# Monte Carlo Study of Imaging Plate Response to Laser-Driven Aluminum Ion Beams
## 1 Introduction  
An imaging plate (IP) is a film-like image sensor that records the incident radiation flux on a thin sheet called a phosphor layer. It is known to be sensitive to energetic charged particles, X-rays, and gamma rays [1; 2; 3; 4]. Since IPs were developed by Fuji film Co. in the early 1980s, they have been widely used in nuclear science and in medicine. While an IP is a passive detector and cannot be used in high repetition rate experiments, IPs have several advantages over other particle detectors: (1) immunity to electromagnetic pulse (EMP), (2) high dynamic range (4-5 orders of magnitude), (3) high spatial resolution (resolving to as low as 10 \(\upmu\)m), and (4) reusability (signals of IPs can be erased with white light) [5; 6; 7; 1]. For radiation detection, Biological Analysis System (BAS) IP types are commonly used [8]. Specifically, BAS-MS, SR, and TR IPs were primarily designed for high sensitivity, high resolution, and detection of beta particles from tritium, respectively [9; 10]. BAS IPs typically consist of three or four layers with various thicknesses: a protective layer, a phosphor layer, a support layer, and a magnetic layer. The BAS-TR IP has no protective layer [11], and the lack of a protective layer makes BAS-TR IP particularly well suited to measure heavy ions with short ranges within matter. Incoming ions deposit kinetic energy in the 50 \(\upmu\)m thick phosphor layer [11]. Beneath the 250 \(\upmu\)m thick support layer, there is a 160 \(\upmu\)m thick magnetic layer which allows for magnetic attachment of the IP to the scanner [12]. When an IP is exposed to radiation, the electrons of Eu\({}^{2+}\) in the phosphor layer are ionized and trapped in FBr or FI sites forming metastable states. The lifetimes of the metastable states range from 10 min to several days. When a scanner irradiates the phosphor layer with 2 eV photons from a laser diode, the electrons in the metastable stateare re-excited and recombine with Eu\({}^{3+}\) and emit 3 eV photons. That emission is known as the photostimulated luminescence (PSL). To use an IP as a quantitative radiation detector, it is necessary to calibrate the PSL relative to the spectral intensity of radiation. For ions, the spectral response function of an IP is measured in units of PSL per incident ion of a given energy. This must be done for each type of radiation and IP and scanner combination, because each combination has in general different responses [5; 7; 13; 14; 15; 16]. Researchers have measured the response of IPs to various radiation types such as electrons [5; 7; 13; 17; 18; 19; 20; 21], protons [12; 14; 22; 23; 24], and X-ray [16; 25] for a wide energy range. The measurement of IP responses to energetic heavy ion beams, however, is more challenging because it is difficult to produce heavy ion beams with sufficient particle fluence for IP calibration at a range of energies in a reasonably short time. For this reason, the published studies of IP response to ions are limited to a few types of heavier elements such as deuterium [23; 26], helium [16; 23], carbon [6], and titanium [27]. Since [PERSON] et al. [28] proposed a linear model predicting the response of IP, some researchers have calculated the IP response by calculating stopping powers using Monte Carlo simulation codes. [PERSON] et al. designed an exponential model and calibrated the IP response to protons, electrons, photons, and \({}^{4}\)He ions, using Geant4 [14; 16; 29]. Recently, [PERSON] et al. reported on the responses of BAS-MS, SR, and TR for 1-200 MeV protons [12] and for 40-180 MeV electrons [21] using Geant4. [PERSON] et al. used FLUKA to calibrate the responses of BAS-MS and SR to 150 keV-1.75 MeV electrons [5]. In this paper, we report on the IP response to aluminum (Al) ions for the first time, with an incident kinetic energy as high as 222 MeV. We have used an Al ion beam driven by an intense laser pulse as the ion source, and detected these ions using BAS-TR IPs for the measurements.