Qualitative Analysis of Long-Lived Residual Radioisotopes in 18 MeV Proton Bombarded Enriched Water Target

Residual radioisotope analysis as a result of cyclotron-based 18F production is of paramount importance since it relates to the radiation safety of patients as well as radiation workers. In this investigation, 18-MeV proton beams were employed to irradiate enriched water (H2 18O) target for 18F production while Talys Evaluated Nuclear Data Library (TENDL) 2017 were used to study the origins of the radionuclide impurities. Gamma rays emmitted by the residual radionuclides were detected using a gamma ray spectroscopic system following a month of decay while their origins were analyzed from the TENDL 2017 nuclear cross-section calculations. Experimental results indicated that several long-lived radionuclides such as 109Cd, 57Co, 57Ni, 58Co and 56Co were recorded by the gamma ray spectroscopic system. The long-lived residual radionuclides were presumably due to proton interactions with Havar window and Silver body. Using the TENDL 2017-calculated nuclear cross-sections, it was discovered that several nuclear reactions responsible for the residual radioisotopes include 109Ag(p,n)109Cd which corresponded to the generation of 109Cd radioisotope, 60Ni(p,α)57Co and 58Ni(p,2p)57Co reactions for the formation of 57Co, 58Ni(p,d)57Ni reaction for the production of 57Ni radioisotope, 58Fe(p,n)58Co reaction for the generation of 58Co, and 56Fe(p,n)56Co reaction for the formation of 56Co. This experimental result can be used as a reference for future production of 18F and other radioisotopes should Havar window and silver body are used in the target system.


Introduction
Recent progress on radioisotopes production has been concerning various types of radioisotope production applicable for diagnosis such as 68 Ga [1][2][3][4][5], 99m Tc [6][7] and therapy such as 153 Sm, 186 Re [8][9][10][11][12][13][14][15][16][17] and many others. Due to emmission of positron particle, 18 F radionuclide has been used in Positron Emission Tomography (PET) in nuclear medicine. Fluorine-18 ( 18 F) is currently produced only by cylotron with proton beam energy ranging from 7 MeV to 18 MeV which has been widely reported elsewhere, in which enriched water (H2 18 O) is used as the target [18][19][20]. Previous research suggested that the intensity of radionuclidic impurities depended strongly on the proton dose [21]. The target system in 18 F production usually consists of a window made of havar foil or Nb foil [22][23], a target body made of silver or titanium [22][23] and the cooling fluid. Interaction between proton beam and havar window and target body could result in radioactivity impurities. In addition the cyclotron vicinity could be activated by secondary neutrons as a result of radionuclide production which has been reported elsewhere [24]. Using 11-MeV proton beam bombarded to 95% enriched water (H2 18 O) target, Długosz-Lisiecka [25] reported formation of 109 Cd and 107 Cd via 109 Ag(p,n) 109 Cd and 107 Ag(p,n) 107 Cd in the target body respectively. Previous investigation by Kambali et al observed other impurities such as 56 Co, 52 Mn, 54 Mn and 110m Ag in 11-MeV proton irradiated enriched water target following 1 hour cooling (after End-of Bombardment) [23]. Kohler et al [26] investigated radionuclide impurities in 18-MeV proton irradiated enriched water and detected various impurities such as 7 Be, 48V, 31 Cr, 52 Mn, 54 Mn, 55 Co, 56 Co, 57 Co, 58 Co, 57 Ni, 89 Zr, 92m Nb, 93m Mo, 95 Tc, and 96 Tc. Furthermore, they identified that the impurities came from proton interaction with Nb window and Nb window impurities. Since the maximum impurities allowed in F-18 is not more than 0.1% according to the European Pharmacopeia [27], the window separating high vacuum cyclotron and the target as well as the target body should be chosen to minimize the impurities. In this work we report the use of Ti and Havar foils as the windows while Niobium is used as the target body. A 97% pure enriched water (H2 18 O) target is bombarded with 18-MeV proton beam while radionuclidic impurities are detected using a gamma ray spectroscopic system. The radioisotope impurities are identified from their gamma ray emissions. In addition, the TENDL 2017 nuclear cross-section data are used to study the origins of nuclear reactions responsible for the experimentally observed radionuclide impurities.

Materials and Methods
In this work, the cyclotron employed to generate 18 MeV protons was located in Mochtar Riady Comprehensive Cancer Center (MRCCC) Siloam Hospital, Jakarta, Indonesia. The protons were bombarded to 2 ml enriched water (H2 18 O) target with beam current of 30 µA and irradiation time of 1 hour, which yielded 2.2 Ci 18 F at the end of bombardment. The beam windows were made of a combination 12.5 µm thick natural Ti foil and 50 µm thick Havar foil which consists of cobalt (42.5 %), chromium (20 %), iron (18.1 %), nickel (13 %), tungsten (2.8 %), molybdenum (2 %), manganese (1.6 %), carbon (0.2 %), beryllium (0.04 %), and some other trace elements. The Ti window was placed in front of Havar window so that the Ti window was directly in contact with vacuum chamber. While irradiation of Ti could result in radioactive isotopes, it was expected that they would not contaminate the enriched water target since Havar window was put between the Ti window and enriched water target. The target vessel was composed of a niobium (Nb) cavity which was housed in a stainless steel body. The cooling water flew in the cavity to cool down the Nb material. The whole target system is illustrated in Fig. 1 Fig. 1. Target system for 18 F production at MRCCC Siloam Hospital, Jakarta, Indonesia.
The gamma rays emitted by residual radioisotopes were detected using a gamma ray spectroscopic system consisting of HPGe detector and analytical software and devices following 1 month decay. The spectroscopic system was calibrated using 3 radioactive sources, e.g. 60 Co, 137 Cs and 241 Am. The radioactive isotopes left over in the enriched water target were then identified from the gamma ray energies and their half lives. In order to study the origins of the residual radioisotopes, the TENDL 2017 data were employed to study nuclear cross-sections and nuclear reactions responsible for the radioisotope formations. In addition, the TENDL 2017 data have been previously used to study cyclotron-based radioisotope production elsewhere [28][29][30][31][32][33][34][35][36].

Results and Discussion
Gamma ray spectrum recorded by the spectroscopic sytem following 1 month decay is given in Fig. 2, which indicates several gamma rays emitted by residual radioisotopes. The identified radioisotopes include 109 Cd, 57 Co, 59 Ni, 57 Ni, 58 Co, 54 Mn, 56 Co and 52 Mn. The half lives of the residual radioisotopes are between 35.6 hours and 461.9 days.The X-ray recorded at 74.97 keV is presumably due to KL3 transition of Pb [37] as the shielding material. A complete list of nuclear data for the identified longlived residual radioisotopes is shown in Table 1, which include their nuclear reaction possibilities.  Based on the the TENDL 2017 nuclear cross-sections, 109 Cd is produced from 109 Ag(p,n) 109 Cd nuclear reaction as seen in Fig. 3 (blue circles), whereas 57 Co may be generated from both 60 Ni(p,α) 57 Co and 58 Ni(p,2p) 57 Co nuclear reactions, though 58 Ni(p,2p) 57 Co nuclear reaction may contribute to higher intensity of 57 Co than that of 60 Ni(p,α) 57 Co nuclear reaction since it has much higher cross-section at 18-MeV protons. The presence of 57 Co was previously discussed by Kohler et al [26], while 109 Cd was earlier reported by Długosz-Lisiecka [25]. Strong gamma ray emission is also seen at 511 keV which presumably due to positrons (β + ) emitted by 56 Co, 57 Co, 58 Co and 57 Ni that directly interact with electrons and eventually emit gamma ray at 511 keV from the interaction. Again, based on the TENDL 2017 cross-section sections, three nuclear reactions, i.e. (p,g), (p,np) and (p,d) may contribute to the gamma rays captured by the spectroscopic system. Of the three reactions, 58 Ni(p,np) 57 Ni has the highest cross-section while the two others, i.e. 56 Fe(p,g) 57 Ni and 58 Fe(p,d) 57 Ni have very low and insignificant cross-section; thus it rules out any possibility of the two reactions responsible for the production of 57 Ni. To sum up, it is clear that 57 Ni is as a result of 58 Ni(p,np) 57 Ni nuclear reaction. Previous research by Kohler et al [26] also detected 57 Ni in the 18 F production.  The detected 58 Co is clearly due to 58 Fe(p,n) 58 Co reaction, which is proven by the very high nuclear cross-section according to the the TENDL 2017 data. Quite similar result also occurs to 56 Co which is generated by 56 Fe(p,n) 56 Co nuclear reaction. Both 58 Fe(p,n) 58 Co and 56 Fe(p,n) 56 Co nuclear cross-sections can be seen in Fig. 5. Furthermore, the present of 58 Co and 56 Co was previously reported by Kohler et al [26]. After 2.5 years of the 18 F production, the residual radioisotopes were measured again, and it was found that only 57 Co radioisotope was recoded while the others were not detected. This result agrees with the data captured 1 month after irradiation in which gamma rays emitted by 57 Co were the strongest among others. Comparison between the two data are shown in Fig. 6. It should be noted that in this work, radionuclide impurities resulted from proton bombardment of Nb cavity was not observed. Kohler et al [26] reported 89 Zr radionuclide impurity which was produced from 93 Nb(p,x) 89 Zr. However, since the half life of 89 Zr is only 3.27 days, it could no longer be detected in this investigation. Fig. 6. Comparison of gamma ray spectrum observed after 1 month decay (red line) and 2 years decay (blue line). Note that the y-axis is in arbitrary unit.

Conclusion
Long-lived residual radioisotopes in 18 F production have been detected, identified and analyzed from their gamma ray spectrum following 1 month decay. Several radioisotopes were identified, such as 109 Cd, 57 Co, 57 Ni, 58 Co and 56 Co. The TENDL 2017 nuclear cross-sections have been employed to study the origins of the impurities. It was concluded that 109 Cd was produced from 109 Ag(p,n) 109 Cd reaction, while 57 Co formation was presumably due to 60 Ni(p,α) 57 Co and/or 58 Ni(p,2p) 57 Co reactions. In addition, 57 Ni, 58 Co and 56 Co radioisotopes were as a result of 58 Ni(p,d) 57 Ni, 58 Fe(p,n) 58 Co and 56 Fe(p,n) 56 Co nuclear reaction respectively. This experimental report agrees with previous reports elsewhere.