Handbook of Modern Coating Technologies

The use of slow positrons for diagnostics of materials

A positron microscope with a microprobe built around it is actually known to a relatively narrow circle of specialists engaged in relevant research with the application of these facili­ties [133]. The positron annihilation technique has been described in a recent review [134], which, however, does not consider transmission or scanning positron microscopes or the microprobe. Preparation of the positron beam occurs as follows. Initially less than 1% of the positrons from the beam are decelerated, the remaining positrons from the beam of these monoenergetic positrons must be separated. To slow down positrons, it is necessary to sepa­rate the source and the sample in space, as well as create a beam guidance system

Annihilation

ihermalization/

fraction ~ 0.13

diffusion

Monoenergetic positrons ~ 1 x 10^-4

(°> 1.03

1.02

S

1.01

a

7

1.00

to

0.99 0.98

FIGURE 5-13 (A) Spectrum of the positron emission for ²²Na source. dWIdf is the number of positrons per energy channel E. Distribution of the positron energy after moderation in tungsten is demonstrated by the narrow curve centered at 3 eV. (B) Scheme of positrons deceleration by tungsten foil with orientation (100) in transmission geometry. Most positrons have high residual energy when they fly out of the moderator. A smaller fraction of positrons stops and annihilates in the foil. Positrons can be spontaneously emitted when they reach the surface during diffusion because of the negative work function of tungsten. The deceleration efficiency is ~10~⁴.

(Section 2.4.2). Monoenergetic positrons can then be accelerated to energies in the range from few electron-volts to several dozen kiloelectron-volts [135]. Based on the data on the interaction of positrons with the near-surface region, it is possible to construct a distribution function of defect concentration with depth (defect depth profiling). The principles underly­ing defect profile measurements and their applications in various techniques using slow pos­itron beam are described in Sections 5.4 and 5.5.

• Positron source and moderation

Fig. 5—13A presents the emission energy spectrum of an ²²Na radioactive source. Various materials are utilized as moderators, bearing in mind that energy values equaling the (nega­tive) positron work function can be obtained in many solids. The transmission geometry where a thin moderator foil is located above the source capsule is preferred in most cases. The average penetration depth of the positron is much greater than the thickness of the foil. Therefore only a small fraction of positrons is thermalized and diffuses there. Such positrons are emitted from the foil spontaneously if they reach a surface during diffusion. Moreover, their kinetic energy corresponds to the thermally expanded work function Ф+. (Fig. 5—13B). Materials made from elements with large atomic numbers appear more suitable for modera­tion due to the increased mean diffusion length to the distance of the thermalization ratio. The most preferred materials for the purpose are a (100)-oriented single-crystalline tungsten foil a few mm in thickness and a tungsten single crystal with (110) orientation to be applied in backscattering geometry.

The foil initially contains a small number of positron traps. To increase the number of defects on the foil surface on which positrons can be captured, it should be preliminarily annealed. The ratio of the number of delayed positrons to the number of particles trapped on the foil is called the deceleration efficiency. For a tungsten single crystal with orientation (110), the working function Ф+ of positrons was measured to be about —3.0 eV. In this case, a 3 X 10^—3 deceleration efficiency was achieved [136—138]. Typically the deceleration

• Experimental arrangement for measuring Doppler broadening of the annihilation peak of g-quanta on the 511 keV line. The signal enters a Ge-detector through the preamplifier, then passes through the ADC to the MCA.
• Doppler broadening spectrum of the annihilation peak in Zn-doped gallium arsenide illustrates positron trapping in defects of plastically deformed GaAs(Zn). The areas under the A_s and A_w curves of the annihilation peak determine the line shape parameters S and W, respectively. The curves for initial and plastically deformed GaAs are normalized to an equal area. (E) Slow positron beam system (POSSY) (sectional view) at Martin Luther University, Germany. Source-moderator device (S) emits positrons, which fall into the E X B Then positrons enter the collimator and speed up toward the test sample. Guidance coils (m) create a circular magnetic field. Vacuum around the source is maintained in the range of 10^—6 Pa. By stepwise differential pumping in the sample chamber of the vacuum system, a pressure of the order of 10^—9 Pa can be achieved. (F) Calculated Makhov profiles P(z;E) in silicon according to formula (6) for four incident positron energies at the following parameters A 5 40 mg/ cm² keV^—r, m 5 2, and r 5 1:6. The average penetration depth z is shown by dashed lines. (G) S parameter. (H) Experimentally determined parameters A and r for the Makhov profile (6) [133]. ADC, Analog ± digital converter; MCA, multichannel analyzer.

efficiency is ~10^_4. For polycrystalline tungsten foils, lower retardation efficiencies are possi­ble, as shown by Brusa et al. (see references cited in articles [135,139]).

When a thin layer of krypton or neon was deposited on a carrier foil at low temperatures, solid-state moderators with an inert gas were obtained [133,140]. Such moderators have an extremely high (up to 10^_2) moderation efficiency (a fact that has not yet found a convincing explanation). It is assumed that an internal electric field plays an important role in determin­ing the drift direction of individual positrons diffusion motion. One option for a positron moderator using an electric field is the use of SiC [141,142] (see references in articles [133,143]). Silicon carbide can be used as an electrostatic moderator because it is the only known semiconductor with a negative work function of the positrons. For sources of the pos­itron beam used for Doppler broadening measurements and conventional positron lifetime spectroscopy, it is necessary to increase the power due to the low efficiency. In addition, since the activity of the source reaches 5 X 10⁹ Bq (135 mCi), intensive radiation protection is required.