Energy spectrum, neutron

Cf spontaneous fissions have a fast neutron energy spectrum, shown in Figure 3, with an average energy of 2.2MeV. On average, 3.76 neutrons are emitted per spontaneous fission. The neutron emission rate is 2.34 X 10 n/(s-g)... 

If the emitted particles are neutrons, the emitted neutron energy spectrum has the form... 

Sands DG, De Laeter JR, Rosman KJR (2001) Measurements of neutron capture effects on Cd, Sm and Gd in lunar samples with implications for the neutron energy spectrum. Earth Planet Sci Lett 186 335-346 Schaeffer OA (1975) Constancy of galactic cosmic rays in time and space. 14th Inti Cosmic Ray Conf 3508-3520... 

MeV threshold, are therefore much lower with the revised neutron energy spectrum. [Used by permission of the American Geophysical Union, from Yatsevich and Honda (1997), Journal of Geophysical Research, Vol. 104, Fig. 2, p. 10294.]... 

That is, an average cross section is used, even though the overbar that indicates averaging is normally dropped. From now on, Eq. 14.13 will be used without the overbar, but the reader should keep in mind that a is an average over the neutron energy spectrum. 

MEASUREMENT OF A NEUTRON ENERGY SPECTRUM BY PROTON RECOIL... 

Detection of neutrons by proton recoil is based on collisions of neutrons with protons and subsequent detection of the moving proton. Since neutrons and protons have approximately the same mass, a neutron may, in one collision, transfer all its kinetic energy to the proton. However, there is a possibility that the struck proton may have any energy between zero and the maximum possible, as a result of which the relationship between a neutron energy spectrum and a pulse-height distribution of the struck protons is not simple. It is the objective of this section to derive a general expression for this relationship. The sections that follow show its application for specific detectors. 

The task of neutron spectroscopy is to obtain the neutron energy spectrum general methods used to unfold this equation are discussed next. 

The LSL-M2 program package determines the neutron energy spectrum based on information obtained from a combination of neutron flux calculations and threshold foil activation measurements. The results of LSL-M2 are used primarily for the determination of radiation damage to reactor components and... 

Assuming that the threshold-reaction cross sections are ideal step functions, as shown in this figure, indicate how the neutron energy spectrum could be obtained. There are N such cross sections with thresholds at ,, = i, v and i = A = constant. 

A = decay constant of the radioisotope produced After irradiation, the activity A t) is counted and the flux is determined from Eq. 16.34. Depending on the foil used (reaction involved), information about the neutron energy spectrum may also be obtained. Information about the neutron spectrum < (E) is necessary for the determination of the neutron dose equivalent H. 

It is obvious that the neutron energy spectrum of a reactor plays an essential role. Figure 19.4 shows the prompt (unmoderated) fission neutron spectrum with 2 MeV. In a nuclear explosive device almost all fission is caused by fast neutrons. Nuclear reactors can be designed so that fission mainly occurs with fast neutrons or with slow neutrons (by moderating the neutrons to thermal energies before they encounter fuel). This leads to two different reactor concepts - the fast reactor and the thermal reactor. The approximate neutron spectra for both reactor types are shown in Figure 19.4. Because thermal reactors are more important at present, we discuss this type of reactors first. 

With an external proton beam both of these experimental sources of energy spread can be avoided. The type of neutron energy spectrum one obtains are illustrated by the measurements of Cassels et al. at 145 Mev, of Nelson et al. at 220 Mev, of Goodell et aL at 350 Mev and of Nedzel at 410 Mev. 

Irradiation in test reactors with mixed neutron energy spectrum produces basically the same damage morphology as in commercial reactors. Available size and volume of specimens are similar to or larger than those in surveillance of commercial reactors. These are great advantages of test reactor irradiation, although influences of differences in flux and neutron... 

A basic requirement for accurate NAA is the correct characterization of the irradiation facility (Becker 1987). Local and temporal neutron flux density gradients as well as gradients in the neutron energy spectrum of the irradiation position must be well understood and known for each irradiation. O Figure 30.5 illustrates the gradients in an irradiation capsule as measured by flux monitor foils. A difference of 1 mm in sample positioning will result in a 0.6% relative difference in the measured concentration. 

In principle, in a reactor, three components of the neutron energy spectrum (O Fig. 57.3) can be distinguished (Erdtmann 1976 Erdtmann and Petri 1986) ... 

Model (2) is an earlier version oi the Model (1) calculation. It is identical with Model (1) except, for the thermal constants which are calculated using i lMaxwellian thermal neutron energy spectrum rather than the Wigner-Wilkins spectrum. 

A critical experiment was performed at room temperature with 12 TORAX-V superheater fuel elements surrounded by a peripheral region containing 1228-1525 U02 fuel pins, as shown in Fig. 1. Measurements were needed to supplement the available computational information since the neutron energy spectrum, effective neutron cross sections, flux intensity and power production are poorly defined by thieory near interfaces such as that between the superheater and the peripheral region, and also in the presence of voids such as exist in the superheater region (26% void). 

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