Fission debris refers toHeavy nuclear fissionThe nuclear debris generated when.The probability of heavy nuclei breaking into two pieces is much higher than that of breaking into three or more pieces.Generally, the mass distribution of two fragments is uneven, one is heavier and the other is lighter, and the mass distribution presents two obvious peaks.
For lighter nuclei such as bismuth and heavier nucleitwo hundred and fifty-sevenFm has only one peak, and its symmetrical fission is dominant.When fission fragments are separatedCoulombRepulsion can have great kinetic energy, such asthermal neutroncausetwo hundred and thirty-fiveU fission, the average kinetic energy of fragments can reach 170 trillionElectron volt(MeV), accounting for more than 80% of the total energy released by fission.Fission productMost of them are very unstable neutron abundancesHomotopyThey undergo a series of decay, either emitting neutrons orBeta decayConcomitantTransition。
γ - ray study of fission fragments neutron measurement technique
Announce
edit
neutronOne of the detection methods ofNuclear fissionMethod, which means that neutrons can produce nuclear fission by interacting with heavy nuclei, and neutrons can be detected by recording the light and heavy fission fragments of heavy nuclei.The energy released by neutron induced fission is very large, and the energy of two fission fragments is about 165 MeV in total.The signals generated by recording the ionization of fission fragments are easily connected withGamma rayseparate.Using fissile material, it can be made into a detector for detecting neutrons, such as a fission chamber.For the measurement of fission fragments, fission chambersSolid state track detectorAnd other means to record fission fragments directly.However, for the measurement in a narrow space, the use of the fission chamber is limited.In our experiments, we need a detector with a smaller volume, and use the nuclear fission method to measure neutrons.Therefore, a small detector can be made of polyester film sandwiched with uranium pieces, and fission fragments can be measured indirectly by measuring the γ activity of fission fragments captured on the polyester film.[1]
measuring principle
In the experiment, after the neutron of 14MeV source interacts with the experimental system, the neutron is measured at different measuring points on the surface of the iron ball.The uranium fission chamber can be selected for measurement.The uranium fission chamber is used to directly record fission fragments and obtain fission counts.The fission reaction rate can be obtained after the fission count is corrected.Fission fragments can also be recorded by an indirect method, that is, by measuring the γ activity of fission fragments through the combination of uranium flakes and polyester films, fission counts can be obtained through conversion.Fission fragmentalGamma ray, divided into prompt gamma ray and delayed gamma ray.The delayed gamma ray is less than 10－11It is difficult to observe the emission of S in such a short time.Delayed gamma rays are unstable fission fragment emissionBeta raybecomestable isotope It is emitted when.Attenuation approximation and time t of delayed gamma ray emissivity－1.2Is proportional, so it is easy to observe experimentally.The delayed gamma rays of fission fragments are measured experimentally.The integral γ count emitted by fission fragments above a certain energy threshold is proportional to the number of fission fragments.Therefore, fission reaction number NfAnd integral gamma count N emitted by fission fragmentsγIt has the following relations
。In the formula, the coefficient K can be calibrated by the fission chamber in advance, and K is also called the conversion coefficient.
To measure NγThe fission fragments that have emitted uranium flakes can be collected, that is, captured.Use the capture sheet (polyester film) to cling to both sides of the uranium sheet, and the fission fragments are absorbed or collected by the capture sheet.After irradiation, remove the uranium piece, measure the integral γ activity on the two capture pieces, give the fission reaction number from the above formula, and obtain the absolute fission reaction rate after correction.The detector made in this way is also called capture detector. Because of its smaller size, it can measure the reflected neutrons in a narrow space.
Through the capture detector measurement, on the one hand, it can replace the fission chamber measurement in specific occasions (such as in a limited space), on the other hand, it has a strong count. For the weak neutron field, it has a high counting efficiency.
Detector composition
Figure 1 Structure diagram of capture detector
The capture detector is composed of polyester film and uranium sheet.The polyester film and the uranium sheet can form a sandwich biscuit type.Here, it is required that the polyester film should be closely attached to the uranium sheet;The diameter of capture sheet is larger than that of uranium sheet to reduce the influence of boundary effect.In addition, the boundary effect between the uranium sheet and the capture sheet shall be kept unchanged during irradiation.If more than one uranium sheet is used, the uranium sheet should be centered and not misplaced. In order to solve these problems and improve the accuracy of measurement, a pair of aluminum compression rings are designed.The inner diameter of aluminum compression ring is 22mm, and the outer diameter is 31mm. They are used in pairs.The structure diagram of capture detector is shown in Figure 1depletd uraniumAnd its size is 24 mm × 0.5 mm.The capture plate is 30mm thick and 50 μ m.
No matter how the detector is placed, the structure shown in Figure 1 will not dislocate multiple uranium pieces, and can be compacted. The polyester film is easy to take and place, which can reduce the cooling time.The gamma rays of fission fragments captured on the sheet are measured with a NaI (Tl) detector.After the capture sheet is taken out, it is placed in the positioning box to ensure that the capture sheet is not misplaced and is pressed tightly.The positioning box is placed on the NaI (Tl) detector to measure the gamma ray on the capture film.
conclusion
By measuring theGamma rayTo measureneutronMethod.The measurement conditions are determined.The average scale conversion coefficient of different experimental systems is consistent within ± 2.5%.The measurement results of this method and fission chamber are compared, and they are consistent within the experimental error range.This method is feasible for the study of weak neutrons.
Simulation of neutron spectrum and angular distribution of fission fragments
Announce
edit
Nuclear fissionIt is a very complex physical process. Nuclear scientists have conducted long-term, in-depth and extensive research on nuclear fission.In order to better observe the fission physical process and obtain more fission physical data in experiments, many laboratories around the world have established fission multiparameter experimental measurement systems.Such systems can simultaneously measure fissionPrompt neutron, promptGamma ray、X-ray, kinetic energy of fission fragments, and can determine the mass of fission fragments.Therefore, it is a very effective tool in the experimental study of fission physics.For the experimental measurement and research of fission fragment nuclear prompt neutron spectrum with fixed mass number and fixed charge number by using the fission multi parameter measurement system, an important problem is that it is necessary to distinguish which fragment emits the measured neutron, that is, it is necessary to know the emitted neutronNeutron spectrumRelation with neutron emission angle (included angle between fission fragment movement direction and neutron emission direction), especially when usedNeutron detectorWhen measuring the emission neutron spectrum of a given fission fragment nucleus, it is necessary to understand the interference degree caused by the neutrons emitted by the complementary fission fragment nucleus. The Monte Carlo method is more convenient for analyzing and calculating such physical problems.[2]
