Effect of pulse irradiation on the evolution of damage structure

https://doi.org/10.1016/j.nimb.2020.06.002Get rights and content

Abstract

Charged particle beams from accelerators driven by radiofrequency or with beam scanners possess time structures (i.e., pulse beam operations). However, it is not clear whether the irradiation effects of these pulse beam operations are the same as those of continuous beams, although the average damage rate is the same in both cases. In this study, the difference between continuous beams and pulse beams, and the effect of the repetition frequency and pulse duration on the defect accumulation are evaluated by the rate equation analysis for Al and Fe. The growth rate of interstitial type dislocation loops in Al is simulated with fixed duty ratios (constant pulse duration over the irradiation period) at 453 K. The effect of pulse duration on a constant period of 1 s is simulated in Fe, changing the pulse duration from 1 to 10−6 s, while the damage (dpa) caused by one pulse remains the same. The results are analyzed and the effects of pulse duration and repetition rate on the damage structure evolution are demonstrated in relation to materials parameters.

Introduction

Irradiation experiments are essential for the development of nuclear materials. High energy charged particles such as ions and electrons, which are produced in accelerators are frequently employed for these experiments. This is because the irradiation conditions and damage rates in the case of ions and electrons can be controlled more easily as compared to those of neutron irradiation, which employs nuclear reactors. Ion/electron beams from accelerators driven by radiofrequency (RF) or accelerators with beam scanners possess time structures (periodic/discontinuous intensities). An example of a pulse train is shown in Fig. 1. The pulse frequency is 1/T and H is the beam intensity, but we consider H to be the damage rate (dpa/s) in a pulse duration τ. It is not clear if the irradiation effects of these pulse beam operations are the same as those of continuous beams, although the average damage rate is the same in the two cases. A pulse (macro pulse) consists of many bunches (micro pulses) in the case of RF-driven accelerators, but the effect of these bunches is not considered in our study.

The concentration and migration efficiency of point defects at the initial stage of irradiation are shown schematically in Fig. 2 [1], [2], [3] for intermediate sink density [4]. The migration efficiency is the product of the concentration, Ck, and mobility, Mk, of point defects, k, and it plays an important role in the nucleation and growth of point defect clusters [1]. For example, the growth rate of interstitial type dislocation loops is expressed by ZSI_IMICI -ZSI_VMVCV, where Z, MICI and MVCV are the number of reaction sites, the migration efficiencies of interstitials and vacancies, respectively. At the initial stage of irradiation, the concentrations of interstitials and vacancies are the same, but the migration efficiency of interstitials exceeds that of the vacancies by time L2 in Fig. 2, because of the high mobility of the former. Hence, if the pulse duration L is shorter than time L2, the pulse train will fulfill the conditions for the interstitial migration efficiency to be dominant. The interstitial dominant conditions during irradiation were demonstrated experimentally by Arai et al. [5]. They utilized the scanning ability of high voltage electron microscopy for the generation and observation of irradiation induced defect clusters. They employed two methods: the flip-flop method in which the electron beam reverts to the observation area of the transmission electron microscope intermittently, and the cycle beam method, where the electron beam traces circular and intermittent paths around the observation area.

The point defect process is, however, far more complex, and the accumulation of vacancies and the thermal decomposition of cluster change the defect structure evolution with increasing irradiation. In this paper, the difference between continuous and pulse beams, and the effect of repetition frequency and pulse duration on the defect accumulation are presented. The rate equation analysis is used for evaluating the time structure effect.

Section snippets

Method

The model used for the calculations is almost the same as that used in previous studies [6], [7], [8], [9], and is based on the rate theory. It describes the reaction rates among point defects and their defect clusters. The main difference between the present model and the previous ones is that there is no introduction of solute atoms, and only pure metal cases, Al and Fe, are treated.

The following assumptions are made in the calculations:

  • (1)

    Mobile defects are interstitials, di-interstitials,

Constant τ/T

The growth rate of interstitial type dislocation loops defined as ZSI_IMICI + ZSI_I2MI2CI2 − ZSI_VMVCV − ZSI_V2MV2CV2 in Al and Fe was simulated with fixed duty ratios (pulse duration over irradiation period: τ/T in Fig. 1.) Fig. 3 illustrates the case of Al irradiated at 453 K with a damage rate in pulse duration, H, of 0.1dpa/s and τ/T = 1/6. These conditions were the same as those in the electron irradiation experiment conducted by Arai et al. [5]. Increasing the pulse frequency resulted in

Concluding remarks

For comparison of the damage structure evolution by neutron, ion and electron irradiation, past studies typically focused on the damage rate [12] and the deposited energy by one particle [13]. Our simulations demonstrated the importance of pulse duration and repetition rate in the damage structure evolution, which are strongly related to the material parameters. The effect of pulse irradiation should be analyzed, taking into consideration the materials parameters and the irradiation conditions.

References (13)

  • T. Yoshiie et al.

    Nucl. Inst. Meth. Phys. Res. B

    (2015)
  • T. Yoshiie et al.

    J. Nucl. Mater.

    (2008)
  • T. Yoshiie et al.

    Nucl. Inst. Meth. Phys. Res., B

    (2013)
  • T. Yoshiie et al.

    Nucl. Inst. Meth. Phys. Res. B

    (2017)
  • P.J. Barton et al.

    J. Nucl. Mater.

    (1977)
  • S. Ishino et al.

    J. Nucl. Mater.

    (1990)
There are more references available in the full text version of this article.

Cited by (0)

View full text