Production of Positrons via Pair Creation from LCS Gamma-rays and Application to Defect Study in Bulk Materials

Gamma-rays and Application to Defect Study in Bulk Materials Fuminobu Hori1,∗, Kouji Ishii1, Taishi Ishiyama1, Akihiro Iwase1, Shuji Miyamoto2, and Mititaka Terasawa2 1 Department of Materials Science, Osaka Prefecture University, 1–1, Gakuen-cho, Naka-ku, Sakai, Osaka 599–8531, Japan 2 Laboratory of Advanced Science and Technology for Industry, University of Hyogo, 3–2–1 Kouto, Kamigori, Ako-gun, Hyogo 678–1205, Japan


Introduction
Positron annihilation techniques are very powerful methods for nondestructive tests of various materials such as fatigue damage, high energy particles irradiation damage and impurity clusters in steels and semiconductors and so on [1][2][3][4][5].In recent years, many positron beam facilities, such as positron micro-beam, polarized positron beam, positronium time of flight spectroscope and positron microscope, have been constructed and developed [6][7][8][9][10][11].Typically, the maximum positron energy is a few tens of keV at these positrons beamlines.Such slow positrons are annihilated at the near surface, with a maximum range of a few microns, in solid materials.In contrast, highly energetic positrons with an energy of MeV order can penetrate several millimeters in bulk materials, such as metals and semiconductors.Therefore, radioactive isotopes such as 22 Na (E max = 0.54 MeV) are commonly used for positron annihilation techniques as a positron source and, 72 Se (E max = 3.3 MeV) and 68 Ge (E max = 1.9 MeV) have been mainly used for defects research in bulk materials.For instance, the implantation depth of 0.54 MeV positrons, which is the maximum energy of positrons emitted from 22 Na, is calculated to be around 100 μm in bcc iron.Moreover, the energy profiles of these positrons from radioactive sources have wide distributions.Also, the positron emission intensity is not constant but decays gradually.Therefore, an energetic positron beam is the best way to get defect information in bulk samples.
In almost all positron beam apparatus, either a radioactive isotope with beta-decay or pair production using linear accelerators and nuclear reactors are used as the positron source.However, in those methods, positrons are first moderated then reaccelerated.Theoretically, electron-positron pair creation is the simplest method to obtain energy controlled positrons.Recently, some groups have successfully generated highly energetic positron-electron pairs by pair creation from inverse Comp-  ton scattering gamma rays at some facilities [12][13][14].In the pair creation scheme, the positron energy spectrum, which is widely spread, depends on the gamma ray (photon) energy.By using an analyzing magnet system, we can obtain nearly-monochromatic positron energy at the position of the target material.We have developed a simple pair creation positron beam apparatus with energy of MeV order by laser Compton scattering (LCS) gamma ray from synchrotron radiation electron and incident laser light.In this system, no moderation and reacceleration of positron is needed, and we can obtain positrons with an energy of the order of keV to MeV directly.Also, the positron energy distribution and positron production rate can be controlled by the wavelength and power of the incident laser.Here, we demonstrate an application of this system to the study of defects associated with fatigue damaged thick material.In this paper, the results of positron annihilation Doppler broadening for cyclic fatigue stress applied iron samples without destruction by using this high energy LCS-positron apparatus are presented.

Experimental setup 2.1 Generation of positron
The basic idea of the positron generation and positron annihilation system is shown in Fig. 1. Figure 2 shows a schematic illustration of the LCS gamma ray generation and positron annihilation apparatus at the beam-line (BL01A) of the electron storage ring at the NewSUBARU SR facility at LASTI, University of Hyogo.LCS gamma rays an energy of 16.7 MeV have been generated from laser light with a wavelength of 1064 nm from a Nd: YVO 4 laser, which has a power output of 4.5 W, and 1 GeV (200 mA) electron beam circulating in the storage ring [15].The energy of the LCS gamma rays E γ was calculated by the scattering energy conservation using the following equation [16,17]: (1) where g is a relativestic coefficient, c is the velocity of the light, E L is the energy of laser photon, E k is the kinetic energy of the electron, m e is the rest mass of the electron and θ is the scattering angle of the scattered photon against the direction of the electron.The generation rate of LCS gamma rays is about 10 5 MeV −1 • s −1 [18].As shown in Fig. 3, the LCS gamma ray goes through a collimator of 6 or 3 mm in diameter and irradiates a 3 mm thick Pb plate mounted on the edge of an electromagnet.The positron production rate of this pair creation system was already measured as 1 positron per 23 photons, and the estimated positron flux is about 3600 s −1 [18].The generated positron and electron pairs can be bent to opposite directions according to the magnetic field and are then separated, while the penetrated gamma ray photons are not influenced by the magnetic field.An imaging plate (IP) is set at the sample position to record the positron electron pair and incident gamma ray before positron annihilation measurement.Figure 4 shows an image of the generated positron, electron and gamma ray distribution on the IP.Applying a magnetic field of B = 0.2 T causes a separation of around 12 cm between the positron and electron positrons behind the Pb target.From the bending distance from the central point on the image, we can calculate the positron energy around the peak position to be about Fig. 4. Generated positron and electron profile on imaging plate after separation pass through magnetic field.
8 MeV.The bulk sample is placed where the peak position of positron profile was measured by the IP.Each sample was mounted on a x-z-rotate movable stage.Annihilation gamma rays were emitted from the sample and were detected by a high purity Ge detector.To reduce the background, the detector was covered by 50 mm thick lead blocks and the annihilation gamma rays were restricted by a 30 mm diameter aperture.As can be seen in Fig. 5, the 511 keV gamma signal, which is the signal of positron annihilation, was measured when the laser photons were incident into the storage ring.
In contrast, an extremely low background and no peak at 511 keV was observed when the sample was removed.This shows that generated positrons were successfully implanted and annihilated in the bulk sample.The rate of positron annihilation events in the iron sample was estimated to be of the order of 1000 s −1 from the counting rate of positron annihilation gamma rays detected by the Ge detector located 10 cm from the sample.This result is in good agreement with the report of positron production measured at the same facility by Horikawa et al. [18] showing that positron annihilation takes place in the sample.

Sample preparation
Pure iron samples were prepared for fatigue stress test with a thickness of 2 mm and a width of 10 mm as shown in Fig. 6.In order to anneal out the effective lattice defects and dislocations for positron methods, the samples were annealed at 1073 K for 30 min in a vacuum of 1 × 10 −4 Pa.Fatigue tests were performed at room temperature with a SHIMADZU-EHF-F1 fatigue machine.The conditions of the fatigue test are described elsewhere [19].The expected fatigue life for these samples corresponds to the number of stress cycles (100 %Nf = 2.3 × 10 5 ) at a stress amplitude of 450 N • mm −2 .This value was estimated by repeating the fatigue test until fracture with several samples and calculating the average result.

Doppler broadening measurement
Doppler broadening measurement using a Ge detector was performed for on unstressed, Nf = 10 % and 100 % (fatigue fractured) fatigue damaged iron specimens by using LCS-positron apparatus as mentioned above.The stopping profile of positrons with energy E is given by with the mean penetration depth given by

Fig. 1 .
Fig. 1.Schematic illustration of LCS gamma ray generation and positron electron pair creation system.

Fig. 2 .
Fig. 2. Schematic illustration of experimental set up for positron annihilation Doppler broadening measurement and high energetic positron generation via pair creation by using storage ring.

Fig. 3 .
Fig. 3. Schematic illustration of LCS gamma ray generation and positron electron pair creation system.

Fig. 5 .
Fig. 5. Positron annihilation gamma energy spectrum emitted from the iron sample (solid line) placed at the peak position of positron intensity on IP and removed sample (broken line) when the laser beam is active.

Fig. 7 .
Fig. 7. Positron annihilation Doppler broadening spectra of before and after fatigue cyclic stress applied iron samples.