Recoil electronic

Fig. 6. Schematic illustration of the relationships of the original y-ray and the scattered radiations for Compton scattering where E is the energy of the incident photon, E is the energy of the recoiling electron, and E is the energy of the scattered photon.

For 7-ray energies below 1 MeV (the range of interest) there are two principal modes of interaction with matter — Compton scattering and photoelectron absorption. Compton scattering is the elastic scattering of the 7 photon by an orbital electron in which part of the incident 7-energy is imparted to the recoiling electron. 

Compton Effect—An attenuation process observed for x- or gamma radiation in which an incident photon interacts with an orbital electron of an atom to produce a recoil electron and a scattered photon whose energy is less than the incident photon. 

Photoeiectron, Auger-Electron (by Photoelectric Absorption) Compton Recoil Electron (by Compton Scattering)... 

Although the photon has no mass at rest, its inertial mass m = hv/c2 gives a linear momentum of mt. Experimental evidence for this linear momentum is found in the observation of recoil electrons in Compton scattering, or on a much larger scale in the solar wind which blows the tails of comets away from the sun. (It should not be forgotten that material particles also contribute to the solar wind.)... 

The density of excitation and ionization is not necessarily the same for all radiation qualities. For example, it is greater along the track of an a-par-ticle than for an electron track. For a primary-recoil electron produced by Co 7-rays in water, the distance between successive ionizations is about 1000 A. TTie ionized track is, therefore, sparse. At each point of ionization, secondary electrons give rise to further ionizations, forming a group of ion-pairs. In contrast, a-particles form a continuous track as a result of overlapping between the spheres of ionization. 

Reaction ( ) is the primary ionization-process initiated by a recoil electron. It is thought that, subsequently, HaO is converted to a hydroxyl radical within 10 seconds ... 

According to this theory, hydrogen and hydroxyl free radicals would be distributed along the track of the original particle or primary-recoil electron, with the hydroxyl radicals situated near the track. The location of the hydrogen atoms is less certain, and they may be situated several A. units away from the site of electron formation. 

Irradiation Procedures. G(Fem) of 15.6 for the ferrous sulfate dosimeter was determined in our laboratory by Hochanadel and Ghorm-ley (II). G-values reported here are based on total energy absorbed by the solutions. The energy absorbed in concentrated sodium nitrate solutions relative to the ferrous sulfate dosimeter was taken to be in the ratio of electron densities since energy absorption in 60Co irradiations is caused essentially only by Compton recoil electrons. 

For all detector types, the transfer of energy to the detector material as the photoelectrons and recoil electrons slow down occurs through excitation or ionization of electrons in neighboring atoms within the detector structure. What occurs next depends on the specific molecular structure of the detector. We can distinguish between structures such as noble gases, photoconductors, fluorescent phosphors, and photostimulable phosphors. 

The resolution of Nal (Tl) spectrometers depends on the energy of the radiation. At energies of about 10 MeV when the response in the crystal is mainly due to pair production, the resolution is about 10%. A slight difficulty in interpretation arises from the presence of subsidiary peaks due to escape of one or both of the pair annihilation quanta. At lower energies the pair lines are also found but the Compton distribution becomes more important and the pair peaks are superimposed on this. At energies below the pair threshold there are pulses from the Compton recoil electrons and from photoelectrons and as the quantum energy decreases the photoelectric peak becomes more important but the resolution decreases, as E, until it is only of the order of 20 to 30% for 50keV quanta. 

The ambiguity due to the occurrence of multiple peaks in Nal (Tl) spectrometry for energies above the pair threshold can be removed, at the expense of sensitivity, by the use of a three-crystal spectrometer. In this instrument pulses from the centre crystal are only accepted when there is simultaneously a pulse due to an annihilation quantum in each of two side crystals. The difference between single- and three-crystal spectra is shown in Fig. 12. For energies below the pair threshold improved resolution can be obtained by using a two-crystal coincidence spectrometer to measure the pulse height due to Compton recoil electrons (Hofstadter and McIntyre ). 

A1 seems to be a satisfactory liner and to stand up imder irradiation not only in contact with water but also in contact with diphenyl. A neater solution of the lining problem is provided for, however, by Creutz s discovery of the easy formation of a nice dip-coat of Zn on U. This dip coat is so thin (3 mils) that its neutron absorption is not greater than that of the A1 sheath. Such a thin sheath is not impermeable to /3-rays from the U which have to be considered together with the Compton recoil electrons. 

This can be resolved into the energy that appears as scattered radiation e, and the energy impart to the recoil electron e, so that e = e, + ,. With this interpretation we can define a scattering cross section according to and where... 

It should be emphasized that the absorption coefficient is a much more restricted concept than the attenuation coefficient. Attenuation also includes the purely elastic process in which the photon is merely deflected and does not give up any of its initial energy to the absorber. In a photoelectric interaction, the entire energy of the incident photon is absorbed by an atom of the medium, while in the Compton effect, some energy is absorbed and appears in the medium as the kinetic energy of a Compton recoil electron the balance of the incident energy is not absorbed and is present as a Compton scattered photon. 

The fraction of energy transferred to the kinetic energy of a recoil electron in each Compton interaction can be calculated from... 

In summary, it should be emphasized that (a) the Compton process is almost independent of atomic number (b) its probability decreases with increase of the photon energy (c) the fraction of the energy transferred per collision to the kinetic energy of a recoil electron increases with increase in photon energy and (d) in soft tissue, the Compton process is much more important than other interaction processes for photons in the range 100 keV to lOMeV. 

The SiO, usually a dark brown powder, is used for the evaporation source because it is a very easily evaporated oxide using conventional resistive heating. In the case of Si02, high temperature heating by the electron beam, for example, is indispensable, and is not suitable for TFT array substrates due to the damage by recoiling electrons. 

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