Section one: Thermal Neutron Detection

Question one:

It can be said that neutrons are indirect ionizing radiation. Additionally, neutron has not charge andits electromagnetic interactions are very week. Neutrons just interact with atomic nuclide and also pass throughout matter basicallyunrestricted. The interaction of Thermal neutron is withinnucleus and charge particles are produced by interaction production such as,Alpha particle, proton and recoil nucleus. These particleswill deposit energy and ionizing the atoms. They are also direct ionizing radiation.

Q.2

Neutrons can be detected using helium-3-filled gas proportional counters. A typical counter consists of a gas-filled tube with a high voltage applied across the anode and cathode. A neutron passing through the tube will interact with a helium-3 atom to produce tritium (hydrogen-3) and a proton. The proton ionizes the surrounding gas atoms to create charges, which in turn ionize other gas atoms in an avalanche-like multiplication processthe resulting charges are collected as measureable electrical pulses with the amplitudes proportional to the neutron energy. The pulses are compiled to form a pulse-height energy spectrum that serves as a “fingerprint” for the identification and quantification of the neutrons and their energies.

Q.3

(C): If the tube of Proportional counter is very large all reactions will be occurred in gas. And if the dimension of the detector is smaller than the range of charged particles (proton and triton), which produced in reaction, either particles will hit the wall and a smaller pulse will be created in spectrum. The two products that created from neutron interaction will travel in opposite direction if proton hit wall the triton will deposit its full energy in gas whereas if triton hit the wall the proton will deposit its full energy in the gas. Therefore we will see wall loses for only one reaction product at a time.

(D): the gamma ray primarily interact with wall of detector and secondary electrons are produced. And because electrons havelow stopping power in the gases,small fraction energy of electron will be deposited in the gas and most energy will be deposited in the opposite wall. Therefore, gamma ray interaction will produce a low pulses that lie in the most right of spectrum and by using simple amplitude discrimination, these gamma radiation can be eliminated without effect of neutron detection efficiency.At high rare, pile up can result in appearing peak amplitudes for gamma rays which are considerably larger than any individual pulse. Short time constants may be good to minimize the gamma ray pile up but might be lead to minimizing in the neutron-induced pulse amplitude due to incomplete charge integration. At very high gamma rates, chemical changes may be occurred in 3He gas as result of disassociation whichleads to pulse height spectra degradation from neutron-induced events. In sever degradation case, the gamma and neutron induced events may cannot be separated. The permanent damage to the tube may be occurred in extreme cases.

Q.5

(b) One peak is of lithium in excited stated and the other one is in ground state

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Q.6The fission counter detect the fragment that produced by fission. In the most common type of fission counter, the interior surface of the detector is coated with fissile isotope. When fission takes place, one of the fission fragments is emitted toward the center of the counter and is detected. The other stops in the fissile deposit or the wall of the counter. The counting rate of fission counter is proportional to the fission rate, which in turn is proportional to the neutron flux. One advantage of fission counter is that the fission reaction has a large amount of energy (200 MeV) and 160 MeV of it will be as kinetic energy of reaction fragments. This make the neutron induced fission reaction has much larger magnitude than reactions types or other event such as background or counter contamination. For this characteristics extremely low background can be achieved and neutron counting can be done at very low counting rate.

Section Two : Fast Neutron Detection

Q.1: Fast neutron interacts with scintillator material (such as organic material) via elastic scattering and neutron transferred its energy to proton. The energy of recoil proton absorbed in scintillator and converted to visible light. The visible light interact with the cathode of multiplier tub that attached to the crystal and electron will be produced and then these electrons are multiplied through several dynode that lie between cathode and anode then a signal is produced.

Q.2: because they are Hydrogen enriched material

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Q3: The scintillatoryield has two component fast and low components. The fast component has fast time decay whereas slow component has slow time decay. These components depend on the specificionization of the ionizing particle (different mass, charge). Heavier particles have higher specific ionization and produce more delayed fluorescence light. The most common method used for pulse-shape discrimination in organic scintillators is based on passing the photomultiplier-tube pulse through a bipolarshaping network, usually a double-delay line. The zero-amplitude crossing of this bipolar pulse depends on the risetime and shape of the initial pulse is independent of amplitude. Thus a measure of the pulse shape is given by the time interval between the leading edge of the initial pulse (which is independent of pulse shape) and the zero crossingof the bipolar-shaped pulse.

Another common approach to pulse-shape discrimination involves integrating the charge contained in the early risetime part of the pulse and comparing it to the integral of the charge in the late part of the pulse. In a recent application of this method, the anode pulse from the photomultiplier tube is split and the early and late parts of the pulse are separately gated into integrating analog-to-digital converters.

Q.4:

(a) and (b)

( C ):Incident angles of elastic scattering collision give rise to somewhat flat and

Rectangular shape. The DT fusion can generate relative high energy while collision.

21H + 31H ?42He + 10N (Q ~ 17.6 MeV)

With the angle increases, the neutron energy will transfer more energy to the target nuclei, which can be detected easily. Therefore, the spectrum with high energy ended of the plateau corresponds to a big angle collision (head on collision). On contrary, the low energy stems from small angle collision or even glancing off as the angle is 0??.

(d)The gamma rays are produced from neutron capture. The fast neutron is moderated to thermal energies. The probability of capture by a nucleus generally increases as the neutron energy decreases. Therefore, a neutron can easily interact with a nucleus and then gamma rays are released with relative high energy than the thermal energy from the exciting nucleus.

(e) Organic scintillators are made of low atomic number materials, such as hydrogen

and low photoelectric cross section because it is proportional to the atomic number.

Therefore, Compton scattering dominates over the photoelectric effect which leads to

Photopeak.

Q.5:According to Richard Olsher el al in their paper “WENDI: AN IMPROVED NEUTRON REM METER”, One type of these dosimeter is Neutron rem meter it is designed to be used in real-time measurement of neutron dose equivalent. It is used when the spectra are unknown or poorly characterized. It designed to have fluence-to-dose conversion function. It consists Polythelenen moderator assembly, which is surrounding the thermal neutron detector, such as BF3 counter tube. As the older version of this detector has several limitation (in curate in intermediate energies, angular dependence and no useful high-energy response) a new design has been created. In this design, the use of both neutrongeneration and absorption to contour the detector response function. This done by adding Tungsten or tungsten carbide (WC) powder to polyethylene moderator with expressed the purpose of generating spallation neutrons in tungsten nuclei and therefore increases the high-energy response of the meter beyond 8 MeV.

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