Reduction of contaminants originating from primary beam by improving the beam stoppers in GARIS-II
Sota Kimura, Daiya Kaji, Yuta Ito, Hiroari Miyatake, Kouji Morimoto, Peter Schury, Michiharu Wada
RReduction of contaminants originating from primary beamby improving the beam stoppers in GARIS-II
S. Kimura a, ∗ , D. Kaji a , Y. Ito b,a , H. Miyatake c , K. Morimoto a , P. Schury c , M. Wada c a Nishina Center for Accelerator Based Science, RIKEN, Wako, 351-0198, Japan b Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan c Wako Nuclear Science Center (WNSC), Institute of Particle and Nuclear Studies (IPNS),High Energy Accelerator Research Organization(KEK), Wako, 351-0198, Japan
Abstract
Two independent beam stoppers have been developed for improving the beam separation of the gas-filled recoil ionseparator GARIS-II. Performance evaluation of these supplemental beam stoppers was performed by using the
Pb( O , Th reaction. A160-fold enhancement of the signal-to-noise ratio at the GARIS-II focal plane was observed.
Keywords:
Gas-filled recoil ion separator; GARIS-II; Beam separation; Fusion-evaporation reactions
PACS: Z = 83) to Ac ( Z = 89) [8, 9] as well as isotopes from Es( Z = 99) to No ( Z = 102) [10] have already been carriedout successfully.SHE-mass-I requires the energetic beams from GARIS-II to be converted to low-energy beams before they canbe analyzed with the MRTOF-MS. This is accomplishedvia a cryogenic gas-cell (GC) and an ion transport sys-tem that includes a multiple ion trap suite [8]. Due tospace charge effects, the ion extraction efficiency of the GCwould be reduced as the incoming beam rate increases [11].A wide range of fusion evaporation reactions, with the pri-mary beam intensity exceeding 2 p µ A in some cases, havebeen used to produce exotic isotopes for the SHE-mass-Imass measurements. In several cases, unwanted particleswith non-negligible intensities were observed to reach theGARIS-II focal plane, possibly be reducing the system ef-ficiency and even posing a risk of damage to the GC. Thus ∗ Corresponding author
Email address: [email protected] (S. Kimura) an enhancement in the suppression of unwanted particleswas required. We report here on the development of suchan improved suppression system.The primary beam and the evaporation residues dif-fer in terms of their behavior in GARIS-II based on their Bρ -values and angular distributions immediately after thetarget. Their Bρ -values are generally similar to each other,as discussed above, although there remains a capability toseparate them by the small differences of their trajectoriesat the exit of the first dipole (D1) of GARIS-II. Evapora-tion residues have large angular distributions compared tothe primary beam, due mainly to multiple scattering in thetarget materials and recoils of evaporated light-particlesin de-excitation processes. As such, primary beam and Stopper2 Stopper1
MovableRotatable
Figure 1: Schematic view of the primary beam stoppers. The D1chamber is shown in cross-section. The blue arrow indicates thebeam direction. This figure presents an operational state whereStopper1 has an effective area of 24 cm while Stopper2 is at the0 cm position. Preprint submitted to NIM A Short communication July 7, 2020 a r X i v : . [ phy s i c s . i n s - d e t ] J u l igh gainLow gain × × ↓ Th O ↓ Pb elastic ↓ p i l e - u p e v e n t s ← Pb elastic
0 20 40 60 80 100 N o r m a li ze d c n t s / do s e ( a r b . u . ) Energy (MeV)10
0 5 10 15 20 N o r m a li ze d c n t s / do s e ( a r b . u . ) Energy (MeV)
Figure 2: Energy spectra of the silicon detector array. (Blue line)Energy spectrum without either stoppers or primary beam chopping.(Black line) Energy spectrum with primary beam chopping. (Redline) Energy spectrum with both stoppers and without primary beamchopping. evaporation residues could be separated by utilizing thedifferences in their angular distributions.For separating the primary beams and the evaporationresidues, two water-cooled primary beam stoppers, “Stop-per1” and “Stopper2”, were designed and installed in theD1 chamber of GARIS-II as shown in Fig. 1. Stopper1,installed at the entrance of the D1 chamber, consisted ofa copper plate of 8 cm width and 3 cm height. It waspossible to vary the effective area it presented to the beamby rotating the structure. Stopper2, located near the D1chamber exit, was made of a 40 cm wide and 6 cm highcopper board mounted on a linear manipulator to changeits position; tantalum fins on its surface prevented scatter-ing of impinging particles.The performance of the stoppers were evaluated by us-ing the
Pb ( O , Th reaction. A 4.84 MeV/u O beam with average intensity of 16 pnA was providedby the RIKEN linear accelerator RILAC. The Pb targetsegments were prepared by evaporation onto 60 µ g/cm carbon backings until an average thickness of 450 µ g/cm was achieved. The target segments were mounted on a 30cm diameter target wheel [12] that was nominally rotatedat 2000 rpm during irradiation.An array of silicon detectors (HAMAMATSU S3204-09), arranged in a 3 × Th and contaminants reaching theGARIS-II focal plane. Signals of the detector array were -3 -2 -1
0 5 10 15 20 R e l a ti v e i n t e n s it y Stopper1 effective area ( cm ) Th O PbContam 0 5 10 15 20Stopper2 position ( cm )
Figure 3: Results of both stoppers’ performance evaluations. Thefigures indicate the dependencies of the counting rates on the Stop-per1 effective area (left) and on the Stopper2 position (right). Sizeof the all error bars are smaller than the points. Red line representsthe result of
Th transport simulations. N o r m a li ze d i n t e n s it y ( a r b . u . ) Polar angle (degree) Th Pb O Th reaching Si array
Th reaching Si array under stopper1 workingy-axisx-axis
Figure 4: Simulated polar angle distribution right after the target.(Black line) Simulated result of
Th without gate. (Blue line)
Th result gated with a condition that reaching the silicon detectorarray installed at the GARIS-II focal plane. (Red lines) Same gateas the blue line, but simulated result under the assumption thatstopper1 fully works. (Green line) Simulated result of O. (Orangeline) Result of
Pb assuming zero-degree recoil. (Dotted lines)GARIS-II acceptance of x- and y-axis. See the text for the details ofsimulations. processed with both high- and low-gain circuits. The pri-mary beam was chopped to measure the α -decay from Th under low-background conditions. The beam chop-ping sequence was chosen to be 0.2 sec beam on and 0.2 secbeam off. Energy spectra as measured by both high- andlow-gain circuits are shows in Fig 2. The high-gained datawere used for counting the
Th events, where the inten-sities were determined by Gaussian fitting. The intensitiesof the contaminants were obtained from integration of thelow-gain data down to 17 MeV, which is a border betweenthe
Pb elastic events and the pile-up events of fast decayof
Th’s granddaughter
Rn ( t / = 2 . µ s). Becauseof this ambiguity, low energy contaminant events are notincluded in the present analysis. Counts of O and
Pbevents were determined via Gaussian fitting of the low-and high-gained data, respectively.The evaluation results are shown in Fig. 3. In thepresent case, the 4+ state of the O beam after the tar-2ets has a Bρ -value that is close to the optimum for Th: Bρ ( O) /Bρ ( Th) = 0 . Th of ∼
20 was de-termined without either stopper. As the Stopper1 effectivearea began to increase, the intensity of the contaminantsrapidly decreased; in contrast, the
Th counting rate var-ied only gradually (left panel of Fig. 3). As a result, wecould suppress ∼
97% of the contaminants from reachingthe focal plane by using Stopper1, while ∼
80% of
Thwas still transported. The same measurements were con-ducted by changing the Stopper2 position as shown in theright panel of Fig. 3. When measuring the Stopper2 per-formance, the Stopper1 effective area was set to its maxi-mum value of 24 cm . The Stopper2 measurement showsresults similar to those obtained with Stopper1. Thus, inthe end, we can improve the signal-to-noise ratio by a fac-tor of ∼
160 by using both stoppers.Red line shown in Fig. 3 indicates the simulated resultof
Th transport in GARIS-II based on Geant4 [13], in-cluding consideration of recoil of evaporated particle in thede-excitation process, charge exchange processes betweenthe GARIS-II buffer gas and ions, scattering by materi-als including the buffer gas, and the GARIS-II geometryincluding both stoppers and optics. The simulation re-produces the trend of reduction of
Th transmission ef-ficiency by the use of Stopper1. Figure 4 shows the sim-ulated polar angular distribution right after the target.This indicates that
Th events only having limited polarangle can reach the GARIS-II focal plane. The reductionrate of
Pb elastic events by Stopper1 can be explainedvia Fig. 4. Assuming that the
Pb events reaching theGARIS-II focal plane have the same angular distributionas the
Th events reaching the GARIS-II focal plane, thereduction rate can be represented by a ratio of the area sur-rounded by the red line to the orange-colored mesh areaand is found to be 45%. This value is consistent withthe experimental value of 47 . . O cannot be explainedin the same way. The reason for this is incompletely un-derstood and further investigation is necessary. Becausethe reaction used in the present study is very asymmet-ric and the evaporation residues have wide initial angulardistributions, the peak lies the outside of the acceptableregion. This implies that the beam stoppers could evenbetter improve the signal-to-noise ratio for more symmet-ric reactions.The contaminant events can be sorted into two cate-gories: peaks and continuous. As indicated in Fig 2, bothstoppers mainly reduce the continuous components. Forthe production mechanisms of the continuous componentshaving wide energy range, some mechanisms such as deepinelastic reaction, scattering with beam line inner walls,etc., can be considered. But it is difficult to identify theorigin of the continuous components based the informationof energy spectra only. Thus further investigations are nec-essary to understand the origin of the contaminants andachieve a maximally low-background environment at the GARIS-II focal plane.In order to improve the signal-to-noise ratio at theGARIS-II focal plane, two independent beam stoppers havebeen developed. Performance evaluations of the beamstoppers were performed by using the
Pb ( O , Threaction. This study indicates a 160-fold enhancement ofthe signal-to-noise ratio at the GARIS-II focal plane hasbeen achieved by using the beam stoppers. We demon-strate that a simple mechanism can drastically reduce thecontaminants at the GARIS-II focal plane while maintain-ing the intensity of the desired fusion-evaporation reactionproducts. An initial application of the beam stoppers isreported in [14].We would like to express our sincere gratitude to theRIKEN Nishina Center for Accelerator-Based Science andthe Center for Nuclear Science at the University of Tokyofor their support of the present measurements. This studywas supported by the Japan Society for the Promotion ofScience KAKENHI, Grant Number 24224008, 15H02096,15K05116, and 17H06090.
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