Positron annihilation spectra

Data-processing System for Positron Annihilation Spectra on Mainframe and Personal Computers (Risp National Laboratory, Denmark, Ris0-M-274O). 

Singh, J.J., Clair, T.L. (1984) Moisture dependence of positron annihilation spectra in Nylon-6 .JVMc/. Instr. and Meth. in Phys. Res. 221,427. 

Kirkegaard, P. Pedersen, N.J. Eldrup, M., PATFIT-88 A data- processing system for positron annihilation spectra on mainframe and personal computers., 1989, Riso National Laboratory, Roskilde. 

Kirkegaard, R, Pederson, N. J., and Eldrup, M., PATFTT-88 A Data-Processing System for Positron Annihilation Spectra on Mainfram and Personal Computers, Tech. Rep. Ris0 -M-2740, Ris0 National Laboratory, Roskilde, Denmark, 1989. 

P. Asoka-Kumar, M. Alatalo, V.J. Gosh, A.C. Kruseman, B. Nielsen and K.G Lynn, Increased elemental specificity of positron annihilation spectra , Phys. i ev. Leff.,1996,77,2097. 

When an electric field was applied across the chamber some positrons annihilated prematurely, following field-induced drift to one of the electrodes. In this case the free-positron component of the lifetime spectrum was field dependent the maximum drift time, rmd, was given by the end-point of the lifetime spectrum and was due to thermalized positrons which had traversed the entire drift length l. The drift speed was then v+ = 1/rmd and the mobility could be found from... 

Recent progress in the study of positron scattering and positron annihilation processes is reviewed in Refs. [11,170-172]. Experimentally, the positron sources from radioisotopes and from electron accelerators are quite weak and have a broad spectrum of energies as compared with electron sources, making precise measurements of positron collision processes extremely difficult. Such measurements, however, have become possible recently due to the... 

A discussion of the coincidence technique with some general applications has been published by Wahlgren, Wing and Hines 71>. Many of the early applications of the technique made use of the fact that 64Cu is one of the few radionuclides produced by thermal neutron irradiation for which the 0.511 MeV positron annihilation photopeak is a prominent feature of the spectrum. Copper has been determined in meteorites 72> and copper ores 73,74) ]-,y coincidence counting of 04Cu annihilation radiation. The rapid and selective nature of the determination may have important applications in the on-line sorting of copper ores. 

Fig. 7. Energy spectrum of 662 keV photons detected in Csl at 77 K from the 137Cs (3 emitter (left) showing the photo peak and the Compton plateau. The low energy peak is due to photons back-scattered from the container. A similar spectrum is obtained for 1275 keV photons from the 22Na 0+ emitter (middle). In this case one also observes the 511 keV line from positron annihilation and its corresponding Compton plateau. The resolution is better than 6 % at 511 keV. The right spectrum shows the response of the photodiode to 22 and 88 keV X-rays from 109Cd. A Csl light yield of 26,000 photons/MeV at 511 keV is derived from this spectrum, assuming about 6000 electron-hole pairs for 22 keV X-rays. This is however a lower limit, as it assumes 100 % quantum efficiency for the photodiode...

The analysis of the positron annihilation lifetime spectra is a very important aspect of using the PAL techniques to analyze polymers. Without proper data analysis interpretation of data might be misleading and important scientific information will be lost. In PAL studies of polymers the PAL spectrum can be analyzed in two ways (1) a finite lifetime analysis or (2) continuous lifetime analysis. In the finite lifetime analysis the PAL spectra is resolved into a finite number of negative exponentials decays. The experimental data y(t) is expressed as a convoluted expression (by a symbol ) of the instalment resolution function R(t) and a finite number (n) of negative exponentials ... 

Figure 13.11 Positron annihilation lifetime spectra for porous MSQ films generated by mixing MSQ film with different triblock copolymers PI 03, F88, P105, and F38. A spectrum for non-porous MSQ is listed for comparison [28].

Regarding the intensity, the higher value corresponds to the intermediate component, ti, which represents approximately the 90% of the total intensity. This agrees with the results obtained in previous studies carried out with porous carbons [12] and carbon fibers from mesophase pitch [11]. In the first study [12] only the intermediate component (ti) was found from the lifetime spectrum. These results indicate that, in carbon materials with high surface area, most of the positron annihilation takes place on the surface of the porosity. In the second case [11], i.e., PALS in carbon fibers, two components in the lifetime spectrum were found. The first component with high intensity (97%) and lifetime of 367 ps was attributed to positron annihilation in pores. The second one with a lifetime of 1130 ps corresponds to the annihilation of positronium atoms (i e., o-Ps). 

The experimental set-up consists of a positron source ( Na), a scintillation counter, to detect the y radiation from the positronium decay, and electronic peripheral equipment to analyse the time spectrum of the positron annihilation. 

If the source is a positron emitter, a peak at 0.511 MeV is always present. The positron-emitting isotope Na is such an example. It emits only one gamma with energy 1.274 MeV, yet its spectrum shows two peaks. The second peak is produced by 0.511-MeV annihilation photons emitted after a positron annihilates (Fig. 12.8). 

Interaction by pair production results in a spectrum that includes escape peaks at the full energy minus 511 keV and minus 1022 keV, when either one or both of the positron annihilation photons do not interact with the detector. Compton scattering of these photons adds to the continuum. 

FIGURE 10.3 Positron lifetime annihilation spectrum of polypropylene. 

Typically, therefore, a PALS spectrum consists of a minimum of three components the short-lived p-Ps component with intensity 7i and lifetime ti = 125 ps a free positron annihilation component, with intensity I2 and lifetime T2 and the o-Ps component, with intensity I3 and lifetime T3. Theory predicts the ratio /3//1 = 3, but as discussed in Chapter 11, certain effects may lead to a decrease in this ratio. The theoretical basis for relating the o-Ps lifetime to free volume is based on a model proposed by Tao [1972], in which < -Ps is assumed to be trapped in a potential well of... 

Figure 1. An example of the positron lifetime spectrum in polymers a polycarbonate. The abscissa is the channel number of the pulse height analyzer and is proportional to time, and the ordinate is the logarithm of the count at each channel. The spectrum is composed of three exponential decay component as shown by the dotted lines, and the longest lived one is the o-Ps annihilating in vacancies.

Positron annihilation lifetime spectroscopy (PALS) studies the lifetime spectrum of ortho-positrons after being injected into the sample [3,4]. This lifetime depends on the probability of the ortho-positronium (o-Ps) particle (a hydrogen-like bound state formed by a positron-electron pair) to be quenched and annihilate. This probabihty is higher in condensed matter than in vacuum. Of all the probe methods PALS is nowadays probably the most versatile one and the most widely used. The o-Ps particle is the smallest probe available and can thus detect the smallest free volume elements furthermore, the method furnishes information on the average free volume size and on the FV size distribution. 

A positron lifetime spectrum, as shown in Fig. 9, can usually be dissolved into two components. As indicated schematically in Fig. 10, the short-lived component with an associated lifetime and intensity can be attributed to the annihilation of the free positron, the annlhy.atlon of the products formed in the reaction of "hot" ortho Ps and the self annihilation of para-Ps, whereas the long-lived component displaying a lifetime Tj and intensity I2 is attributed to the annihilation of thermalized o-Ps. 

A spectrum is built up from at least a million lifetime measurements from individual positrons. Standard computer codes are available for the decomposition of the spectra including POSITRONFIT in which a least squares fit is used to fit a model spectrum with a given number of decay components to the observed spectrum. Maximum entropy techniques have also been used to determine the most probable underlying distribution of trap lifetimes. As the number of different traps increases, interpretation becomes increasingly difficult. Several research groups have published positron annihilation lifetime data on irradiated RPV steels (see Section 9.11.1). 

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