Introduction

To build the next generation high power radioactive ion beam (RIB) facility, EURISOL (European Isotope Separation On-Line), is a part of the Nuclear Physics European Collaboration Committee (NuPECC) long-range plan. It is intended to use liquid Hg or LBE (Lead bismuth eutectic) as proton to neutron converter in the proposed EURISOL facility. Studies on the impact of high-energy protons (~1–1.5 GeV) on liquid Hg have been carried out as a task of EURISOL design study [1]. However, handling of tons of liquid Hg has some practical difficulty, which may be overcome using LBE as target material. Hence, it is important to have a detailed experimental knowledge on the production of radionuclides in the thick LBE targets on impact of high-energy (1.4 GeV) protons. This knowledge is important from multiple angles, to understand fundamental nuclear reactions in the high-energy domain, from the health physics point of view for risk assessment, etc. Moreover, these co-produced radionuclides in the LBE targets can be huge source of isotopes having applications in various fields. For example, accelerator-driven systems (ADS) are designed with LBE target, as well as LBE coolant instead of Na-coolant. Therefore the behavior of these radionuclides in LBE is also of interest [2].

In 1991, Ambe et al. introduced the multitracer technique, which is a breakthrough in tracer studies [3]. A multitracer solution contains a large number of radioactive species, all in their no-carrier-added state and with high specific activity. This multitracer solution allows to mimic natural condition and to study multielemental multidimensional natural dependence. Multitracer solution has been so far prepared by bombarding gold target by high energy 12C/14N/16O projectiles (80–135 MeV/u) followed by radiochemical separation of bulk gold target [3–5]. Therefore radiochemical separation of bulk lead and bismuth targets after bombardment by high-energy protons will also lead to a multitracer solution which could be used in various studies in fundamental sciences.

Earlier several attempts were taken by the scientists to study the impact of high-energy protons on lead or bismuth targets. Rémy et al. [6] in association with CERN and Saclay (Saturne) studied the cross-section for binary and ternary fission induced by high-energy protons (0.6, 2, 3, 18 and 23 GeV) in thin uranium and lead targets. Gloris et al. [7] determined the cross sections for proton induced reaction on natPb targets of thickness about 120 mg/cm2 each, by stacked foil technique within the energy range of 80 MeV to 1.6 GeV at the Laboratoire National Saturne/Saclay. They reported the cross sections of around 68 radionuclides starting from 46Sc (T 1/2 = 83.8 days) to 207Bi (T 1/2 = 31.55 years) and concluded that at lower energies the radionuclides are produced by pre-equilibrium dominated reactions, hence the product masses are near to the target nuclei such as natPb(p,xn)205Bi. But at higher energies, the masses of the product radionuclides are nearly half of the target mass; hence, the mode of production can be related to fission, fragmentation and deep spallation processes, such as, the production of 83Rb (T 1/2 = 86.2 days) and 103Ru (T 1/2 = 39.26 days). Titarenko et al. [8] made experimental as well as computer simulation studies using HETC, GNASH, LAHET, INUCL, CEM95, CASCADE and ALICE codes on the yields of residual product nuclei in 209Bi thin targets irradiated by 130 MeV and 1.5 GeV protons at the ITEP U-10 proton synchrotron. Based on the production of radionuclides by 1.5 GeV protons, they demonstrated different production channels, namely, spallation reactions (mass of the product nuclei ranged from ~125–210 amu), fission reactions (~30–140 amu); whereas the production of radionuclides with masses ~125–140 amu are most probably governed by both the channels. The number of radionuclides identified in 1.5 GeV proton bombardment on bismuth targets was around 102. Glories et al. [9] again extended the energy of the protons to 2.6 GeV on Pb targets of thickness 125 mg/cm2 at the SATURNE II synchrocyclotron of the Laboratoire National Saturne at Saclay and at the cyclotron of the Svedberg Laboratory at Uppsala and reported a wide range of radionuclides starting from 7Be (T 1/2 = 53.12 days) to 207Bi. Around 129 radionuclides were identified in the proton irradiated Pb targets. They determined 2,000 individual cross sections for 127 reactions and discussed the different modes of production of the radionuclides like multi-fragmentation, three different types of asymmetric and symmetric fission and deep spallation. Using external beam of the ITEP U-10 proton synchrotron, Titarenko et al. [10] also measured the cross section of the produced radionuclides by proton irradiation on 206,207,208,natPb and 209Bi targets of thickness 127–358 mg/cm2 for 11 proton energies within the range of 0.04–2.6 GeV and compared their results with the data obtained at other laboratories as well as with theoretical simulations by seven numerical codes. They found that the predictive powers of the codes varied but were satisfactory for most of the radionuclides in the spallation region.

Glasbrenner et al. [11] investigated the production of Po radionuclides by irradiating LBE targets of 2 mm thickness and 16 mm diameter with protons having 71 and 590 MeV energy. They found the formation of 206Po and 208Po radioisotopes in the targets and concluded that the specific activity ratio of 206Po/208Po is dependent on the energy and intensity of the proton beam. Using the FLUKA Monte Carlo code, Sunil et al. [12] estimated the neutron yield, neutron ambient dose equivalent, induced activity and the remnant gamma ambient dose equivalent for 0.1–1 GeV protons incident on thick LBE target. Radionuclides such as 195Au (T 1/2 = 186.09 days), 201Tl (T 1/2 = 72.912 h), 203Pb (T 1/2 = 51.873 h), 205Bi (T 1/2 = 15.31 days), 206Bi (T 1/2 = 6.243 days), 207Bi (T 1/2 = 31.55 years), 208Po (T 1/2 = 2.89 years), 210Po (T 1/2 = 138.37 days) and 3H were found to be the important radionuclides in the target handling perspective. Formation of radionuclides in the LBE targets is highly dependent on the energy of the protons employed as it opens several reaction channels, which were studied earlier in either Pb or Bi targets.

This article reports the first experimental study on the production of radionuclides in the 1.4 GeV proton induced reactions on thick LBE targets of different thickness and lengths. It aims to compare the production and activity of various radionuclides in variable lengths of LBE targets. This paper also discusses the production of radiotoxic Po and At radionuclides in the LBE targets.

Experimental

Preparation of the LBE targets

Eight cylindrical thick LBE targets having a fixed diameter of 6 mm with varying lengths from 1 to 8 mm were prepared by melting pure LBE pieces. The chemical composition of LBE targets is Pb:Bi = 45:55 by weight. The weight of the targets was varying from 0.43 to 2.83 g with the increase in length (Table 1). To avoid the risk of contamination from the irradiated samples, each target was sealed by two-component glue (Araldite) before irradiation. The layer of glue was about 0.5 mm covering the total surface of the LBE targets.

Table 1 Details of 1.4 GeV proton irradiation on the LBE targets
Full size table

Irradiation

The RaBIT (rapid proton beam irradiation transport) irradiation facility at CERN-ISOLDE was used to irradiate the sealed targets by 1–3 pulses of 1.4 GeV protons (Fig. 1). The RaBIT facility consists of a pneumatic transport system installed in a class A laboratory that allows to send samples in containers (referred as shuttle in this article) in front of the beam line and bring these back. Typically the shuttles are made of plastic but during these experiments it was observed that the mechanical integrity of those was compromised. Therefore, it was decided to continue the irradiations using shuttles made of a more robust material such as aluminum. The total intensity of the proton beams for the first two targets (1–2 mm length) was 3.18 × 1012 protons/sample and for the other six targets (3 to 8 mm length) was in between (3.0 and 3.43) × 1013 protons/sample. The beam intensity was measured with the help of a current integrator placed before the target. Details of 1.4 GeV proton irradiation on varying lengths of the LBE targets are shown in Table 1. Due to safety restrictions on the dose rate, targets were cooled for 24 h before the first measurement. Therefore, we lost information on the production of short-lived radionuclides; except for the short-lived daughter products of long-lived parents. A regular series of time resolved γ-spectra were collected off-line for each irradiated target for the first 15 days after proton bombardment. A p-type HPGe detector (ORTEC) of 2.1 keV resolution at 1.33 MeV in combination with digital spectrum analyzer (DSA 1000, CANBERRA) and Genie 2 k software (CANBERRA) were used for the off-line γ-ray spectrometric measurements. The energy and efficiency calibration of the detector was performed using standard sources of known activity like 152Eu (T 1/2 = 13.528 years), 133Ba (T 1/2 = 10.551 years) and 60Co (T 1/2 = 5.27 years). The short-lived product radionuclides in the 1.4 GeV proton irradiated LBE targets were identified by the characteristic photo peaks and decay data. The yields of long lived (T 1/2 > 5 days) radionuclides were calculated from the series of γ-spectra taken after 54–60 days and also after 359–368 days of the end of bombardment (EOB) using similar measurement set up. Some of the activities of the long-lived radioisotopes were not determined because of the presence of long-lived parent isotopes, compared to the daughter and improper peak positions in the gamma spectrum. It was difficult to quantify the activity of short-lived radionuclides due to the high Compton background in the initial spectra. Some of the radionuclides could not be confirmed in the sample due to overlapping photo peaks and similar half-lives with other radionuclides.

Fig. 1
figure 1

a Location of RaBIT (rapid proton beam irradiation transport) set up at the ISOLDE facility. b The RaBIT pneumatic transport system, c schematic representation of the plastic/aluminum containers (shuttle) with a loaded sample. The external diameter is 10 mm

Full size image

Results and discussion

The radioisotopes identified ranged from 7Be (53.12 days) to 207Po (5.8 h). The identified short-lived (T 1/2 < 5 days) radionuclides has been listed in Table 2. The list includes 206,207Po, 203−207Bi, 200,201,203Pb and other successive lower Z elements. Production of 7Be is questionable as it may also be produced by the interaction of 1.4 GeV proton beam with the two-component glues that was used to seal the targets. However, Gloris et al. [9] also reported production of 7Be when bombarded lead targets by 2.6 GeV protons.

Table 2 List of radioisotopes identified in 1.4 GeV proton irradiated LBE targets (T 1/2 < 5 days)
Full size table

The total numbers of identified radionuclides in target lengths 8 to 1 mm are 111, 101, 103, 98, 87, 86, 56 and 45 respectively. Hence, the number of radioisotopes in the LBE targets steadily decreased with the decrease in length of the samples of fixed diameter from 8 mm to 1 mm. This signifies that the production of radioisotopes in the LBE targets is highly dependent on the mass of the LBE target. In other word, as the target length increases more numbers of isotopes are observed. The radioisotopes and their measured activities (long-lived >5 days) at the experimental condition have been tabulated in Table 3. The activities of each radioisotope have been normalized with respect to total proton intensity of 3 × 1013. The calculated activities at EOB of the long-lived (T 1/2 > 5 days) radioisotopes produced in the LBE targets were higher in sample length 8 mm compared to other samples which also signifies that the production of radioisotopes and their yield is dependent on mass of the LBE targets.

Table 3 Produced long-lived radioisotopes (T 1/2 > 5 days) with their visible activity in 1.4 GeV proton irradiated LBE targets
Full size table

It should be kept in mind that the measured activity is not the absolute activity; even it may differ largely in some cases. This is because the activity produced inside the target is largely attenuated, even some time it can be below the detection limit. For example, from our earlier study it has been found that liquid Hg shows self-shielding of 90 % for 141 keV γ-ray of 99mTc/99Mo [13]. Therefore the activity reported in this paper is the minimum activity that would be produced in the given experimental condition and can be termed as “visible activity”. For example, due to less shielding and less self-attenuation, visible activities in sample 1 and 2 are comparatively higher. Only radiochemical treatment, i.e. dissolving the target, separation of the bulk, etc., followed by quantification should give correct estimate of the ‘actual’ activity.

The modes of production of the radionuclides include multiple mechanisms, namely, fission, fragmentation, spallation, etc., which was also observed by other scientists while using Pb or Bi as target materials at higher proton energies [7–10]. The produced radionuclides so evidenced in the proton irradiated LBE target undergo various decay processes such as electron capture, alpha decay, β- decay, isomeric transitions, etc., hence it is difficult to predict accurately the mechanism of formation of each individual radionuclides.

Po formation

Among the various Po radioisotopes, we observed only the formation of 206Po (T 1/2 = 8.8 days) and 207Po (T 1/2 = 5.8 h). Since, proton irradiated LBE targets were cooled for 24 h before the first measurement, short-lived Po radioisotopes (having T 1/2 between 2–3 h) were not identified. We could not identify 208−210Po radioisotopes due to absence of their suitable characteristic photo peaks. The 206,207Po isotopes may be produced through secondary particle formation, like 209Bi(p,π − xn)206,207At (ε)206,207Po or 204,206,207,208Pb(p,π − xn)206,207Po types of reactions, typical in thick targets, as observed and advocated by Tall et al. [14]. However, it is not possible to comment on the formation of 206At (T 1/2 = 30.6 min) or 207At (T 1/2 = 1.81 h) radionuclides based on our measurements. The 206,207Po radioisotopes may also be produced directly through 209Bi(p,4n)206Po or 209Bi(p,3n)207Po reactions. Table 3 shows that the activity of 206Po is considerably high in thicker targets, which reduces the probability of formation of polonium isotopes through secondary particle reactions, as the yield of the secondary reactions are generally smaller. Tall et al. [14] also predicted for “small” amount of Po radioisotopes that could be formed through secondary particle reactions.

At formation

We have not observed any signature of At radionuclides in any of the samples. However, it was not possible for us to observe short-lived 204–208At radionuclides due to 24 h cooling time. The high activity of 206Po does not vouch its formation through the decay of 206At. Tall et al. [14] observed the formation of 204–210At radioisotopes on irradiating thick LBE targets with 1–1.4 GeV protons at CERN-ISOLDE. The target used by them consisted of lead–bismuth contained in a tantalum cylinder of 20 cm in length and 1 cm in radius, 75 % filled, of mass 547 g. The modes of production of At radionuclides were assisted either by (p,π − xn) charge exchange reactions on 209Bi or by secondary reactions involving 3He and 4He. The masses of LBE targets used in our experiment were between 0.43 and 2.83 g, two orders of magnitude less compared to that used in ref [14]. The production yields of the secondary reactions are generally small. Therefore it may also happen that the amount of At produced through secondary particle reactions in our experiment is too tiny and below the detection limit.

Applications

As indicated earlier, the high-energy proton irradiated thick LBE targets are huge source of radionuclides, which are useful in practical applications, e.g., in the field of medical science, industry and technology. In literature, various reports are found for the production of these radionuclides by low energy nuclear reactions (5–7 MeV/u). In this section, we discuss some specific applications of the radionuclides identified in the LBE targets. For example, the long-lived 65Zn (T 1/2 = 244.3 days) is used as a tracer to study the bio-kinetics and distribution of zinc in various organs and subcellular compartment of the body [15, 16] whereas 72As (T 1/2 = 26.0 h), 86Y (T 1/2 = 14.74 h) and 89Zr (T 1/2 = 78.41 h) find their applications in PET imaging [17–19]. Efforts have been made to validate various production routes of different neutron deficient Tc radionuclides like 93−96Tc. 96Tc (T 1/2 = 4.28 days) has been proposed for use in prevention of coronary restenosis [20, 21]. The ruthenium radionuclides such as 97Ru (T 1/2 = 2.9 days) are used in cisternography. The 97Ru-bleomycin labeled compound is used in diagnosis of tumors and also as a chemotherapeutic agent [22] while 103Ru (T 1/2 = 39.2 days) is used to study the migration of 103Ru radionuclide in tumor cells and liver [23, 24]. One of the mostly used radionuclides 111In (T 1/2 = 2.80 days) finds its applications in the labeling of cellular blood components, monoclonal antibodies, myocardial damage detections, localization of abscess in polycystic kidneys, radiolabeled immunoglobulin therapies, imaging for cancer, etc. Apart from this, 111In is also used as a probe in γ–γ time differential perturbed angular correlations (TDPAC) studies [25, 26]. Similarly, rhenium radionuclide such as 181Re (T 1/2 = 19.9 h) owing to its suitable nuclear properties can be useful in radioimmunotherapy, radionuclide synovectomy, bone pain palliation, etc., [27] whereas, 101mRh (T 1/2 = 4.3 days) can be used as anti-tumor agent for radiotherapeutic use [28]. The lanthanides 153Gd (T 1/2 = 240.4 days) finds application in single photon emission computed tomography (SPECT) used for diagnosing neurological disorders, cardiac blockages or abnormalities, and primary and metastasized cancer. 153Gd is also used in industrial radiography, for ascertaining the strength and integrity of structural materials and piping systems [29–31]. Recently terbium radionuclides, especially 149Tb (T 1/2 = 4.11 h) is under focus as an excellent radionuclide for targeted therapy. However, Maiti [32] decisively showed in her recent paper that no low energy nuclear reaction, i.e. reaction using light or heavy ion projectiles are suitable for production of terbium radionuclides in large quantity required for clinical purpose [32, 33]. We found several terbium radionuclides (e.g., 151,153,154Tb, listed on Table 2), therefore it is expected that α-emitting 149Tb will also be produced in large quantities in proton irradiated LBE targets, which we could not observe because the radionuclide has no suitable γ-energies. 157Dy (T 1/2 = 8.14 h) is used as a scanning agent for bone and bone marrow [34, 35], while 169Lu (T 1/2 = 34.06 h) is the parent isotope of 169Lu/169Yb generator system. 169Yb (T1/2 = 32.02 days) is gaining interest in brachytherapy [36, 37]. 191Pt (T 1/2 = 2.802 days) is used to investigate the effect of labeled 191Pt-cisplatin in terms of the antitumor effect and general toxicity on tumor in the body [38]. 195mHg (T 1/2 = 41.6 h) is the parent radioisotope of 195mHg/195mAu generator system and has potential use in angiocardiography [39, 40]. Similarly, 205Bi (T 1/2 = 15.31 days) is used as a tracer in biodistribution studies and in the design and testing of α-emitting radiotherapeutic agents [41]. Apart from the medical uses, the other radioisotopes such as 75Se (T 1/2 = 119.8 days) are also used as tracers to study the selenium adsorption properties in bentonites (clay), migration in plants and fruits [42, 43], whereas 95Nb (T 1/2 = 34.97 days) is used as a 95Zr/95Nb chronometer for dating nuclear events [44, 45]. 133Ba (T 1/2 = 10.5 years) is used as a tracer to study perturbed angular correlation parameters in bovine serum albumin, sorption of radionuclide in estuarine sediments [46].

Conclusion

This is the first experimental study on the production of radioisotopes by the bombardment of 1.4 GeV protons on thick LBE targets and assessment of the produced radioisotopes by γ-spectroscopy. The production of radionuclides in the LBE targets depends on its mass and intensity of the 1.4 GeV proton beam. Production of 206,207Po is evident in all the samples (1–8 mm). The high activity of polonium radionuclides favors their direct production mechanism rather than production through secondary particle reactions. In the present experimental condition we have not observed any “visible” At activity. The production of various At radionuclides were observed by Tall et al. [14] in very thick LBE targets and was interpreted as secondary particle reactions. David et al. also presented model for production of At by double charge exchange reactions [47]. This experiment therefore leaves ample opportunity to define the limit of “thick” targets from where secondary particle reactions are initiated. The production of various radionuclides via direct nuclear reaction, fission, spallation, fragmentation also opens a window for future prospective regarding the complex nuclear process occurring at the high-energy proton irradiated thick LBE target.

Apart from the valuable information in fundamental science, the product radioisotopes have various applications in the field of medical sciences and technology. The 1.4 GeV proton irradiation on LBE targets for 1 s to 1 min 51 s, produced activities at the end of bombardment (EOB) is in the order of 102–103Bq for few radioisotopes such as 7Be (T 1/2 = 53.12 days), 74As (T 1/2 = 17.77 days),171Lu (T 1/2 = 8.24 days),172Lu (T 1/2 = 6.70 days), 183Ta (T 1/2 = 5.1 days) etc. Therefore, it is possible to achieve large-scale production of these radioisotopes, alternative to reactor with high specific activity.