2 Experimental methods

The neutron irradiation experiment was carried out using the petawatt laser for fast ignition experiments (LFEX) laser system at the Institute of Laser Engineering (ILE) at Osaka University^[ Reference Kawanaka, Miyanaga, Azechi, Kanabe, Jitsuno, Kondo, Fujimoto, Morio, Matsuo, Kawakami, Mizoguchi, Tauchi, Yano, Kudo and Ogura ^{38 ]}. To generate and accelerate ions we focused four laser pulses provided from LFEX on a target with an intensity of approximately 1.5 $\times$ 10 ${}^{19}$ W/cm ${}^2$ and with a duration of 1.5 ps in full width at half maximum (FWHM). Figure 2 shows the experimental setup of the target installed in the vacuum chamber. The angle between the laser axis and the normal vector of the target foil was 42 ${}^{\circ }$ . As the target, a deuterated carbon [(C ${}_8$ D ${}_8$ ) ${}_n$ , henceforth referred to as CD] foil with a thickness of 5 μm was used. A plasma was induced by the laser and, subsequently, ions of protons and deuterons were accelerated from the plasma via TNSA^[ Reference Fuchs, Antici, d’Humières, Lefebvre, Borghesi, Brambrink, Cecchetti, Kaluza, Malka, Manclossi, Meyroneinc, Mora, Schreiber, Toncian, Pépin and Audebert ^{39 –} Reference Yogo, Mima, Iwata, Tosaki, Morace, Arikawa, Fujioka, Johzaki, Sentoku, Nishimura, Sagisaka, Matsuo, Kamitsukasa, Kojima, Nagatomo, Nakai, Shiraga, Murakami, Tokita, Kawanaka, Miyanaga, Yamanoi, Norimatsu, Sakagami, Bulanov, Kondo and Azechi ^{41 ]}. In the TNSA, a target foil is irradiated by a laser with an intensity higher than or equal to 10 ${}^{18}$ W/cm ${}^2$ to make a plasma. The generated plasma is extended into a vacuum for the opposite side (rear side) to the laser irradiation side to generate a charge separation field, by which ions on the rear surface of the target could be accelerated up to several tens of MeV. The accelerated protons and deuterons have continuous energies, of which the spectral density decreases with increasing energies. The maximum energies of the protons and deuterons using LFEX are typically 30 and 20 MeV, respectively^[ Reference Golovin, Mirfayzi, Shokita, Abe, Lan, Arikawa, Morace, Pikuz and Yogo ^{42 ]}. A Be block with a size of $\phi$ 10 mm $\times$ 5 mm was installed 4 mm downstream from the CD foil. The accelerated ions generated neutrons by p+ ${}^9$ Be and d+ ${}^9$ Be reactions^[ Reference Kleinschmid, Bagnoud, Deppert, Favalli, Frydrych, Hornung, Jahn, Schaumann, Tebartz, Wagner, Wurden, Zielbauer and Roth ^{12 –} Reference Mori, Yogo, Hayakawa, Mirfayzi, Lan, Abe, Arikawa, Golovin, Wei, Honoki, Nakai, Mima, Nishimura, Fujioka and Kodama ^{14 ]}. High-energy neutrons are predominantly generated by the ${}^9$ Be(p, n) ${}^9$ B reaction with high-energy protons. Around the Be block, a semi-cylindrical polyethylene moderator with a volume of 157 cm ${}^3$ in a stainless-steel case was located. The thicknesses of the top and side were 5 mm and the thickness of the bottom was 2 mm (see Figure 2). To produce ${}^{67}$ Cu, a natural Zn target with a size of $\phi$ 10 mm $\times$ 5 mm and a weight of 2.86 g was placed behind the Be target. The neutron energy spectrum was measured by the time of flight (TOF) method. A benzophenon-doped BBQ liquid scintillation detector with a size of $\phi$ 60 mm $\times$ 60 mm with a photomultiplier tube (PMT; Thron EMI, 9902KBT)^[ Reference Abe, Hosoda, Arikawa, Nagai, Kojima, Sakata, Inoue, Iwasa, Iwano, Yamanoi, Fujioka, Nakai, Sarukura, Shiraga, Norimatsu and Azechi ^{43 ]} was located 8.3 m downstream from the center of the target chamber, and the angle between the laser axis and the TOF beam line was approximately 15 ${}^{\circ }$ , corresponding to 57 ${}^{\circ }$ away from the normal vector of the Be target. The output signal from the PMT was recorded by an oscilloscope shot by shot and the neutron energy spectrum for each laser shot was obtained from each signal^[ Reference Mirfayzi, Kar, Ahmed, Krygier, Green, Alejo, Clarke, Freeman, Fuchs, Jung, Kleinschmidt, Morrison, Najmudin, Nakamura, Norreys, Oliver, Roth, Vassura, Zepf and Borghesi ^{44 ]}.

Figure 2 Experimental setup for the laser shot to generate neutrons. The laser is focused on the CD foil target. The Be neutron converter is placed 4 mm downstream of the CD foil. Behind the Be target, the Zn target was set in the hole at the center of the front surface.

Unstable isotopes and isomers were produced via nuclear reactions of the neutrons and stable isotopes of Zn. Eleven minutes after the laser shot, the Zn target was moved from the target chamber to obtain the $\gamma$ -ray signals. The $\gamma$ -rays emitted from the produced nuclides were measured with an HPGe detector. The efficiency of the HPGe detector relative to a $\phi$ 3 inch $\times$ 3 inch NaI(Tl) scintillation detector was approximately 70%. A multichannel analyzer (TechnoAP, APG7400A) was used to acquire the pulse height of the amplifier, and the dead time was less than 0.07%. Copper plates with a thickness of 5 mm and lead blocks with a thickness of 10 cm were used for background radiation shielding. Standard $\gamma$ -ray sources of ${}^{152}$ Eu and ${}^{133}$ Ba were used for calibration of the detection efficiency and energy of the HPGe detector. The $\gamma$ -ray measurements of the laser-neutron irradiated Zn sample and background with a non-irradiated Zn target were conducted for 120 and 99 h, respectively, with the same background shield.

Table 1 Produced nuclides and their half-lives, $\gamma$ -ray energies, emission probabilities of the $\gamma$ -rays, nuclear reactions, numbers and activities.

3 Experimental results

Figure 3 shows the energy spectrum of the generated neutrons, which was measured using the TOF method with the scintillation detector located 57 ${}^{\circ }$ from the normal vector of the Be block. The maximum energy is approximately 17 MeV, and the number of neutrons decreases with increasing energy. The neutron yield was 8 $\times$ 10 ${}^9$ neutrons/sr in the energy range from 1 MeV to the maximum energy. The maximum energy is high enough to induce the ${}^{67}$ Zn(n, p) ${}^{67}$ Cu and ${}^{68}$ Zn(n, np) ${}^{67}$ Cu reactions. Note that the total number and the maximum energy are expected to be lower than those of the neutrons emitted forward, respectively.

Figure 3 Fast neutron spectrum obtained from the TOF measurement. The neutron energies reached 17 MeV.

The produced radioisotopes and isomers were identified using the analysis of $\gamma$ -rays radiated following $\beta$ decay or internal decay of the isomers. For some nuclear species, the decay curves of $\gamma$ -rays were also used for identification. The number of a produced nuclide was evaluated from the peak area in the energy spectrum. Figures 4(a)–4(c) show the $\gamma$ -ray spectra from the neutron-irradiated Zn sample, the background and their subtraction in the three different energy ranges. In these spectra, the $\gamma$ -rays of ${}^{63}$ Zn, ${}^{65}$ Zn, ${}^{69}$ Zn^m, ${}^{64}$ Cu, ${}^{66}$ Cu and ${}^{67}$ Cu are observed. Because the peaks of the 93 and 185-keV $\gamma$ -rays radiated from ${}^{67}$ Cu in the energy spectrum were overlapped with the $\gamma$ -rays from ${}^{234}$ Th and ${}^{235}$ U in the background, the background spectrum was subtracted from the Zn sample spectrum. Figures 4(d)–4(f) show the $\gamma$ -ray spectra measured for 8.1 h, 5.1 h and 8 min, respectively. In Figures 4(d) and 4(e), $\gamma$ -rays radiated from ${}^{71}$ Zn ^m (T ${}_{1/2}$ = 4.0 h) and ${}^{65}$ Ni (T ${}_{1/2}$ = 2.5 h) are observed, respectively. In contrast, when the measuring time is as short as 8 min, the $\gamma$ -ray peak of ${}^{68}$ Cu ^m with a half-life of 3.8 min appears [see Figure 4(f)]. From the energy spectra and decay curves, ${}^{63}$ Zn, ${}^{65}$ Zn, ${}^{69}$ Zn ^m , ${}^{71}$ Zn ^m , ${}^{64}$ Cu, ${}^{66}$ Cu, ${}^{67}$ Cu, ${}^{68}$ Cu ^m and ${}^{65}$ Ni were identified. Table 1 shows the properties and measured activities of the radioisotopes. Note that the only upper limit of the radioactivity of ${}^{71}$ Zn was obtained by taking the square root of background around 910.27 keV. Although the 511.56-keV $\gamma$ -ray with a highest emission probability of 32% is radiated from ${}^{71}$ Zn, it is difficult to use it for the estimation of radioactivity because its energy is nearly equal to 511 keV, which is one of the strongest background $\gamma$ -rays.

Figure 4 $\gamma$ -ray spectra measured for 120 h, 8.1 h, 5.1 h and 8 min. (a)–(c) The $\gamma$ -ray spectra integrated for 120 h. The background signal measured for 99 h was normalized to the target measurement of 120 h. (d) The $\gamma$ -ray spectrum measured for 8.1 h, where peaks corresponding to ${}^{71}$ Zn ^m are observed. (e) The $\gamma$ -ray spectrum for 5.1 h, where peaks for ${}^{65}$ Ni are observed. (f) The $\gamma$ -ray spectrum for 8 min, which shows the ${}^{68}$ Cu ^m peak at 526 keV.

5 Summary

In this study, we have demonstrated production of a medical radioisotope, ${}^{67}$ Cu, using LFEX at Osaka University. Neutrons with energies of up to 17 MeV have been generated using $\mathrm{p}$ + ${}^9$ Be and $\mathrm{d}$ + ${}^9$ Be reactions on the secondary target located behind the ion acceleration target. The neutron energy spectrum at 57 ${}^{\circ }$ is obtained from the TOF method. The radioactivities of the generated unstable isotopes and isomers were evaluated from $\gamma$ -ray energy spectra measured with an HPGe detector. The ${}^{67}$ Cu radioactivity of 1.0  $\pm$  0.2 Bq was obtained, which is about 50 times higher than the value calculated using the PHITS simulation code. This may originate from the fact that the neutron energy spectrum was measured at 57 ${}^{\circ }$ and, thus, the maximum energy and total number are lower than those of the neutrons irradiating on the Zn target. We estimated radioactivity of ${}^{67}$ Cu using a future laser system and the PHITS code, obtaining the result that it is possible to produce 270 MBq of ${}^{67}$ Cu using a high-frequency laser for 10,000 s irradiation. This suggests that the medical radioisotope may be produced in a compact laser system inside of a hospital in the future.