Radiotracers from heavy-ion fragmentation

Nuclear Instruments and Methods in Physics Research BlO/ll North-Holland. Amsterdam

963

(1985) 963-966

RADIOTRACERS FROM HEAVY-ION FRAGMENTATION D.J. MORRISSEY Notional

Superconducting Cyclorron Loboratory and Department

of Chemistry, Michigan

Store University, East Lansing, MI

48824.

USA

48824,

USA

J.A. NOLEN Jr Notronol Superconducting Cyclotron Laboratory

and Deportment

of Physics, Michigan

Slate University,

EOSI Lansing, MI

J.M. TIEDJE Department

of Crop and Soil Science. Michigan

State University, East Lansing, MI

48824,

USA

The advent of high energy heavy-ion accelerators has introduced the new mechanism of projectile fragmentation for the production of radiotracers. Projectile fragmentation occurs when the heavy-ion projectile has a sufficient velocity to undergo a strong interaction with a target nucleus without being deflected very much from its initial trajectory. The fragmentation of 14N beams from the K500 superconducting cyclotron at Michigan State is described. a 490 MeV 14N beam was fragmented in a beryllium foil and the reaction products are stopped in (liquid) water. The water provides an additional source of “N through fragmentation of t60, facilitates the conversion of “N atoms to labeled nitrate and allows the rapid transfer of the source out of the accelerator vault.

1. Introduction The fragmentation of high energy heavy-ion projectiles can be used to produce nuclei at the limits of stability [1,2] as well as provide secondary beams of radioactive nuclei (31. Fragmentation occurs when the projectile has a kinetic energy large enough that a strong interaction can take place between the target and the projectile nuclei without significantly perturbing the projectile velocity vector. As both projectile and target are fragmented by the interaction, suitable choices can allow specific radionuclides to be produced as both projectile and a target fragments. The present manuscript describes the production and characterization of 13N by the reaction of 14N with a target system that contained beryllium and water. 13N has been produced in (p, n) reactions [4] for biological studies by the Cyclotron Laboratory at Michigan State University for a large number of years [5]. The proton beams necessary for the production reactions are no longer available and a new technique had to be developed. The production of radioisotopes by projectile and target fragmentation has the advantages offered by a large target thickness due to the long range of high energy beams. The largest fragmentation cross sections are for the removal of only one neutron or proton from the precursor nucleus. Thus, the best target-projectile combination to produce 13N is 14N + “N. A gas target is not practical as the beam fragments should be brought to rest in the target and thus combine with the target

0168-583X/85/503.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

fragments. A more suitable target is water as it contains only oxygen isotopes, which lead to “N, and protons, which cannot be fragmented. The cross sections for the formation of projectile fragments in high energy reactions have not been measured for a broad range of nuclei nor for a large range of bombarding energies but rather must be estimated with a geometrical model of the reaction [6]. The calculated cross section for r3N production in the present

IO -

14N BEAM I60

FRAGMENTS

TARGET

FRAGMENTS

8fl P = 6Z

$ g4 Cz

1oo~u

0

50’~<100 25
m El_ 0 0 ;;

2-

I 2

4

,

I

I

I

I

6 6 IO NEUTRON NUMBER

I

1 I I2

Fig. 1. The calculated cross sections for formation of various isotopes by the fragmentation of 14N by 160, see text. The cross sections have contributions from the fragmentation of both the projectile and the target (MSU-84-4451. X. BADIOISOTOPES/TARGETS

964

D.J. Morrissey et al. / Radiotracers from heauy -ion fragmentation

reaction is 135 mb, consisting of a relatively large contribution from 14N (91 mb) and a significant contribution from I60 (44 mb). The distribution of the calculated cross sections from this reaction is shown in fig. 1. The figure emphasizes the important feature of fragmentation reactions that a wide range of products are formed with similar cross sections. Because the relative amounts of product nuclei and the total reaction cross section depend on the nature of the projectile and target, the reaction system can be tailored to enhance a specific isotope. For example, the ratio of i3N to “‘C is predicted to be 2.2 : 1 for a Be target as opposed to 1.9 : 1 in the present study. However, the cross section for i3N production would be only 82 mb in the former case as there is no contribution from the target. In any event, pure radioisotopic sources require chemical processing.

In the first series of production runs, 14N beams from the NSCL K500 superconducting cyclotron were used with energies in the range of 20 to 35 MeV/nucleon. We will describe the most extensive data which was obtained at 35 MeV/nucleon. The fragmentation target was placed at the first focal point of the NSCL beam line, a convenient location after the first horizontal bending magnet. The target consisted of a Be foil (in the range of 0.25 to 2.03 mm thick) backed by a 1 cm3 volume of liquid water. The l4 N beam and its fragments penetrated the foil and stopped in the water. The water serves several purposes, it produces 13N through the of the fra~entation of 160 it facilitates in~~oration i3N atoms into labeled nitrates and it can rapidly transport the radioactive products through the accelerator shielding. The initial version of the target system prepared sources in a batch process. A schematic diagram of the

water transfer system is shown in fig. 2. Highly purified water was supplied from a local storage tank through a series of remotely controlfed solenoid valves. The system was pressurized with He gas in order to avoid dissolving N, in the water. In sequence, the beam stop was filled with water and irradiated for short periods of time. The beam intensity was usually between 10 and 30 particle naoamps (pnA), being limited by constraints on cyclotron operation. After irradiation the beam stop water was flushed through the accelerator shielding walls in a thin tube, approximately 20 m long, within 10 to 20 s of the end of bombardment. The activity of the source was routinely monitored by placing an aliquot into a NaI(Tl) well counter. Such detectors are sensitive to the annihilation radiation from positron emitters. Multiscales of the outputs from up to 12 detectors could be cohected simultaneously by a computer controlled CAMAC system. Briefly, the outputs of the NaI(Tl) detectors were passed through Ieading edge discriminators (EG&G T122/NL) and then into two CAMAC scalers (LECROY MODEL 2551). The scalers were cycled between data collection and readout by a CAMAC real time clock (EG&G ORTEC RC014). The contents of the scaler buffers were read out via a serial highway 4 times a second, added together and stored in a VAX 11/750 computer. Leastsquares decay curve analysis showed the presence of “0, i3N, “C, ‘*F and a long lived component, most likely ‘Be. The studies with the batch system showed that the buildup of radioactivities was not linear with time of bombardment, even for short times (l-3 min). This could be due to variations in the thickness of the liquid phase due to heating. Large variations would allow the beam fragments to pass through the water beam stop and become imbedded in the rear cover plate. Recall that stopping a 35 MeV/nucleon 14N beam delivers 5 W of power at the low current of 10 pnA. The strongest sources were produced by adding the water samples from a sequence of short irradiations, nominally three successive 3 min bomb~dments. A second target system has been developed which continuously circulates water through the beam stop. The volume of water in the new beam stop was increased to 4 cm3 and the total volume of flowing water was approximately 30 cm3. The initial test of this target system produced sources which were a factor of 3 to 5 stronger than those from the batch system.

3. Results and discussion Fig. 2. Schematic diagram of the batch transfer target system. The conditions of the valves (0 = open, C = closed) are indicated for filling the beam stop, bombardment and flushing the source through the shielding walls [MSU-83-4171.

The bombardment of the Be/H,0 batch target produced both gaseous and aqueous radioactivities. During test runs the water and pressurizing He gas were transferred into an evacuated flask. The activities of the

D.J. Morrissey et al. / Radiotracers jrom heavy-ion fragmentation

Fig. 3. Typical decay curves of the activities observed in the aqueous and gaseous phases, see text. The thick lines indicate the total activity while the thin indicate the activity of the individual components in each phase (MSU-84-4421.

and gas phases from a typical source are shown in fig. 3. The decay of the total activity (heavy line) was resolved into its components (thin lines) by a leastsquares fit to the data. The measured samples represent l/14 and l/200, by volume, of the aqueous and gaseaqueous

ous phases, respectively. The distribution of activities between the two phases and the values of the source strengths are given in table 1. The “N batch sources were typically 4 mCi/ppA. (The number of milliCuries per microampere is equal to the product of the number of target atoms, the cross section and the saturation factor which provides a convenient basis for comparison of production mechanisms.) The present value of a few mCi/ppA represents about 10% of the calculated fragmentation cross section discussed earlier. This reinforces our finding of insufficient buildup of activity with bombardment time. The initial source strengths produced by the circulating target were approximately 20 mCi/ppA and are within a factor of 2 of the predictions. We take this to be good agreement because the calculated cross sections are not that reliable. The data presented in table 1 and fig. 3 show that

965

r3N is the predominant component in the aqueous phase and that essentially no 13N is found in the gaseous phase. On the other hand, “C is split between the two phases, representing nearly all of the gaseous radioactivity. As our goal was to produce “N, all of the gaseous radioactivities were usually discarded. The chemical makeup of the aqueous phase was studied by high pressure liquid chromatography (HPLC). Small samples, = 0.1 cm-‘, were injected into a chromatograph and separated with a strong anion-exchange column (Partisil SAX) with a pH 3.0, 0.05M phosphate buffer. The fractions were observed by UV absorption and in a sample loop placed in a NaI(Tl) well counter. The UV absorptions of the carrier-free radioactivities were masked by an extremely large peak which was not significantly radioactive. This strong absorption was not identified but must be due to a radiolysis product. Typical traces of the NaI(T1) count rate as a function of time after injection into the HPLC are shown in fig. 4. The curve at the top of fig. 4, obtained by injecting an untreated sample of beam stop water into the HPLC approximately 4.5 mm after the end of bombardment, shows that the “N is distributed among ammonium ions (NH:. 6%), nitrite ions (NO;, 24%), and nitrate ions (NO;, 70%). This pattern was reproduced (within 10% of the quoted values) and is completely consistent

‘0°0- NH:

Table 1 Distribution of activities Isotope

‘SF “C 13N “0

Activity (mCi/PrA)

Aqueous phase (S)

Gaseous Pha= (W)

0.6f0.2 4.4 f 0.2 3.9f0.4 2.7 f 0.1

4.Oil.O 7.7 * 0.2 32.1 f0.3 13.3*0.1

1.0* 1.0 30.3*0.3 1.0*1.0 9.9*0.5

Fig. 4. A HPLC trace obtained after injection of an untreated (raw) sample of beam stop water is shown. In addition, the effects of chemical treatments on the distribution of activity in the liquid phase are also shown. The baselines of the data have been shifted as follows, raw source +700 cps. evaporation in base +450 cps, evaporation in acid +200 cps, and peroxide treatment - 50 cps [MSU-84-4431.

X. RADIOISOTOPES/TARGETS

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L&J. Morrissey et al. / Radiotracers from heavy -ion fragtneniarion

with previous studies by Tilbury and Dahl [7] of the chemical forms of 13N produced by proton irradiations of water. We estimate than the dose delivered to the water in the batch target in a 3 min bombardment is roughly 0.2 eV/molecule. The dose delivered to the circulating water target was significantly lower, approximately 0.04 eV/molecule. The dist~bution of 13N shifted towards nitrite, as expected (NH:, 20%; NO;, 66%; NO;, 14%). Two additional peaks intervene between the ammonium and nitrite peaks. The later and smaller of the two contains “C, most likely as HCO; . The other peak contains “N but the chemical form is not known. The other curves in fig. 4 show the distribution of activities after several chemical treatments. The curve labeled BASE shows the activities remaining after a source was evaporated to dryness in base, a process which removes ammonia. The curve labeled ACID shows the activities remaining after another source was evaporated to dryness in acid, a process which removes HCO;, NO; and most NO;. The bottom most curve shows the conversion of all species to nitrate by treating a source with hydrogen peroxide (30% solution) and then evaporating to dryness.

yield, a second target system with a continuous water flow was developed. This new target system increased the source strength by a factor of 3 to 5. The distribution of activities between the gas and aqueous phases were reported and chemical composition of the t3N in the aqueous phase was characterized. The latter composition was found to be consistent with ~st~butions previously reported for proton irradiations of water. We would like to thank the National Superconducting Cyclotron Laboratory staff for making these experiments possible. We would also like to acknowledge the efforts of Richard Au who developed the data acquisition software, Christoph Meingast and Steve Bricker for their work on the target systems, and Charles Rice and Greg Walker for their assistance in making the HPLC measurements. The work was supported by the National Science Foundation under Grants PHY 80-17605, PHY 83-12245 and DEB 80-2168.

References [l] H.H. Heckman and F.S. Goldhaber, Ann. Rev. Nucl. Sci.

28 (1978) 161. [2] T.J.M. Symons, Y.P. Viyogi, G.D. Westfall, P. Doll, DE. 3. Conclusion We have applied a new technique to the production of short-lived radioactive sources. The technique relies on nuclear reactions in which both the beam and target nuclei are fragmented. The production of 13N was the primary goal of the present study, and so experiments were performed with a t4N beam and a water target. The sources were produced in a batch process using a stationary water target. This system delivered activities at the level of a few mCi/ppA. Because this source strength represents only about 10% of the expected

Greiner, H. Faraggi, P.J. Lindstrom, D.K. Scott. H.J. Crawford and C. McParland, Phys. Rev. Lett, 42 (1979) 40. (31 J.R. Aionso, A. Chattetjee and CA. Tobias, IEEE Trans. Nucl. Sci. NS-26 (1978) 3003. [4] SM. Austin, A. Galonsky. J. Bortins and C.P. Walk, Nucl. lnstr. and Meth. 126 (1975) 373. (51 J.M. Tiedje. R.B. Firestone, M.K. Firestone, M.R. Be&h, H.F. Kaspar and J. Sorensen, Adv. Chem. Ser. 197, eds., J.W. Root and K.A. Krohn (Amer. Chem. Sot. 1981) ch. 15. [6] D.J. Morrissey, W.R. Marsh, R.J. Otto, W. Loveland and G.T. Seaborg, Phys. Rev. Cl8 (1978) 1267. [7] R.S. Tilbury and J.R. Dahl, Rad. Res. 79 (1979) 22.