Radiation that changes direction as it passes through material is called what?

Bremsstrahlung Radiation

Bremsstrahlung is the German word for braking, or slowing down. When a high-speed projectile electron from the cathode passes the nucleus of a tungsten atom in the target, the positively charged nucleus exerts an attractive force on the electron. A strong nuclear electric field inhibits penetration of the electron into the nucleus but causes the electron to decelerate and change direction (Fig. 1-15). This deceleration results in a loss of kinetic energy, which is converted into EM (x-rays). The quality (or energy) of radiation released is contingent on the amount of deceleration and kinetic energy possessed by the incoming electron (measured in kVp). Deceleration is affected by the proximity in which electrons randomly approach the nucleus and size of the nucleus. Electrons directly striking the nucleus and giving up 100% of their kinetic energy generate the highest energy x-rays.

Bremsstrahlung x-rays have a spectrum of energies with an average energy somewhere below, but proportional to, the peak kilovolts used. Primary control of x-ray beam quality, or overall penetrating power, is a result of the effect of peak kilovolts on bremsstrahlung interactions. The quantity of bremsstrahlung x-rays is related more to milliampere-seconds (tube current) than peak kilovolts. Higher values of milliampere-seconds release more electrons to the target. Most of the x-rays produced in a diagnostic beam are of bremsstrahlung origin.

Effect of Energy

Electron beam characteristics change with beam energy. Representative beam data are presented in Table 6-13, central axis PDD in Figure 6-41, and isodose curves in Figure 6-42. Several key observations apply:

PDD increases as energy increases because the electron range increases with energy. The values for d90, d80, and Rp (see Table 6-12) all increase with energy.

Surface dose is high, usually in the 70% to 90% range (see Table 6-13), and it increases as energy increases (an opposite effect from photon beams) because of changes in the amount of lateral electron scattering and its dose contribution at dm.

In general, dm starts at shallow depths and increases as energy increases because the electron range increases with energy.

As energy increases, the region around Dm becomes quite broad. In this case dm is defined at a selected point.

The steepness of the dose fall-off region lessens with increasing energy. Increased scattering occurs, and a wider energy spectrum is created with a distribution of electron ranges. The originally monoenergetic electron beam is degraded into a wide spectrum beam with varied practical range.

The amount of bremsstrahlung contamination increases with energy because the probability for radiative interaction increases with electron energy. Bremsstrahlung contamination can be minimized but not eliminated totally. Structures lying beyond the electron range still receive dose from bremsstrahlung x-rays.

The approximate d80 (depth to the 80% isodose line in centimeters) in tissue can be found by dividing the nominal electron energy (in million electron volts) by 3. The d80 of a 9-MeV electron beam is 9/3, or about 3 cm. This number is useful for indicating the largest therapeutic depth, although many clinicians consider the d90 as the maximum effective range.

Recombination radiation

In addition to Bremsstrahlung radiation, a hot plasma also emits recombination radiation. This radiation is emitted when a free electron is captured in a bound state of an ion. If E is the kinetic energy of a free electron and χn is the ionization potential of the energy level in which the electron is captured, the radiation is emitted with a photon energy of hv=E+χn (Figure 9). As the free electron has a continuous energy distribution, the emitted radiation spectrum is also continuous for hv≥χn. Further, since the recombination can occur in different energy levels of the ion, the overall spectrum is quasi-continuous showing discontinuities at energies equal to the ionization potential energies of various levels. The overall shape of the spectrum is similar to that of plasma Bremsstrahlung radiation shown in Figure 8.

Interestingly, whereas in an X-ray tube, the radiation is on the longer wavelength side of the Duane–Hunt limit, here the spectrum is on the shorter wavelength side of the ionization potential.

7.3.1 Terminology and definitions

Elastic scattering and bremsstrahlung production are generally negligible by comparison with the collision process for heavy charged particles at energies up to 500 MeV (Evans, 1955). Therefore the total mass stopping power, S/ρ, for particles such as protons, alpha particles and pions is essentially equal to the collision mass stopping power:

(7.14) Sρ=1ρdEd1≈[Sρ]col

After a negative pion has lost its kinetic energy by ionisation it may cause fragmentation of the nucleus. Molecular structure plays a considerable role and trace concentrations of elements can produce disproportionately large effects (Jackson and O'Leary, 1984).

9.01.2.4.6 Radiation length

At high energies where bremsstrahlung is dominating and proportional to energy, the energy loss may be approximated by

[35] dEdx=−kE;dEE= −kdx;Ex=E0e−kx

where k a constant varying with the target material and x is the depth in the material. The radiation energy is then decreasing exponentially for high electron energies. The length over which the electron energy is reduced with a factor of 1/e due to radiation losses is called radiation length, XR. XR decreases with increasing atomic number (Figure 13).

The Continuous Spectrum

To the continuous spectrum the name Bremsstrahlung or ‘braking’ radiation is often applied, and gives a good clue to the way in which the radiation is produced, i.e., by the slowing down of the cathode-stream electrons by the atoms of the ‘target’ material.

What happens in the bombarded target is extremely complicated and its explanation beyond the scope of this book. However, it is possible to present a fairly simple mechanistic picture, which though not precise in detail, enables the main features to be understood.

As a generalization it may be said that two types of interaction (‘collisions’) are involved. In the first the impinging electrons ‘interact’ with the electrons of the atom (the electron ‘cloud’), whilst the second is between the electrons and the nuclei. When electrons approach fairly close to atoms they will be repelled, and therefore deflected, by the ‘cloud’ and, to some extent, will be slowed down. The small amount of energy so lost will be transferred to the atom involved, so that by this process the target material gains energy; it is heated up. Sometimes, however, an electron will penetrate through the ‘cloud’ and approach the nucleus itself. In the powerful electrical field of the nucleus the electron behaves rather like a comet entering our solar system; it swings around the sun and departs in a completely new direction. Thus the electron suffers a considerable change in direction and at the same time a large reduction in its speed and, therefore, energy. The energy lost in this type of interaction is emitted as a high-energy photon, that is to say an X-ray. How much energy is emitted depends not only on the details of the ‘collision’ but also on how much energy the electron retains after any previous ‘collisions’. A wide range of photon energies may thus be produced, varying downwards from the ‘collision’ in which the electron loses its energy in a single ‘collision’—a rare but not impossible event. Fig. 24 illustrates some of the type of interactions that may occur when electrons strike the target. Electron 1, after slight deflexions by two atoms, penetrates the ‘cloud’ of a third, suffers a marked change of direction, and emits an X-ray photon. With some residual energy it continues, suffers another minor deflexion before giving up all its residual energy in a final nuclear ‘collision’. Electron 2, typical of many, suffers a long series of ‘collisions’ with the electron ‘clouds’, which result in small energy losses—and heat production—but no X-ray photons. Electron 3 is quite different. It passes very close to the nucleus and is brought to rest, thus giving up all its energy. Such a ‘head-on’ type ‘collision’ produces the maximum energy photon possible. Finally electron 4 makes three nuclear ‘collisions’, giving up roughly a third of its energy in each.

Fig. 24. –Some interactions between electrons and atoms in an X-ray tube target, and their results. H indicates heat-producing ‘collision’.

indicates photon-producing ‘collision’.

It must be stressed that Fig. 24 only indicates the types of interaction, and does not represent the direction of emission of the photons or how often they occur, both of which features of the X-ray production process depend upon the energy of the electrons—i.e., upon the applied voltage—as will be described later. However, the diagram illustrates the important fact that X-rays are not produced solely at the surface of the target. Electrons penetrate through many atomic layers and the X-rays have therefore to pass through some thickness of target material before emerging into space. This has a marked influence on the spatial distribution of radiation from an X-ray tube, a point to which we will return later.

1 INTRODUCTION

Besides the well known electron-ion bremsstrahlung in pure plasmas, other atomic radiation processes must be considered in power balance calculations of hot plasmas. While electron-electron bremsstrahlung is not likely to be very important at the temperatures of interest, an increase in the average charge (Zeff) of the ions due to the presence of more or less highly charged impurity ions already leads to an increase in the bremsstrahlung emission by a factor Zeff, compensating the increase in Ohmic heating power. However, as long as the thermal energy kT of the plasma electrons is not much larger than ionization energies χ of the various impurity ions, recombination radiation is actually more important, and when the ions are not completely stripped, this radiation tends in turn to be dominated by (mostly) resonance line radiation.

Since any of the early but fairly realistic estimates of these various radiative contributions to the power loss of plasmas (see, e.g., Griem [1]) it has been clear that very small percentages of incompletely stripped ions would significantly alter the power balance. More detailed experimental and theoretical work in the past decade has substantiated this conclusion. Moreover, Burgess [2] has pointed out that the composite process of electron capture associated with excitation of one of the bound electrons and followed by radiative decay of the doubly-excited state of the resulting ion (dielectronic recombination) strongly shifts the ionization-recombination balance in favor of lower ionization stages. It therefore accentuates the line radiation.

Since for a given plasma composition bremsstrahlungs power and recombination radiation are relatively simple to estimate and often not too important, the emphasis in the following sections will be on line radiation. For practically all strong lines it is safe to assume that radiative decay of the upper states involved is immediate and much more likely than any other process. Then one has for the power density Pz in a given spectral line

(1)PZ= EZXZNZNe,

where E is the excitation energy, X is the excitation rate coefficient to be discussed in Sec. 2, and Nz and Ne are ion and electron densities, respectively. All ions can essentially be assumed to be in their ground states (or its various fine-structure levels), so that the remaining atomic physics task is to obtain the distribution of ions of a given atom over various charge states. Besides by possible transport terms, this distribution is governed by the set of rate equations

(2)dNZdt = (IZ-1NZ-1+RZ+1NZ+1 -IZNZ-RZNZ)Ne,

to be supplemented by the condition that the sum over all Nz correspond to the total element abundance. The production terms in eq. (2) correspond to ionization of the preceding charge stage and recombination on the following stage, while the destruction terms account for ionization of stage z and recombination into the next lower stage. The various rate coefficients, Iz for ionization and Rz for recombination, will be discussed in Sees. 3 and 4, respectively. In the final section, theoretical predictions of line radiation losses are summarized.

Industrial use of sources

3.1.2 Electron interactions

There are two electron interactions with matter namely, characteristic and Bremsstrahlung X-ray, which are important in the X-ray generation. When an incident electron collides with an inner orbital electron in the atom, excitation happens as the inner orbital electron (e.g., K shell) absorbs a small amount of energy and jumps to a higher energy level. However, if the energy transferred to the orbital electron is adequate, the electron is released from the atom with its vacancy filled by an outer orbital electron. This ionization event results in an emission of characteristic X-ray/photon with a discrete energy value. Bremsstrahlung interaction, on the other hand, is radiative. It is a deflection of incident electrons by atomic nucleus, resulting in Bremsstrahlung photon emission known as “braking radiation.” Because the incident electron can be bended around the nucleus with various distances, the emitted photon has a continuous energy distribution. Such radiative interaction is important in high-energy electron and high atomic number material. It should be noted that electron traveling in the tissue can lose a very small or large amount of energy leading to a very small or large deflection in a single interaction. These large variations of energy transfer and deflection angle along the electron path cause the well-known electron range-straggling in the tissue.