Nuclear Instruments and Methods in Physics Research B 317 (2013) 227–230

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Nuclear Instruments and Methods in Physics Research B

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First online production of radioactive ion beams at VECC

0168-583X/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.nimb.2013.06.040

⇑ Corresponding author. Tel.: +91 3323184462.E-mail address: vaishali@vecc.gov.in (V. Naik).

Vaishali Naik a,⇑, Alok Chakrabarti a, Mahuwa Bhattacharjee a, Prasanta Karmakar a, Sampa Bhattacharjee b,Arup Bandyopadhyay a, Siddhartha Dechoudhury a, Dodi Lavanya Kumar a, Manas Mondal a, H.K. Pandey a,T.K. Mandi a, D.P. Dutta a, Tapatee Kundu Roy a, Debasis Bhowmik a, Dirtha Sanyal a, Ayan Ray a,Md. Sabir Ali a, S.C.L. Srivastava a, P.Y. Nabhiraj a

a Variable Energy Cyclotron Centre (VECC), Sector-1 Block-AF, Bidhan Nagar, Kolkata 700064, Indiab UGC-DAE CSR, Kolkata Centre, III/LB-8, Bidhan Nagar, Kolkata 700098, India

a r t i c l e i n f o

Article history:Received 24 April 2013Received in revised form 31 May 2013Accepted 19 June 2013Available online 4 July 2013

Keywords:RIBECRRadioactive ion beamGas-jet recoil transport

a b s t r a c t

Radioactive ion beams (RIB) have been recently accelerated for the first time at the VECC-RIB facility.Beams of 14O (71 s), 42K (12.4 h), 43K (22 h) and 41Ar (1.8 h) have been produced by bombarding 1 atmnitrogen and argon gas targets with 1 micro-ampere proton and alpha particle beams from the K130cyclotron. Radioactive atoms were transported 15 m away to the ECR ion-source using a gas-jet transportsystem. Typical measured intensity of RIB at the separator focal plane is few times 103 pps. About3300 pps of 1.4 MeV 14O was measured after acceleration through a 3.4 m long RFQ linac. The detailsof the gas-jet coupled ECR and RIB production experiments are presented.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

An ISOL type radioactive ion beam (RIB) facility is being builtaround the K130 cyclotron at VECC. The aim is to deliver ions withenergy in the range from 1 keV/u to 1 MeV/u for mass numberA � 100. The aim is complete the R&D steps involved in develop-ment of accelerators for the proposed national RIB facility ANURIB(Advanced National Facility for Unstable and Rare Isotope Beams)[1] planned at the upcoming new campus of VECC. The physicsexperiments with the present facility will focus on nuclear spec-troscopy of short-lived nuclei and material science studies. Apartfrom this, the facility will be used as a test bench for developmentof new beams, on-line studies on yields from thick targets and tar-get-ion-source characterization.

A general layout of the RIB facility is shown in Fig. 1. The RIBfacility consists of three stages – production, ionization/separationand post-acceleration. Radioactive atoms will be produced bybombarding thick targets with proton or alpha-particle beamsfrom the cyclotron and ionized to 1+ charge state in an integratedtarget/ion-source placed in the cyclotron vault. The low energy 1+RIB will be thereafter transported 3 m across the shielding wall tothe RIB hall and injected into a charge breeder.

The target/ion-source for the 1+ RIB will be a surface ionizersimilar to the pioneering ISOLDE design [2] or a coil based ECR

(electron cyclotron resonance) ion-source with internal target sim-ilar to that at Ganil [3]. Both these target/ion-sources are in thedevelopment stage but the n+ ion-source, separator and post-accelerators have already been installed. In parallel, thick targetdevelopment activities have also been started. Major thrust isbeing put on development of thorium oxide and uranium oxidetargets aimed at photo-fission production of radioactive ion beamsin the ANURIB facility.

The n+ charge breeder is a 6.4 GHz ECR ion-source and leads totwo beam lines. First is the post-acceleration beam line, presentlydelivering beams up to energy of 415 keV/u. The second, materialscience beam-line delivers 29 keV/u beams at the end of a33.7 MHz Radio Frequency Quadrupole (RFQ) linac. More detailsof the facility are described in earlier papers [4,5].

So far only beams of beta-stable isotope such as 16O, 40Ar, 14N,12C, 39K and 56Fe have been accelerated in our facility. While thetarget/ion-source and beam-line in the cyclotron vault is gettingready, we have been working on a new R&D aimed at using thewell known gas-jet recoil transport (GJRT) method [6,7] to feedthe reaction products directly into the ECR ion-source. The tech-nique could be potentially useful for transport of reactive andnon-volatile elements that do not easily diffuse out of thick targets.

The GJRT technique has been extensively used by us at VECC fornuclear spectroscopy of short-lived nuclei [8,9]. The gas-jet systemhas been adapted with the necessary modifications needed for cou-pling with the ECR ion-source. Recently the gas-jet coupled ECRhas been successfully tested online. Here we report the first results

Fig. 1. General layout of the RIB facility at VECC. The arrangement of Faraday cups (FC1–FC3) and gamma-ray detectors for the RIB production experiments is also shown.

228 V. Naik et al. / Nuclear Instruments and Methods in Physics Research B 317 (2013) 227–230

on the acceleration of radioactive ion beams at our facility usingthe gas-jet technique.

2. Results

2.1. Production of beams of 14O, 42,43K and 41Ar

In the GJRT method nuclear reaction products recoiling out ofthe target are thermalized in a gas and are transported with aero-sols via a thin capillary tube to a collection station, in this case theECR ion-source. Previous studies at Grenoble [10] and Heidelberg[11] indicate that the aerosols, moving at sonic velocity in thejet, are not effectively stopped in the ECR plasma and no significanttransport of radioactive ions was observed after the ion-source inthese studies.

Keeping previous studies in mind we used a catcher inside theECR plasma chamber to stop the aerosols. The catcher is made fromcopper coated reticulated vitreous carbon fiber (RVCF) with 4Xcompression [12]. RVCF has been previously used for RIB produc-tion targets and it was presumed that it should withstand the plas-ma environment. The catcher was optimally positioned inside theplasma chamber to obtain minimum microwave reflected power.Prior to online experiments, the system was tested off-line for sta-ble isotopes of potassium.

In a series of cyclotron runs, radioactive isotopes of 41Ar, 42K and43K were produced in 40Ar(a, xpyn) reactions by bombarding argongas target by 40 MeV alpha-particle beam whereas 14O was pro-duced by bombarding 11 MeV protons on nitrogen gas target. Typ-ical proton and alpha-particle beam intensity on target was aroundone micro-ampere and the target pressure was maintained at 1 kg/cm2. The reaction products were transported from the targetchamber to the ECR ion-source via a 15 m long, 1.4 mm innerdiameter tygon capillary.

The low energy RIB was selected in the isotope separator down-stream of the ECR ion-source and gamma-ray activity was mea-sured in HPGe detectors placed at the locations indicated inFig. 1. For the measurements at the ECR entry, the gas-jet wasstopped on a removable stopper whereas after ECR extraction thebeam was stopped on the Faraday cups FC1–FC3 and gamma-rayspectra were measured in detectors placed close to the stopperand Faraday cups. The radioactive isotopes were identified fromtheir characteristic gamma-rays and by following their half-lives.A typical gamma-ray spectrum for 14O measured at FC2 is shownin Fig. 2.

The intensity for the RIB measured at the separator focal planewas few times 103 pps for all the beams except 41Ar. Table 1 givesthe list of RIBs developed and their intensity at various locations.The intensity of the RIB has been deduced from the measured gam-ma-ray intensity after correcting for the detector efficiency andusing the Ic values from the Table of Isotopes. The ratio of measuredintensity at FC1 and before the ECR gives the combined efficiencyfor release from catcher, ionization and extraction from the ion-source. This is measured to be around 15 ± 5% and 21 ± 8% respec-tively for oxygen and potassium radioactive ion beams. It may bementioned that the gamma-ray peak for 41Ar was very weak inthe detector placed at ECR entry. This is expected since it is knownthat rare gas atoms do not easily attach to the aerosol particles andare transported as it is. These atoms therefore do not stick to thestopper surface and may be pumped out. But once inside the ECRplasma chamber, the 41Ar atoms got ionized.

Because of the high gas load from the target chamber, the typ-ical vacuum measured at the ECR extraction end was around8 � 10�6 mbar during the online measurements. The microwavepower in the ECR was kept at 10–15 W as the ion-source wasbecoming unstable at higher power and only 1+ radioactive ionswere extracted in these on-line tests.

In a separate experiment, RIB of 14O was accelerated to 100 keV/u through the RFQ linac. The target chamber gas pressure was keptat 0.8 kg/cm2 and 2+ ions of 14O were transported through the RFQusing 14N2+ as the pilot beam. The intensity for 14O2+ beam wasmeasured as 5 � 103 and 3.3 � 103 pps respectively before andafter the RFQ.

2.2. Preliminary results on ionization of fission products in gas-jetcoupled ECR

To test the catcher technique for non-volatile elements wetransported fission products, produced by bombarding 16 lm thick232Th target with 50 MeV alpha-particle beam, to the ECR ion-source. The target chamber was pressurized with 1 kg/cm2 heliumand fission fragments were stopped on the catcher as in the previ-ous experiments. The beam was extracted from the ECR ion-sourceat 10 kV. The ion-source was quite stable with helium plasma andwas operated with higher microwave power of 100 W. The totalextracted beam current was measured on a Faraday cup FC1 placedat the einzel lens focus after the ECR ion-source. Decay gamma-rayspectra were recorded in HPGe detectors placed at ECR entry andnear the Faraday cup at ECR exit respectively.

Fig. 2. Gamma-ray spectrum measured for 14O at the separator focal plane.

Table 1Intensity of radioactive ion beams measured in this study.

RIB Production route T1/2 I(pps) @ ECR entry I(pps) @ ECR exit Combined efficiencya(%) I(pps) @ separator focal plane

14O 14N(p, n) 71 s 4.4 � 105 6.7 � 104 15 ± 5 5.0 � 103

42K 40Ar(a,pn) 12.36 h 1.5 � 105 3.1 � 104 20.6 ± 8 2.7 � 103

43K 40Ar(a,p) 22.3 h 9.4 � 104 2.0 � 104 21.2 ± 8 1.2 � 103

41Ar 40Ar(a,2pn) 109 min – 4.6 � 103 – 1.3 � 102

a Combined efficiency for catcher + ionization + extraction from ECR = ratio of intensity before ECR and at FC1.

Fig. 3. A portion of gamma-ray spectrum from the decay of fission products measured in HPGe detectors placed at (top) before and (bottom) after the ion-source.

V. Naik et al. / Nuclear Instruments and Methods in Physics Research B 317 (2013) 227–230 229

A portion of the measured spectra for the two cases is shown inFig. 3. The spectrum before/after the ion-source has been measuredfor duration of 120/75 min respectively, with the beam continu-ously on the target. A large fraction of gamma-rays seen beforethe ECR are detected in the detector placed after the ion-source.This indicates that many of the species have diffused through thecatcher and got ionized in the ion-source. However, due to largenumber of species and charge state mixing it was not possible tounambiguously identify the fission products after the analyzingmagnet. Also, at FC1 there will be a contribution from decay ofdaughter products. Therefore assignment of the observed gam-ma-rays is tentative. The preliminary data however indicates that

mainly volatile elements may have been transported. We plan torepeat the measurement in the future to systematically study thesuitability of the technique for various fission products. The planis to use Tungsten coated RVCF catchers with various porosities.Also, a tape transport system will be used to reduce the back-ground from longer-lived isotopes and daughter activities.

3. Summary

We have presented details of a gas-jet transport and catchertechnique for on-line production of RIB in an ECR ion-source. Thetechnique has been tested off-line for stable isotopes and on-line

230 V. Naik et al. / Nuclear Instruments and Methods in Physics Research B 317 (2013) 227–230

measurements have been done for fusion reaction recoils and fis-sion products from light-ion induced reactions using the K130cyclotron beams. Radioactive ion beams of gaseous and alkali met-als such as 14O (71 s), 42K (12.4 h), 43K (22.2 h) and 41Ar (1.8 h) havebeen produced for the first time at VECC using the technique. Acombined efficiency of 15–21% has been measured for diffusionthrough the catcher, ionization and extraction of RIB through theECR ion-source. Preliminary data from alpha-particle induced fis-sion of 232Th indicates the possibility of many fission products dif-fusing through the catcher and getting ionized in the ECR plasma.

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