in Pu-containing nuclear fuels because ofthe β-decay of241Pu and the complex network of reactions that take place in high neutronflux environments. Therefore, the accurate cross section data of241Am and reliable m/g valuesof²⁴²Am are critical in nuclear energy production and nuclear fuel cycle. At present,experimental data reported on the241Am（n,γ） reaction are mainly at thermal energy and a fewat other energy points below300keV. Many of these measurements disagree significantly. Thecross sections in the1¹0keV energy region have5¹5%difference between various dataevaluations and which at higher energies （>300keV） deviate at the50%level or more. So it issignificant for people to undertake new measurements using the latest technology andexperimental equipment to improve the precision on neutron capture cross section of241Am.Activation method is usually used for measuring the （n,γ） reaction cross sections.However, it is only suitable to those residual nuclei with radioactivity and appropriate half-lives.Moreover, the uncertainties of measurements are greatly relying on the decay data of residualnuclei, such as half-lives, branching ratio. So it is difficult to make measuring by usingactivation method except at thermal energy because of the low neutron capture cross section.Prompt gamma-ray method is used to measure the neutron capture cross section by directlydetecting the prompt gamma rays emitted from the residual nuclei de-excitation. Compared toactivation method, prompt gamma-ray method is more generality and can be used to indentifycapture events on an event-by-event basis using information on gamma-ray multiplicity andtotal gamma-ray energy. In this research, the gamma-ray total absorption facility based on40 BaF2crystals is used to study the neutron capture cross section of241Am（n,γ） reaction since inBaF2crystals there are fast and slow components of the luminescence, high efficiency forgamma-rays, good time resolution, low sensitivity for neutron, and so on. Because in CLOVERdetectors there are a good energy resolution, and higher efficiency compared with only oneHPGe module. An eight clover assembly is proposed for studying on measuring the m/g valueof241Am（n,γ）242m/gAm.In order to confirm the feasibility for the experimental scheme proposed above, in thiswork, we have made some studies as the following:（1） Based on the Monte Carlo toolkits Geant4, a code named as GTAFandCLOVER forsimulating the new experimental method has been developed, which containing two geometrymodules for the GTAF facility and the CLOVER assembly respectively.（2） Since the neutron data with Z>92are not provided in Geant4, a program named asENDF2G4has been implemented for converting the evaluated neutron data from the ENDFformat to the format that in Geant4. We obtained the evaluated neutron data of241Am using theENDF2G4program and the energy level data of²⁴²Am according to the information fromENSDF. These data are used as the input data of GTAFandCLOVER for simulating the241Am（n,γ）²⁴²Am reaction.（3） The calculations show that the efficiency of GTAF could reach to over90%forgamma-ray energies below2MeV, and over85%for gamma-ray energies below6MeV. Theneutron absorption rate of GTAF is less than10%for incident neutrons from12.5keV to100keV, which indicates that BaF2crystals are low sensitivity for detection neutrons thatscattered into the detectors from the sample. Note，that neutron captures in BaF2crystals occuron average only after more than10scattering interactions; this means that the events arestrongly delayed in time with repsect to the primary interactions. The background could beboth of the time-independent and time-dependent and the main part of background is thegamma-rays frome the neutrons scattered by sample and captured by the detectors and thematerials around. The calculations indicate that the best material for the neutron absorber is6LiH by combined effects of the hydrogen （which acts as a moderator） and the6Li （whichabsorbs neutrons via the6Li（n,α）T reaction）. The calculations were made for241Am（n,γ）242m+gAm with GTAF above a threshold energy of2.5MeV in the summing energy spectrum, the results for effect loss rate is about7.5%and12.2%by using4cm and8cm6LiH respectively.The fast/slow component ratio in the pulses from a BaF2crystal for gamma-ray and alphaparticle is different. If combinated with the Flash ADC technology, the difference of fast/slowratio can be used to eliminate the background induced by the alpha particles from BaF2crystals.For the pulsed neutron source, the repetition rate of250kHz is required to avoid overlap ofdelayed background events with subsequent pulses. It is feasible for us to measure the crosssection of241Am（n,γ）242m+gAm reaction with GTAF by solving the problems mentioned abovein our experiments.（4） In order to determine the optimal configuration for measuring the value of m/g,simulation were made for various configuration of the CLOVER detectors as a function of theangle θbetween the CLOVER detector and the beam line. The results show that when theangle increases, the absolute efficiency also increases; the efficiency for1MeV gamma rays at80o configuration is about16%and21%corresponding to the entrance size at12.4cm×12.4cmand12.4cm×6.2cm respectively. The sum energy spectrum for241Am （n, γ） reaction has beensimulated roughly with the CLOVER assembly and the angleθat80o. The calculated resultsshow that the efficiency is very low but the peak at5.537MeV and that at5.489MeV can stillbe indentified from the sum energy spectrum.In summary, a simulation system based on Geant4has been developed for us to dosimulation on measuring the cross section of²⁴¹Am（n,γ）^242m,gAm reaction and the m/g value of²⁴²Am with GTAF facility and CLOVER assembly, respectively. After doing a lot of simulations,we have understudied a part of the characteristics about these two detector systems, and obtainedsome valuable results which will provide us some important suggestions and physical parametersin our future experiments.