RADIO-ACTIVATION CAUSED BY SECONDARY PARTICLES DUE TO NUCLEAR REACTIONS AT THE STRIPPER FOIL IN THE J-PARC RCS

In the J-PARC RCS, high residual doses are obtained around the stripper foils. It is caused by not primary particles due to the beam losses but secondary particles due to nuclear reaction at the foil. This radio-activation is an intrinsically serious problem for the RCS which adopts the charge exchange multi-turn beam injection scheme with the stripper foil. In this paper, we report an experimental result of suppression of the residual dose around the foil by controlling a foil fitting rate. In addition, we introduce next plan to measure the secondary particles from the foil in detail. INTRODUCTION The 3-GeV Rapid Cycling Synchrotron (RCS) of the Japan Proton Accelerator Research Complex (J-PARC) accelerates protons from 400MeV to 3GeV kinetic energy at 25 Hz repetition rate. The RCS has two functions as a proton driver for neutron/muon production at the Material and Life science experimental Facility (MLF) and as a booster of the Main Ring synchrotron (MR) for the Hadron experimental facility (HD) and Neutrino experimental facility (NU) [1]. The most important issue in achieving such a MWclass high power routine beam operation is to keep machine activations within a permissible level, that is, to preserve a better hands-on maintenance environment. Therefore we adopt the ring collimator system to remove the beam halo and to localize the beam loss at the collimator area [2]. In addition, a large fraction of our effort has been concentrated on reducing and managing beam losses, in the J-PARC RCS [3]. RESIDUAL DOSE MEASUREMENT To keep a lookout for a sharp increase of activation level in tunnel, we continue a search of high level residual doses after every stopping the beam operation. In addition, we measure the doses on upper-, lower-, inner, and outer-surfaces of vacuum chambers by using the Geiger–Muller (GM) counter. These specific dose distributions characterize the beam loss mechanisms in the RCS. Dose Distribution Along the Ring In April, 2016, serious trouble occurred to the ring collimator system. A collimator control system failure was occurred and then a vacuum leak was occurred at the secondary collimator no.5. In order to restart the user beam operation quickly, the Col-Abs. no5 was replaced for ducts with iron shield. Now, new collimator no. 5 is under construction, and it will be installed into the ring on the summer shutdown period in 2017. After the collimator trouble, we started to measure the residual doses along the ring in detail to confirm a keeping the beam loss localization [4]. Figure 1 shows the schematic view of the RCS ring and recent measurement result of the residual dose distributions along the ring. The result indicates that the beam losses can be localized in the collimator successfully and then there are not high level residual doses along the ring except around the stripper foil. Figure 1: Schematic view of the RCS ring and measured residual dose distributions along the ring. In the RCS, highest residual dose is observed around the stripper foil. The stripper foil is irradiated with not only the injecting H beam but also circulating H beam during the beam injection period. As a result of our preceding studies [5, 6], it is clear that the source of high level residual dose is not the loss of the primary particle at the stripper foil but the secondary particles (proton and neutron) produced by the nuclear reaction due to the interaction between the stripper foil and the beam. ___________________________________________ * Work supported by JSPS KAKENHI Grant Number JP16K05027 † email address yoshimoto.masahiro@jaea.go.jp TUPVA093 Proceedings of IPAC2017, Copenhagen, Denmark ISBN 978-3-95450-182-3 2300 Co py rig ht © 20 17 CC -B Y3. 0 an d by th er es pe ct iv ea ut ho rs 04 Hadron Accelerators A17 High Intensity Accelerators Figure 2: Residual doses and gamma ray spectra of the stripper foil retrieved from the ring. Radio-Activation of the Stripper Foil In the RCS, the stripper foil, which had been irradiated with the beam, can be retrieved from the ring and stored in a closed acrylic case in safety [5]. It has advantages capable of readily handling and individual analysing. Figure 2 shows the residual doses of the stripper foil and the aluminium (Al) frame observed by the GM counter and the gamma ray spectra detected by the portable germanium (Ge) detector with lead collimator. The highest dose is observed at the beam irradiation spot of the stripper foil. On the other hand, relatively high doses are also measured at another area of the stripper foil and Al frame. In order to investigate process of the nuclear reaction in the stripper foil and the frame, produced nuclei are identified from analysis of the gamma ray spectra obtained by using the Ge detector. Because lead shielding blocks are set in front of the foil and the Ge detector head is covered with lead collimators, it aims to shrink and localize the measured area. Nuclides in the stripper foils are mainly Be-7 and Na-22. Maybe Na-22 is contamination from the Al frame because it was an insufficiency piling the lead blocks up for shielding the gamma ray from the Al frame. On the other hand, Al frame is also activated and the nuclide is only Na-22. And then, it is assumed that this activation is caused by the secondary particles produced by the nuclear reactions in the foil. EFFORT TO REDUCE RESIDUAL DOSE Charge exchange multi-turn beam injection scheme with stripper foil is key technique to achieve high power beam with low beam loss during the beam injection. However, our preceding studies indicate clearly that the secondary particles generated by the nuclear reactions due to the interaction between the beam and stripper foil have to cause the high machine activation around the foil. Namely this machine activation around the stripper foil is an intrinsic problem of the charge exchange multi-turn injection with the stripper foil. Figure 3: Transition of the specific residual dose distribution around the stripper foil. Table 1: Injection Painting Parameters and Foil Parameters. 2015/03/17 ( foil hitting rate ~ 41 ) MLF 400kW (100π/100π-Cor.) MR/NU 316kW (50π/50π-Cor.) foil width 30mm foil edge position +13mm 2015/04/22 ( foil hitting rate ~ 18 ) MLF 500kW (150π/100 π -Cor.) MR/HD 24kW (100 π/100 π -Cor.) foil width 30mm foil edge position +9mm 2015/11/14 ( foil hitting rate ~ 13 ) MLF 500kW (150 π/150 π-Anti.) MR/HD 39kW (50 π/50 π-Cor.) foil width 20mm foil edge position +9mm 2017/03/17 ( foil hitting rate ~ 7 ) MLF 151kW (200 π/200 π-Anti.) MR/NU 470kW (50 π/50 π-Cor.) foil width 20mm foil edge position +9mm 0.001 0.01 0.1 1 10 100 c ou nt ra te [1 /s] 200