Neutrons detection

Fast neutron detection sometimes uses a hydrogenous moderator to slow down the neutrons and then employs a low-energy neutron detector as described above. One common fast neutron detector is a Bonner sphere. In this detector, a scintillator is placed in the center of a polyethylene sphere. Radiation transport calculations are used to produce efficiency curves that depend on the energy of the incident neutron. Another common fast neutron detector is a long counter. This detector uses a slow neutron detector (originally a BF3 chamber) at the center of a cylindrical moderator designed so that the detector is only sensitive to neutrons incident from one side. 

Neptunium is not found in nature in any extractable quantities. However, it occurs in uranium ores in exceedingly small concentrations resulting from neutron capture of uranium isotopes. No major application is known for this element. Its isotope, Np-237, is used in neutron detection instruments. 

The activation [672] of Lil with Eu2+ and the use of an activated Lil phosphor as a scintillation detector for slow neutron detection [673] has been investigated. Blue, fluorescent Lil (0.03 mole % Eu) phosphor was found to be the most useful [673] phosphor because of its ease to growth, relatively high light output, chemical stability and good match with spectral characteristics of the 6260 type photomultiplier. Lil (Eu), however, does have an interfering y radiation sensitivity. Fast neutron scintillation spectra of Li6(w, a)H3 in Eu doped Lil crystals has also been investigated [674]. 

Industrial utilization of neptunium has been very limited. The isotope 1 Np has been used as a component in neutron detection instruments. Neptunium is present in significant quantities in spent nuclear reactor fuel and poses a threat to the environment. A group of scientists at the U.S. Geological Survey (Denver, Colorado) has studied the chemical speciation of neptunium (and americium) in ground waters associated with rock types that have been proposed as possible hosts for nuclear waste repositories. See Cleveland reference. 

Figure 2. The neutron spectrum at die NIST BT2 neutron imaging facility. The spectrum is not corrected for die neutron detection efficiency, which increases as die square root of die energy.

The water thickness measurement uncertainty due to neutron counting statistics can be calculated from Poisson counting statistics. For a random process, the standard deviation, Ah in the observed counts I is A/ = y7. The number of neutrons in the incident, or open beam, I0, is the product of the neutron fluence rate (cur2 s 1), integration time T (s), integration area A (cm2), and neutron detection efficiency, ip... 

The most serious problem associated with the use of neutron scattering for nuclear spectroscopy comes from the fact that the resolution for neutron detection is typically rather poor, and the sensitivity to small transition probabilities is also poor when neutron detection is being employed. These difficulties can be alleviated by observing the y rays which de-excite the excited levels rather than the inelastically scattered neutrons. 

In both neutron and y-ray detection, the shielding of the detector is extremely important. Especially in the neutron detection measurements, the long target-to-detector distance (2-4 in) which is required to obtain velocity resolution via the neutron TOF technique means many more neutrons are produced than are actually scattered from the sample and then detected. These extraneous neutrons create a disastrous background unless the detector is adequately shielded. We have accomplished this with a large cylindrical shield which contains a lead cavity surrounded by Li2C03 loaded paraffin. The entrance collimator has steel and lead liners the main detector shield weighs about 2000 kg (see B in Fig. 1). 

Many types of plastic scintillators are commercially available and find applications in fast timing, charged particle or neutron detection, as well as in cases where the rugged nature of the plastic (compared to Nal), or very large detector sizes, are appropriate. Sub-nanosecond rise times are achieved with plastic detectors coupled to fast photomultiplier tubes, and these assemblies are ideal for fast timing work. 

Neutrons have mass but no electrical charge. Because of this they cannot directly produce ionization in a detector, and therefore cannot be directly detected. This means that neutron detectors must rely upon a conversion process where an incident neutron interacts with a nucleus to produce a secondary charged particle. These charged particles are then directly detected and from them the presence of neutrons is deduced. The most common reaction used in neutron detection today is ... 

The flux of neutrons, i.e., the number of neutrons detected per unit area per unit time in a SANS experiment is expressed by... 

The name comes from Neptunus, the Latin name for the god of the sea, but it was named after the planet Neptune, which had recently been discovered. The element was first prepared in 1940 by Edwin M. McMillan and Philip Abelson at the Berkeley Laboratory of the University of California. They irradiated uranium with neutrons to create the new element. Neptunium does not exist in nature and is primarily of scientific interest. It is used in neutron detection equipment. 

The total number of neutrons detected in a time channel between t and t + dt can be expressed as... 

A helium gas tube is shown schematically in Fig. 3.10a. It consists of an earthed steel tube filled with He the pressure is usually around 10 bar. A high voltage, 1800 V, anode runs down the length of the tube. The charged ionisation products caused by the proton and triton ( H", Eq. (3.5), are accelerated towards the anode, causing further ionisation and an avalanche effect. This results in a gain of up to a factor of 10 and single neutron detection is readily achievable. 

Figure 9.13 Four examples of response functions (a) 5-MeV Alpha particles detected by a silicon surface barrier detector (Chap. 13), or 20-keV X-rays detected by a Si(Li) reactor (Chap. 12). ib) 1-MeV Gamma ray detected by a NaI(Tl) crystal (Chap. 12). (c) 1-MeV Electrons detected by a plastic scintillator (Chap. 13). (

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