Physics Research International
Volume 2017 (2017), Article ID 2124383, 5 pages
https://doi.org/10.1155/2017/2124383

Research Article

Dependence of ¹⁸F Production Yield and Radioactive Impurities on Proton Irradiation Dose

Imam Kambali,¹ Parwanto,¹ Hari Suryanto,¹ Nur Huda,² Ferdi D. Listiawadi,² Herta Astarina,² Ratu R. Ismuha,² and Kardinah²

¹Center for Radioisotope and Radiopharmaceutical Technology, National Nuclear Energy Agency (BATAN), Puspiptek Area, Serpong, South Tangerang, Indonesia
²Dharmais Cancer Hospital, Jakarta, Indonesia

Correspondence should be addressed to Imam Kambali; imamkey@batan.go.id

Received 25 February 2017; Revised 24 May 2017; Accepted 7 September 2017; Published 9 October 2017

Academic Editor: Leonardo Golubovic

Copyright © 2017 Imam Kambali et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Radiation safety for patients during positron emission tomography (PET) procedures is affected by the amount of radioactive impurities generated during production of fluorine-18 (¹⁸F) radionuclide. In this investigation, the dependence of ¹⁸F production yield and radioactive impurities on proton irradiation dose is discussed. Enriched water () target was bombarded perpendicularly by 11-MeV proton beams at various proton doses. Experimental results indicated that the ¹⁸F radioactivity yield and the amount of ⁵⁶Co and radioactive impurities depend strongly on the proton dose. In the proton dose range between 2 μAhr and 20 μAhr, the radioactive impurities increased with increasing proton dose. There was no significant difference in the radioactivity yield of both ⁵⁶Co and impurities at low proton dose between 2 and 10 μAhr. However a huge difference was recorded when the dose was increased above 10 μAhr. The experimental data can be used to predict the amount of impurities generated during ¹⁸F production at proton dose of higher than 20 μAhr.

1. Introduction

Current nuclear methods in medical and industrial radioisotope production have been mostly involving nuclear reactors and circular accelerators. Subatomic particles such as neutron, proton, deuteron, ³He, ⁴He, and other heavier particles are generated and then accelerated to certain energies sufficient for production of radioisotopes. At thermal energy, neutrons can be directed into a target to create radioisotopes relevant for nuclear medicine applications, such as ¹⁵⁴Sm, ¹⁶⁶Ho, ¹⁴⁹Pm, ¹⁶⁶Ho, ¹⁶¹Tb, and ¹⁷⁷Lu [1, 2]. Deuteron particles accelerated in circular accelerators or cyclotrons have been recently employed to produce short-lived medical radioisotopes such as , , ¹⁵⁵Tb, and ¹⁶¹Tb [3, 4], whereas ³He particles have been suggested for production of , ⁴⁸V, and ⁴⁸Cr radioisotopes [5].

Proton beams generated from cyclotrons have been widely employed to produce radionuclides relevant for medical applications [6–8] as well as materials studies [9, 10]. One of the most widely used radionuclides in medical diagnoses for positron emission tomography (PET) modality is fluorine-18 (¹⁸F) [11, 12]. Research on theoretical and experimental aspects of ¹⁸F production using small cyclotrons has been carried out elsewhere to study the optimum target system and irradiation parameters [13], radioactive by-products in the cyclotron vicinity [14], and identification of radioactive impurities [15].

The main parameters affecting the types of radionuclidic impurities found in the enriched water target during ¹⁸F production include the type of the window separating the cyclotron chamber and the target system. The most common materials used as the window are Havar and titanium (Ti) foils. Moreover, proton beam energy employed in the bombardment also influences the number of impurities that fell in the postirradiated target. Previous studies discovered that several radioactive impurities such as ⁵⁶Co, ⁴⁸V, ⁵¹Cr, ⁵²Mn, ⁵⁴Mn, ⁵⁴Co, and ⁵⁸Co were found to contaminate the enriched water target when proton beams passed through Havar windows [16]. For Nb window, the most significant impurities detected by Köhler et al. were ⁸⁹Zr, , , ⁹⁵Tc, and ⁹⁶Tc [15].

Most studies on the radionuclide impurities have concentrated on the identification of the impurities without further studying the effect of proton beam current on the amount of the impurities. In the present investigation, the dependence of proton beam dose on the radioactivity yields of ¹⁸F and the impurities is discussed. A medical cyclotron which accelerates 11-MeV proton beams is used to produce ¹⁸F and, in this study, the impurity measurement is performed using a gamma spectroscopy system.

2. Materials and Methods

2.1. Irradiation

A 97% pure enriched water (), purchased from Cambridge Isotope Laboratories, Ltd., was set as the target for ¹⁸F radionuclide production. As much as 1.8 ml enriched water target was placed in a target system consisting of a Havar window and silver body as described elsewhere [14]. An 11-MeV proton beam was directed into the enriched water target at variable proton doses, ranging from 2 to 20 μAhr. The proton dose was varied by varying the irradiation time while keeping the proton beam at a constant current of 30 μA. Note that a new target was prepared for every irradiation, allowing it to be free of contaminations prior to the irradiation. The irradiation procedure was performed using a typical Eclipse 11 cyclotron located at the National Cancer Center (NCC), Dharmais Cancer Hospital in Jakarta, Indonesia, as shown earlier [14].

2.2. Impurity Analysis and Radioactivity Yield

A portable gamma ray spectroscopy system which has been described elsewhere [14, 17] was used to detect and quantify the radionuclidic impurities. The Amptek-USA made spectrometer included a pocket MCA (Type MCA8000A) with the serial number 2278. The pocket MCA was connected to a NaI(Tl) detector. The impurities were recognized from their gamma ray energies as well as half lives.

Theoretical calculations of the end of bombardment (EOB) yield of ¹⁸F radionuclide were performed using the widely used equation [18] and the calculated results were compared with the experimental data by taking into account the radioactivity decays. Moreover, a computerized program was developed [19] using Visual Basic Language. While the ¹⁸F radioactivity was relatively easy to predict using the developed program, the impurity yields were not theoretically calculated in this investigation due to complex irradiation parameters.

3. Results and Discussion

3.1. Radioactivity Yield of ¹⁸F

The gamma ray spectrum of ¹⁸F observed at different integrated beam currents, ranging from 2 μAhr to 20 μAhr, is shown in Figure 1. The figure indicates that the gamma ray peak at 0.511 MeV increases with increasing proton dose. The experimental radioactivity hikes proportionally with increasing integrated proton beam current, which clearly agrees with the calculated radioactivity yield as seen in Figure 1 (inset).

Figure 1: Recorded ¹⁸F spectrum at various proton doses observed 1 hour after the end of bombardment. The experimental radioactivity (circles) and calculated radioactivity (line) are shown in the inset.

At low proton dose of 2 μAhr, the experimental radioactivity yield recorded in the gamma ray spectroscopy system is  GBq as shown in Table 1. The intensity trend follows linear line of with (in which is the ¹⁸F radioactivity and is the proton dose). As the proton dose is increased to 20 μAhr, the ¹⁸F radioactivity yield also goes up to  GBq. The linear regression equation derived in this experiment may be used to predict future ¹⁸F production when the proton beam is increased further above 20 μAhr, presumably the proton energy is kept at 11 MeV, and the radioactivity is measured one hour after irradiation. For example, a proton dose of 50 μAhr is required to produce 85.10 GBq of ¹⁸F radionuclide, whereas up to 107 μAhr proton dose is needed to obtain nearly 185.00 GBq of ¹⁸F.

Table 1: Production yield of ¹⁸F following 1-hour decay.

Proton dose (µAhr)
¹⁸F yield (GBq)

Experiments
Theory

2

3.60

2.5

4.32

6.5

11.23

10

17.28

11

19.01

20

36.01

In general, our experimental results agree with previous investigations by Nye et al. [20] and Wieland et al. [21], in which the radioactivity yield of ¹⁸F increases with increasing proton dose. For example, at proton dose of 3.34 μAhr, our experiment results in ¹⁸F yield of 5.79 GBq compared to 5.25 GBq by Wieland et al. [21].

3.2. Experimental Spectrum and Yield of Radioactive Impurities

Following 2 days of cooling (decay) period, the majority of ¹⁸F radioactivity has decreased which allows better observing the gamma ray spectrum of radioactive impurities. As indicated in Figure 2, two radioisotopes are captured by the gamma ray spectroscopy system, namely, ⁵⁶Co and . The two impurities have been previously discussed in the production of ¹⁸F radionuclide [16], despite no further discussion on the radioactivity dependence on the proton beam dose. The identified gamma rays for ⁵⁶Co are at and 1.238 MeV, whereas, for , the strong peaks are recorded at , 0.885, and 1.4 MeV. In addition, the annihilation peaks at 0.511 MeV originated from positron emissions of ¹⁸F radionuclide and gamma emissions of ⁵⁶Co and impurities with energies over 1.022 MeV are also detected.

Figure 2: Gamma ray spectrum of ⁵⁶Co and impurities following 2 days of ¹⁸F decay period.

As can be seen in Figure 3, in general the radioactivity of both radioactive impurities (⁵⁶Co and ) increases with increasing proton beam dose. For ⁵⁶Co impurity, the radioactivity yield () matches empirical equation (with ), whereas the radioactivity yield of impurity follows equation (with ). Note that, in the equations, represents proton dose expressed in μAhr. Again, the equations may be applied to calculate the radioactivity yields of both ⁵⁶Co and impurities. For instance, using a 50-μAhr proton dose, the expected impurity yields for ⁵⁶Co and are nearly 37 MBq and 5.55 MBq, respectively.

Figure 3: Radioactivity yield of ⁵⁶Co (unfilled squares) and (filled circles) following 2 days of decay period.

As shown in Figure 3 and Table 2, at low proton dose between 2 and 10 μAhr, there is no significant difference in the radioactivity yield of both impurities. For instance at 2 μAhr proton dose, as much as  MBq of ⁵⁶Co is detected, whereas  μCi of is recorded at the same proton dose. However the difference becomes more apparent and wider as the proton dose increases. At 20 μAhr proton dose, only  MBq of is observed whereas around  MBq of ⁵⁶Co is measured at the same proton beam dose, meaning that the amount of  ⁵⁶Co impurity is more than 3 times that of the .

Table 2: Radioactivity yield of ⁵⁶Co and impurities following 2 days of decay period.

Proton dose (µAhr)
Impurity yield (MBq)

⁵⁶Co

2

2.5

6.5

10

11

20

The much higher increase in the ⁵⁶Co radioactivity yield observed in this experiment is most likely related to the following.

( 1) Secondary Neutron Yield Production. During ¹⁸F production, secondary neutrons are generated via (p,n) nuclear reaction. While every incoming proton interacts with the atom it passes through, its interaction does not necessarily generate secondary neutrons. Thus the number of secondary neutrons produced in this process is not directly proportional to the number of the incoming protons. As discussed in previous studies [14, 16], ⁵⁶Co is generated from ⁵⁶Fe(p,n)⁵⁶Co nuclear reaction whereas is produced via ¹⁰⁹Ag(n,γ) nuclear reaction. As a result, the number of radioisotopes is much less than that of ⁵⁶Co radionuclides.

( 2) Silver Holder Geometry. While the Havar window used in this investigation is perpendicularly hit by the incoming proton beam, the silver body is parallel to the beam as discussed in previous studies [14]. Secondary neutrons generated by the (p,n) reactions are most likely to hit any materials perpendicular to the incoming proton beam as highlighted in earlier similar investigation [17]. As a result, it makes the silver holder harder to be hit by the secondary neutrons and any recoiled atoms originated from the silver window. Thus the radioactivity yield detected in the water target is much lower than the ⁵⁶Co yield.

3.3. Impurity-to-¹⁸F Yield Ratio

Based on the experimental results as shown in Table 3, at the end of bombardment (EOB), the average ratio between each impurity and ¹⁸F yield ranges between 1.2 and 6.2%. For impurity, proton dose does not significantly influence the impurity-to-¹⁸F yield ratio. In contrast, the proton dose has significant impact on ⁵⁶Co impurity-to-¹⁸F yield ratio, particularly when the proton dose is increased above 10 μAhr. In this case, the average on ⁵⁶Co impurity-to-¹⁸F yield ratio jumps from 2.9% to 6.2%. The difference in the impurity-to-¹⁸F yield ratio between ⁵⁶Co and corresponds to both secondary neutron yield production and silver holder geometry as discussed in previous section.

Table 3: Ratio of ⁵⁶Co and impurities to ¹⁸F yield at EOB.

Proton dose (µAhr)
Impurity-to-¹⁸F yield ratio (%)

⁵⁶Co

2

2.5

6.5

10

11

20

4. Conclusion

Two radioactive impurities (⁵⁶Co and ) found in the postirradiated water target following ¹⁸F production have been measured for their radioactivities. Experimental results indicate that the radioactivities depend strongly on the proton beam dose. The higher the proton dose the higher the radioactivities. At low proton dose of up to 10 μAhr, there is no significant difference in the radioactivity of ⁵⁶Co and radionuclides. However the radioactivity difference becomes significant as the proton beam dose is increased above 10 μAhr. At this point, ⁵⁶Co radioactivity is a lot higher than that of , which is presumably due to secondary neutron yield production and silver holder geometry as discussed in the Results and Discussion.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This research project is supported by The World Academy of Sciences (TWAS) under the Principal Investigator’s Research Grant no. 15-020 RG/PHYS/AS_I–FR3240287075, as well as the in-kind support from National Nuclear Energy Agency (BATAN) and Dharmais Cancer Hospital. Technical support by the cyclotron technicians and staff at Dharmais Cancer Hospital in Jakarta is also greatly acknowledged.

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