Proton recoil

Gas-filled detectors are used, for the most part, to measure alpha and beta particles, neutrons, and gamma rays. The detectors operate in the ionization, proportional, and G-M regions with an arrangement most sensitive to the type of radiation being measured. Neutron detectors utilize ionization chambers or proportional counters of appropriate design. Compensated ion chambers, BF3 counters, fission counters, and proton recoil counters are examples of neutron detectors. 

Fast neutron exposure determined by counting individual proton recoil tracks. 

The aim of this section is to extract from the measurements the values of the Rydberg constant and Lamb shifts. This analysis is detailed in the references [50,61], More details on the theory of atomic hydrogen can be found in several review articles [62,63,34], It is convenient to express the energy levels in hydrogen as the sum of three terms the first is the well known hyperfine interaction. The second, given by the Dirac equation for a particle with the reduced mass and by the first relativistic correction due to the recoil of the proton, is known exactly, apart from the uncertainties in the physical constants involved (mainly the Rydberg constant R0c). The third term is the Lamb shift, which contains all the other corrections, i.e. the QED corrections, the other relativistic corrections due to the proton recoil and the effect of the proton charge distribution. Consequently, to extract i oo from the accurate measurements one needs to know the Lamb shifts. For this analysis, the theoretical values of the Lamb shifts are sufficiently precise, except for those of the 15 and 2S levels. 

Proton recoil Fast neutrons can knock hydrogen nuclei right out of their atoms. The resulting so-called recoil protons can then cause ionization that can be detected and measured. This reaction can be utilized in gas-filled detectors, nuclear track films and scintillators. 

The response of proton-recoil counters resembles a step function (see Chap. 14). 

The main scattering reaction used is neutron-proton collision, called the proton-recoil method. The knocked-out proton is the particle recorded. 

With the exception of the proton-recoil method, which functions for fast neutrons only ( > 1.0 keV), all the other interactions can be used with neutrons of any energy. However, at every neutron energy, one method may be better than another. The best method will be selected based on the neutron... 

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. 

N Ep) dEp = proton recoil energy spectrum = number of protons produced (by collisions with neutrons) with energy between E and E -t- dE R(E, Ep) dE = response function of the detector = probability that a proton... 

Equation 14.28 has the form of the folding integral (see also Sec. 11.5), while Eq. 14.29 gives the composite response function for the proton recoil spectrometer. 

Wall-and-end effects. Tracks of protons generated close to the wall or close to the ends of the counter have a high probability for incomplete energy deposition and collection of ionization. Proton-recoil tracks close to the wall are truncated by collisions with the wall material before the struck proton deposits all its energy in the counter. Protons being produced close to the end of the counter... 

Electric field distortion. The gas multiplication in a proportional counter depends on the intensity of the electric field. Close to the ends of a cylindrical counter, the strength of the electric field becomes gradually less intense than in most of the counter volume. This effect produces lower pulses from proton recoils at the ends of the counter. Detectors with large length-to-diameter ratio are less affected by this problem. Theoreticcil corrections of this effect have been developed and successfully applied. ... 

Effect of carbon recoils. Neutrons detected by methane-filled counters collide not only with hydrogen nuclei but also with carbon atoms. The ionization produced by carbon recoils is indistinguishable from that produced by protons. However, carbon recoils produce pulses that are smaller than those from protons because of differences in both kinematics and ionization ability. The maximum fraction of neutron energy that can be imparted to a carbon nucleus in one collision is 0.28 (versus 1 for a hydrogen nucleus), and the relative ionization efficiency of a carbon to a proton recoil is about 0.5. Thus, the effect of carbon recoils is to add pulses at the low-energy region of the response... 

Calculate the measured neutron spectrum obtained by the proton recoil method if the detector response is a 6-function and the source spectrum is the square function shown in the figure below. Assume cr(n, p) is constant for the range , < < 2. 

In the previous section we estimated the upper limit on noise from all sources for a single module, based on a 10 event signal. The main backgrounds arise from uranium and thorium decays in the active medium, detector walls, photomultiplier tubes, and surrounding rock, from proton recoil due to muon generated fast neutrons, and from activation of nuclei by fast neutrons and muons. 

Differential neutron spectra were measured In center of a fast-spectrum molybdenum-reflected critical assembly using spherical proton-recoil detectors. The critical assembly was buiU to investigate geometrical variables and reactivity effects of-materials for a small, fast-spectrum conceptual reactor (lithium-cooled, U nitride-fueled). Measurements were made at Atomics Intemational where the critical asseimbly was designed, built, and operated. Calculations were made, at the NASA Lewis Research Center. 

E. F. BENNETT, Fast Neutron ectroscopy by Proton-Recoil Proportional Counting," NucI Sci. Bug., 27,16(1967). 

Proton-recoil measurements have been made at core center, in core near to the core-reflector interface, and in the reflector of ZPR-3 Assemblies 61 and 62. ITiese are homogenized EBR-lI-type cores radially reflected... 

Fig. 2. Comparison of calculated spectra with mq>eri-mental (proton-recoil) spectra at various positions in ZPR-3 Assembly 61.

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