Micro-collimators fabricated by chemical etching of thin polyallyldiglycol carbonate polymer films exposed to oxygen ions

Abstract

One approach for α particle radiobiological experiments is to use solid-state nuclear track detectors (SSNTDs) as support substrates to record the α particle hit positions on the targets. To facilitate accurate characterization of the hit positions as well as the incident α particle energies, micro-collimators are required in these experiments to restrict the incident α particles to those with small deviations from normal incidence with respect to the collimator. In the present paper, we fabricated micro-collimators, which restricted α particles to those with deviations as low as 12°. Specially etched polyallyldiglycol carbonate (PADC) films, which are a kind of SSNTD, with a thickness 70 μm were prepared from commercially available PADC films. These were then irradiated by 4.83 MeV/n oxygen ions generated from the heavy-ion medical accelerator in Chiba (HIMAC), Japan. The irradiated films were chemically etched for at least 2.7 h to achieve etched-through air channels to form the micro-collimators. The micro-collimators formed by etching the irradiated films for 2.7, 3.0 and 4.0 h were experimentally shown to be able to restrict α particles to those very close to normal incidence. In contrast, the micro-collimator formed by etching the irradiated film for 4.5 h started to allow α particles with larger deviations from normal incidence to pass through, which is likely due to overlapping of air channels from excessive etching.

Introduction

Studies on biological effects of α particles are important because (1) α particle irradiation is ubiquitous in our natural environment, which arise from our inhalation of radon progeny [1], [2], [3], [4], [5] and (2) α particles have large linear energy transfer (LET) values and are efficient in causing DNA double strand breaks (DSBs) [6], [7], [8], [9]. In α particle radiobiological experiments where the targets (such as cultured cells or other organisms) are irradiated with α particles, it is necessary to determine the dose absorbed by the targets, which requires the accurate positions and incident energies of α particle hits on the targets.

There are different approaches to determine the number and energy of α particles actually incident on the targets. The first one is to make use of versatile microbeam facilities in which the hit positions and the energies of the incident ions can be precisely controlled but will involve sophisticated equipment and instrumentation [10], [11], [12], [13], [14], [15]. Columbia’s group also developed a stand-alone microbeam without a conventional accelerator as a source of energetic ions, but focused α particles from an α emitter using a compound magnetic lens consisting of 24 permanent magnets arranged in two quadrupole triplets [16].

Another approach is to make use of a radioactive α particle source such as ²⁴¹Am without focusing. However, due to the statistical nature of radioactivity and the random direction of α particle emission, both the hit positions and the energies of the α particles cannot be controlled. For this method, researchers have made use of solid-state nuclear track detectors (SSNTDs) as support substrates of the targets, which can record the hit positions of α particles and reveal these on subsequent chemical etching, to help determine the hit positions retrospectively (see Refs. [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]). A review on SSNTDs was given by Nikezic and Yu [29]. The targets are irradiated with α particles, which pass through the SSNTD substrate to strike the targets in contact with the substrate. However, the incident angle of the α particles on the substrate will vary due to the random direction of α particle emission. Excessively oblique incidence will make accurate determination of hit positions difficult and at the same time will necessitate tedious procedures to determine the resultant energy of the α particles incident onto the target. To tackle this problem, Choi et al. [30] recently proposed the use of a micro-collimator to screen out those α particles insufficiently close to normal incidence with respect to the surface of the support substrate. The micro-collimator was an etched polyallyldiglycol carbonate (PADC) film, which was an SSNTD and commercially sometimes available as the CR-39 detector. PADC films have become popular support substrates for cell culture with track registration capability in that they are transparent, biocompatible [31] and are not dissolved in the alcohol used for sterilizing the substrate.

The micro-collimator fabricated by Choi et al. [30] was 15 μm thick; so the energy loss of α particles while traveling through the air columns in the collimator was minimized. By approximating an air channel as a frustum of a cone, the semi-cone angle was found to be ∼23°. As commented by Choi et al. [30], it is desirable to fabricate mirco-collimators with more cylindrical bores, i.e., those with smaller semi-cone angles, so that the α particles passing through the collimators will be closer to normal incidence. With the help of surfactants (5% DOWFAX 2A1) in the etchant, Choi et al. [30] succeeded to fabricate mirco-collimators with semi-cone angles of ∼20°. In the present paper, we proposed an alternative method to fabricate micro-collimators to restrict passing α particles to those even closer to normal incidence with respect to the substrate surface by making use of the high-energy heavy ions generated from the heavy-ion medical accelerator in Chiba (HIMAC). The fabricated micro-collimators were characterized and the results were discussed.