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Status of the low-energy super-heavy element facility at RIKEN
Nuclear Instruments and Methods in Physics Research B 376 (2016) 425–428
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Status of the low-energy super-heavy element facility at RIKEN P. Schury a,⇑, M. Wada a, Y. Ito a, F. Arai a,b, D. Kaji a, S. Kimura b, K. Morimoto a, H. Haba a, S. Jeong d, H. Koura e, H. Miyatake d, K. Morita a, M. Reponen a, A. Ozawa b, T. Sonoda a, A. Takamine a, H. Wollnik c a
RIKEN, Nishina Center for Accelerator Based Science, Wako City, Saitama, Japan Institute of Physics, University of Tsukuba, Tsukuba City, Ibaraki, Japan c Dept. Chemistry and BioChemistry, New Mexico State University, Las Cruces, NM, USA d Institute of Particle and Nuclear Studies (IPNS), High Energy Accelerator Research Organization (KEK), Ibaraki 305-0801, Japan e Advanced Science Research Center, Japan Atomic Energy Agency, Ibaraki 319-1195, Japan b
a r t i c l e
i n f o
Article history: Received 2 September 2015 Received in revised form 25 February 2016 Accepted 26 February 2016 Available online 22 March 2016 Keywords: Atomic masses SHE Gas cell
a b s t r a c t In order to investigate nuclei produced via fusion–evaporation reactions, especially super-heavy elements (SHE), we have begun construction of a facility for conversion of fusion–evaporation residues (EVR) to low-energy beams. At the base of this facility is a small cryogenic gas cell utilizing a traveling wave RF-carpet, located directly following the gas-filled recoil ion separator GARIS-II, which will thermalize EVRs to convert them into ion beams amenable to ion trapping. We present here the results of initial studies of this small gas cell. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction The present procedure for identification of newly synthesized super-heavy element (SHE) isotopes relies on their decay to known isotopes via a chain of a-decays. But, as the nuclei become more stable near the ‘‘island of stability”, they will become ever more likely to undergo b-decay [1]. Being a three-body reaction, bdecay does not provide the distinct fingerprint that a-decay does. Additionally, the lifetimes should increase, which could make identification via decay more difficult and prone to errors. A more immediate difficulty can already be seen in element113, 115, and 117. In 278 113 Uut and lighter SHE nuclei adecays connect to well-studied, directly synthesizable nuclei, the well-known a-decay chains providing unique fingerprints for identifying the parent nucleus. Heavier elements, produced by socalled hot-fusion, exhibit a-decay chains that terminate in spontaneously fissioning nuclei before reaching well-known isotopes. As such, absolute determination of their identity is greatly hindered. One option for overcoming current and future identification issues would be to move from identification via decay spectroscopy to identification via mass spectroscopy.
⇑ Corresponding author at: Wako Nuclear Science Center (WNSC), Institute of Particle and Nuclear Studies (IPNS), High Energy Accelerator Research Organization (KEK), 2-1 Hirosawa, Wako City, Saitama 351-0198, Japan. E-mail address: [email protected] (P. Schury). http://dx.doi.org/10.1016/j.nimb.2016.02.061 0168-583X/Ó 2016 Elsevier B.V. All rights reserved.
At present, however, atomic masses have been directly measured for only six transFermium nuclei — 252255 No and 255256 Lr [2–4] — with the SHIPTRAP Penning trap [5] at GSI, while recent measurements from TRIGA-TRAP have indirectly improved the mass landscape among transFermium nuclei [6]. While Penning trap mass spectroscopy (PTMS) can achieve tremendous mass resolving power for long-lived and light nuclei, it has some drawbacks for low-yield, short-lived and heavy nuclei. For short-lived heavy nuclei, the maximum revolving power achievable by PTMS is limited by the lifetime of the nucleus to be studied. For example, in the case of 260 No (T 1=2 =102 ms), SHIPTRAP would be limited to a mass resolving power of Rm 82,000 in the case of doubly charged ions (even less for singly charged ions). Furthermore, in TOF-ICR PTMS, a measurement requires sufficiently populating a time-offlight ion cyclotron resonance curve; typically N 100 ions are required to do so; FT-ICR PTMS [7] can use single ions, but has thus far only been amenable to use with ions produced in-trap. While the drawbacks of PTMS are not extremely detrimental to the feasibility for mass measurements of No and Lr, for which production cross-sections are large [2.3 lb and 437 nb in the cases of 208 Pb (48 Ca, 2n)254 No and 209 Bi(48 Ca, 2n)255 No, respectively] and halflives are generally greater than one second [51 s and 31 s for 254 No and 255 Lr, respectively], it will become a strong hindrance for systematic measurements of even Rf isotopes, whose typical half-lives and production cross-sections are both much smaller [16 nb for 208 Pb(50 Ti, 2n)256 Rf, T 1=2 ¼ 6:64 ms]; the new method
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P. Schury et al. / Nuclear Instruments and Methods in Physics Research B 376 (2016) 425–428
of PI-ICR [8] may partially mitigate these restrictions. To fully bypass these drawbacks, we are implementing a multi-reflection time-of-flight mass spectrograph (MRTOF). The MRTOF has a limited mass resolving power, currently Rm 175; 000. However, it can achieve that resolving power in only a few milliseconds for even very heavy ions [9], thereby being able to access even the shortest-lived bound isotopes. Furthermore, as a true spectrograph, it can in principle provide a mass measurement even for extremely low-yield species, as the detection of every individual ion can be viewed as an independent mass measurement. The MRTOF will be the end-stage of a facility for low-energy SHE. It will be preceded by a series of ion traps to prepare the ions for use with the MRTOF, which are further preceded by a small gas cell used to thermalize fusion–evaporation residues (EVR) to convert them to ion beams amenable to ion trapping. In order to use the MRTOF for identification of SHE will require a system with high-efficiency. We have previously reported the performance of one set of ion traps [10] and the MRTOF [9]. Herein we report the performance of the first stage component for such a facility, a small cryogenic gas cell utilizing a traveling-wave RF-carpet [11,12]. 2. Setup The facility will consist of a small (1.8 L) gas cell utilizing a traveling wave RF carpet, a triplet RF ion trap for initial ion storage, a drift-tube based transport region, another triplet RF ion trap for ion preparation, and an MRTOF mass spectrograph. In addition to these primary components, the system will be equipped with multiple beam monitoring stations — Si PIN diodes for a-decay and channel electron multipliers (CEM) for ion counting — as well as a pair of offline ion sources — one at the backside of the initial trap triplet and one at the backside of the final trap triplet. A schematic overview of the planned facility is shown in Fig. 1.
The facility will be located following the gas-filled recoil ion separator GARIS-II [13]. Primary beams with energy <6 MeV/A, accelerated by the heavy ion linac RILAC, impinge thin target windows on a rotating wheel upstream of GARIS-II. The desired EVRs are separated from projectile and target-like fragments by GARIS-II. The gas cell, located 20 cm from the GARIS-II exit window (0.5 lm mylar), is a reentrant chamber within the GARIS-II focal plane vacuum chamber and explicitly designed for cryocooling. Between the GARIS-II exit window and the gas cell’s 2.5 lm mylar entrance window (supported by a 0.6 mm thick stainless steel honeycomb pattern) is a retractable Si PIN diode array for measuring the incoming beam (particle identification and beam profile determination) and a 160 mm 130 mm 4 lm rotatable mylar degrader. The gas cell is mounted inside the GARIS-II focal plane chamber. It is supported from above by the large copper block of a cryocooler cold head. The focal chamber vacuum provides thermal insulation. Inside the gas cell chamber, a Kapton printed circuit board is rolled into a 100 mm diameter tube. The circuit board, consisting of 46 copper strips connected by a series of surface-mount resistors, provides a drift field to transport ions to the RF carpet located at the far end of the chamber. The RF carpet consists of a series of concentric rings (gold surface) of 80 lm width with 80 lm spacing, with a maximum diameter of 80 mm; beyond the last ring is a 10 mm wide annular shield ring. It is printed on 45 lm Kapton. At the center of the circuit board is a 0.36 mm diameter hole for extracting ions. The RF carpet is affixed by TorrSealÒ to an Aluminum Nitride block, mounted on the back wall of the gas cell chamber. A carbon-fiber RF sextupole ion guide (SPIG) is mounted after the RF carpet. The SPIG is supported from the back wall of the focal plane chamber to minimize thermal load on the gas cell. A recess in the back of the Aluminum Nitride block allows the SPIG to be inserted to within 1 mm of the carpet. The presently reported results were made using a Si PIN diode following the top trap triplet (see Fig. 1). This Si PIN diode is on a ladder which includes, additionally, an offline Cs thermal ion source and a CEM.
3. Method
Fig. 1. Schematic overview of the SlowSHE facility for low-energy SHE ions.
A 1 plA 40 Ar11+ primary beam with energy 4.825 MeV/A, accelerated by the heavy ion linac RILAC, impinged thin target windows on a rotating wheel upstream of GARIS-II. The target wheel contained 4 windows of Ta foils (1 lm thickness) and 12 windows of 0.29 mg/cm2 169 Tm sputtered onto 3 lm Ti foils. A rotating shadow wheel was used to ensure that only the desired target windows were irradiated at any given time [14]. When the projectile beam interacts with the target, the projectile and target nuclei can fuse to form a compound nucleus in an excited state. The compound nucleus will then rapidly de-excite by particle emission, typically multiple neutrons (xn), a proton and multiple neutrons (pxn), or an a-particle and multiple neutrons (axn) are emitted. The remaining evaporation residue (EVR), in this case, will pass out from the target with an energy of 30 MeV and an energy spread of several percent. In the case of 169 Tm(40 Ar), the beam at center of target had an energy of 143 MeV and the 4n-channel reaction cross-section should have been 545 lb [15]. For 181 Ta(40 Ar) the beam at center of target had an energy of 180 MeV and the 4n-channel reaction crosssection should have been 1.1 lb [15]. The products were separated in-flight from projectiles and target-like particles using GARIS-II. The separator was filled with Helium gas at 73 Pa. Magnetic rigidity for each reaction product was set to be 1.669 Tm. The ions passed through an exit window of 0.5 lm thick mylar upon leaving GARIS-II.
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To tune GARIS-II, perform initial particle identification, and determine the incoming rate for later use in efficiency determinations, an array of Si PIN diodes (see Fig. 2) was inserted between the rotatable degrader and the GARIS-II exit window. The Si PIN diodes were calibrated using the a-decays of 205 Fr and its daughter 201 At. By examining the observed a-decay spectrum the various nuclei delivered by GARIS-II and the rate of each can be determined. An elastic scattering monitor near the target provides a constant non-invasive monitor of the incoming beam rate. By simultaneously measuring the a-decay spectrum and recording the elastic scattering rate, we could determine normalization factors for each species delivered by GARIS-II. After tuning GARIS-II and identifying the incoming EVR ions via a-decay, and carefully determining the elastic scattering normalization factor for each species, the detector array was removed and ions were allowed to enter the gas cell. The gas cell was initially pressurized to 10 kPa at room temperature. Just prior to the online studies, a Cs thermal ion source installed in the gas cell was used to optimize the transport from the gas cell to the Si PIN diode following the trap triplet; no trapping conditions were applied. The RF amplitudes of the various RF multipole transport components were initially scaled to achieve first-order optimal conditions for the evaporation products. The degrader was initially set rotated to the optimal value as calculated with SRIM [16]. The ladder at the end of the first trap triplet was set to place the Si PIN diode directly in front of the trap assembly. Once an a-decay signal was observed on the downstream detector, the amplitudes of each of the RF multipole transport components were adjusted to optimize the transport. With optimal conditions present, various parameters were systematically adjusted to observe their effect. The most prominent of these are described below. As seen in Fig. 2, the upstream detector array does not ideally overlap with the gas cell. The beam shape seen on the upstream detector array was typically much wider along the x-axis than along the y-axis, with the top and bottom row registering less than 10% of the counts as the central row while the outer columns still registered >70% as many counts as the central column. This makes assigning gas cell efficiency a little tricky. We have chosen to calculate the efficiency based on the ions which actually could enter the gas cell, by prorating the counts registered on the outermost Si PIN diodes based on the fractional area which overlaps with the gas cell entrance window while ions impinging on the dead region between detectors are taken into account by means of 2D Gaussian fitting.
Fig. 2. Drawing of the upstream Si PIN array. The array is larger than the gas cell drift tube, represented by the solid circle. The dashed circle shows the area of the RF carpet. A smoothed color relief plot showing the beam profile gated on a-decays of 205 Fr is superimposed on the image.
4. Results When using the 169 Tm target, the 4n channel EVR 205 Fr was the most intense isotope observed in the upstream Si PIN diode array. The second-most intense isotope was the 3n channel EVR 206 Fr with 1% of the total intensity. Using 205 Fr we investigated the efficiency as a function of the degrader thickness. The results, shown in Fig. 3, indicate a peak efficiency of 28(3)% where the uncertainty is based on counting statistics, which is lower than the 100% observed offline for Cs thermal ions. In addition, the optimal thickness was found to be 1 lm thicker than SRIM calculations indicated. The results in Fig. 3 were performed with attenuated primary beam to limit the incoming rate to 100 cps. To investigate possible space charge effects, the degrader was set to the optimal thickness and the efficiency was measured as a function of incoming ion rate by adjusting the primary beam attenuator; incoming rate was determined from the elastic scattering monitor. It was observed, at room temperature, that the efficiency decreased slightly as the rate increased toward incoming beam rates of 104 cps. After this, the cryocooler was turned on until the system cooled below 0 °C and then the measurement was repeated. The results, shown in Fig. 4, indicate that the cooling slightly improved efficiency while mitigating the possible observed space charge effects. After investigating the space charge effect with the gas cell temperature near 0 °C, the cryocooler was turned back on and the gas cell was allowed to cool to 45 °C to investigate possible further increases in efficiency with reduced temperature. No such increase was observed. The efficiency was measured again as a function of degrader thickness (Fig. 3, red filled points) and found to agree completely with the curve observed at room temperature. When using the Ta foil target, several EVR channels were observed. The highest intensity was seen in the a4n and an channel EVRs 213 Ac (T1/2 = 738 ms) and 214 Ac (T1/2 = 8.2 s) and the p4n channel EVR 216 Th (T1/2 = 26 ms). Also observed were the p2n channel EVR 215 Ac (T1/2 = 170 ms) and the 4n channel EVR 217 Pa (T1/2 = 3.5 ms). The observed a-decay spectrum can be seen in Fig. 5; lighter species are presumed to be decay products. Due to the rich spectrum, and to better ensure calibration, two measurements were made at the upstream detector array. First the primary beam was cycled on and off with a 40 s period and 50% duty cycle for 12 min, the data is shown in green on Fig. 5(top).
Fig. 3. Efficiency as a function of degrader thickness measured for
205
Fr.
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P. Schury et al. / Nuclear Instruments and Methods in Physics Research B 376 (2016) 425–428
With the beam being pulsed with a 40 ms cycle, the a-decay spectrum downstream was also measured for 30 min, as shown in Fig. 5(bottom). The resolution of the downstream detector was not as good as the upstream detector. Nonetheless, we could clearly observe a similar spectrum to that seen upstream. Such measurements were made for various degrader thicknesses and, unlike in the case of Fig. 3, a flat response curve was seen for the extraction efficiency in each case. This may be due to the use of a much thicker target material, which would reduce the average energy to well-below that of 205 Fr for these isotopes, while also broadening the energy distribution. Despite this, we could place lower limits on the efficiency for these shorter-lived species. In the case of 213—215 Ac and 216 Th we can set a lower efficiency limit of >10%. For 217 Pa we can set a lower limit of >3% — which is quite good considering the extremely short half-life. In all likelihood, a thinner degrader would have increased these efficiencies. 5. Conclusions and outlook Fig. 4. Efficiency as a function of incoming beam rate measured for
(Coutns/Elastic) / 2.653 keV
10
Fr.
-3 213Ra (2.7 m) 209Fr (50 s)
10
205
-4
209Rn
208Rn
212Ra 214Ac (13 s) (8.2 s)
211Fr
214Ra
(3.1 m)
(2.4 s)
213Ac (738 ms) 216Th (26 ms)
(28 m) (24 m)
205At (26 m)
215Ac (170 ms)
10-5
217Pa ? (3.5 ms)
10-6
Acknowledgements
102
209Fr:
6646 (50 s) 7215 (8.2 s)
Counts / 1.496 keV
6537 (3.1 m)
214Ac:
7081 7137 (2.4 s)
201At:
6342 (85 s)
214Ra:
1
We wish to express gratitude to the Nishina Center for Accelerator Research and the Center for Nuclear Science at Tokyo University for their support of online measurements. This work was supported by the Japan Society for the Promotion of Science KAKENHI (Grant Nos. 2200823, 24224008, 24740142, and 15K05116).
214Ac:
211Fr:
10
213Ac:
7364 (738 ms)
212Ra:
6899 (13 s)
References
215Ac:
7,600 (170 ms)
216Th:
7,922 (26 ms)
217Pa:
8,336 (3.5 ms)
100 5000
We have tested a small gas cell located after the gas-filled recoil ion separator GARIS-II, using a isotopes of a variety of elements. We observed an efficiency of 30% for 205 Fr and more than 10% for 213215 Ac, and 216 Th. For the extremely short-lived isotope 217 Pa we observed and efficiency of at least 3%, providing evidence of the rapid extraction from the gas cell. These efficiencies will be remeasured using thinner target materials along with a wider range of degrader thicknesses in summer 2016. Space charge effects were also investigated. At room temperature, a slight reduction in efficiency was observed for incoming ion rates approaching 104 cps, but the reduction was not observed when the gas cell was cooled to 0 °C. The next stage of development will be to transport ions from the gas cell to the MRTOF and perform mass measurements. Mass measurements will initially be performed with the beams presented here. After demonstrating success with these subUranium beams, we will move on to No, Lr, and Rf.
5500
6000
6500
7000
7500
8000
8500
Eα (keV) Fig. 5. (top) The a-decay spectra observed on the upstream detector array. Green data comes from 40 s primary beam pulse cycle, while red curve used 40 ms primary beam pulse cycle. (bottom) The a-decay spectra observed on the downstream detector array with 40 ms primary beam pulse cycle. Half-lives of noted isotopes are given parenthetically. Characteristic a-decay energies are given in the bottom panel.
Then, the primary beam was similarly pulsed with a 40 ms cycle and 50% duty cycle (red data) for 60 min. In this way, short-lived species could easily be distinguished.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
A.V. Karpov et al., Int. J. Mod. Phys. E 21 (2012) 1250013. M. Block, Int. J. Mass Spec. 349–350 (2013) 94–101. M. Dworschak et al., Phys. Rev. C 81 (2010) 064312. M. Block et al., Nature 463 (2010) 785. M. Block et al., Eur. Phys. J. D 45 (2007) 39. M. Eibach et al., Phys. Rev. C 89 (2014) 064318. E. Myers, Int. J. Mass Spec. 349–350 (2013) 107–122. S. Eliseev, K. Blaum, M. Block, C. Droese, M. Goncharov, E. Minaya Ramirez, D.A. Nesterenko, Yu.N. Novikov, L. Schweikhard, Phys. Rev. Lett. 110 (2013) 082501. P. Schury, M. Wada, Y. Ito, F. Arai, S. Naimi, T. Sonoda, H. Wollnik, V.A. Shchepunov, C. Smorra, C. Yuan, Nucl. Instr. Meth. Phys. Res. B 335 (2014) 39–53. Y. Ito et al., Nucl. Instr. Meth. Phys. Res. Sect. B 317 (Part B) (December 2013) 544–549. F. Arai, Y. Ito, M. Wada, P. Schury, T. Sonoda, H. Mita, Int. J. Mass Spectrom. 362 (2014) 56–58. G. Bollen, Int. J. Mass Spectrom. 299 (2011) 131. D. Kaji, K. Morimoto, N. Sato, A. Yoneda, K. Morita, Nuclear Instr. Meth. Phys. Res. Sect. B, Part B 317 (15 2013) 311–314. D. Kaji, K. Morimoto, Nucl. Instr. Meth. Phys. Res. Sect. A 792 (21 2015) 11–14. D. Vermeulen et al., Z. Phys. 318 (1984) 157–169. James F. Ziegler, M.D. Ziegler, J.P. Biersack, Nucl. Instr. Meth. Phys. Res. B 268 (2010) 1818–1823.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Status of the low-energy super-heavy element facility at RIKEN P. Schury a,⇑, M. Wada a, Y. Ito a, F. Arai a,b, D. Kaji a, S. Kimura b, K. Morimoto a, H. Haba a, S. Jeong d, H. Koura e, H. Miyatake d, K. Morita a, M. Reponen a, A. Ozawa b, T. Sonoda a, A. Takamine a, H. Wollnik c a
RIKEN, Nishina Center for Accelerator Based Science, Wako City, Saitama, Japan Institute of Physics, University of Tsukuba, Tsukuba City, Ibaraki, Japan c Dept. Chemistry and BioChemistry, New Mexico State University, Las Cruces, NM, USA d Institute of Particle and Nuclear Studies (IPNS), High Energy Accelerator Research Organization (KEK), Ibaraki 305-0801, Japan e Advanced Science Research Center, Japan Atomic Energy Agency, Ibaraki 319-1195, Japan b
a r t i c l e
i n f o
Article history: Received 2 September 2015 Received in revised form 25 February 2016 Accepted 26 February 2016 Available online 22 March 2016 Keywords: Atomic masses SHE Gas cell
a b s t r a c t In order to investigate nuclei produced via fusion–evaporation reactions, especially super-heavy elements (SHE), we have begun construction of a facility for conversion of fusion–evaporation residues (EVR) to low-energy beams. At the base of this facility is a small cryogenic gas cell utilizing a traveling wave RF-carpet, located directly following the gas-filled recoil ion separator GARIS-II, which will thermalize EVRs to convert them into ion beams amenable to ion trapping. We present here the results of initial studies of this small gas cell. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction The present procedure for identification of newly synthesized super-heavy element (SHE) isotopes relies on their decay to known isotopes via a chain of a-decays. But, as the nuclei become more stable near the ‘‘island of stability”, they will become ever more likely to undergo b-decay [1]. Being a three-body reaction, bdecay does not provide the distinct fingerprint that a-decay does. Additionally, the lifetimes should increase, which could make identification via decay more difficult and prone to errors. A more immediate difficulty can already be seen in element113, 115, and 117. In 278 113 Uut and lighter SHE nuclei adecays connect to well-studied, directly synthesizable nuclei, the well-known a-decay chains providing unique fingerprints for identifying the parent nucleus. Heavier elements, produced by socalled hot-fusion, exhibit a-decay chains that terminate in spontaneously fissioning nuclei before reaching well-known isotopes. As such, absolute determination of their identity is greatly hindered. One option for overcoming current and future identification issues would be to move from identification via decay spectroscopy to identification via mass spectroscopy.
⇑ Corresponding author at: Wako Nuclear Science Center (WNSC), Institute of Particle and Nuclear Studies (IPNS), High Energy Accelerator Research Organization (KEK), 2-1 Hirosawa, Wako City, Saitama 351-0198, Japan. E-mail address: [email protected] (P. Schury). http://dx.doi.org/10.1016/j.nimb.2016.02.061 0168-583X/Ó 2016 Elsevier B.V. All rights reserved.
At present, however, atomic masses have been directly measured for only six transFermium nuclei — 252255 No and 255256 Lr [2–4] — with the SHIPTRAP Penning trap [5] at GSI, while recent measurements from TRIGA-TRAP have indirectly improved the mass landscape among transFermium nuclei [6]. While Penning trap mass spectroscopy (PTMS) can achieve tremendous mass resolving power for long-lived and light nuclei, it has some drawbacks for low-yield, short-lived and heavy nuclei. For short-lived heavy nuclei, the maximum revolving power achievable by PTMS is limited by the lifetime of the nucleus to be studied. For example, in the case of 260 No (T 1=2 =102 ms), SHIPTRAP would be limited to a mass resolving power of Rm 82,000 in the case of doubly charged ions (even less for singly charged ions). Furthermore, in TOF-ICR PTMS, a measurement requires sufficiently populating a time-offlight ion cyclotron resonance curve; typically N 100 ions are required to do so; FT-ICR PTMS [7] can use single ions, but has thus far only been amenable to use with ions produced in-trap. While the drawbacks of PTMS are not extremely detrimental to the feasibility for mass measurements of No and Lr, for which production cross-sections are large [2.3 lb and 437 nb in the cases of 208 Pb (48 Ca, 2n)254 No and 209 Bi(48 Ca, 2n)255 No, respectively] and halflives are generally greater than one second [51 s and 31 s for 254 No and 255 Lr, respectively], it will become a strong hindrance for systematic measurements of even Rf isotopes, whose typical half-lives and production cross-sections are both much smaller [16 nb for 208 Pb(50 Ti, 2n)256 Rf, T 1=2 ¼ 6:64 ms]; the new method
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P. Schury et al. / Nuclear Instruments and Methods in Physics Research B 376 (2016) 425–428
of PI-ICR [8] may partially mitigate these restrictions. To fully bypass these drawbacks, we are implementing a multi-reflection time-of-flight mass spectrograph (MRTOF). The MRTOF has a limited mass resolving power, currently Rm 175; 000. However, it can achieve that resolving power in only a few milliseconds for even very heavy ions [9], thereby being able to access even the shortest-lived bound isotopes. Furthermore, as a true spectrograph, it can in principle provide a mass measurement even for extremely low-yield species, as the detection of every individual ion can be viewed as an independent mass measurement. The MRTOF will be the end-stage of a facility for low-energy SHE. It will be preceded by a series of ion traps to prepare the ions for use with the MRTOF, which are further preceded by a small gas cell used to thermalize fusion–evaporation residues (EVR) to convert them to ion beams amenable to ion trapping. In order to use the MRTOF for identification of SHE will require a system with high-efficiency. We have previously reported the performance of one set of ion traps [10] and the MRTOF [9]. Herein we report the performance of the first stage component for such a facility, a small cryogenic gas cell utilizing a traveling-wave RF-carpet [11,12]. 2. Setup The facility will consist of a small (1.8 L) gas cell utilizing a traveling wave RF carpet, a triplet RF ion trap for initial ion storage, a drift-tube based transport region, another triplet RF ion trap for ion preparation, and an MRTOF mass spectrograph. In addition to these primary components, the system will be equipped with multiple beam monitoring stations — Si PIN diodes for a-decay and channel electron multipliers (CEM) for ion counting — as well as a pair of offline ion sources — one at the backside of the initial trap triplet and one at the backside of the final trap triplet. A schematic overview of the planned facility is shown in Fig. 1.
The facility will be located following the gas-filled recoil ion separator GARIS-II [13]. Primary beams with energy <6 MeV/A, accelerated by the heavy ion linac RILAC, impinge thin target windows on a rotating wheel upstream of GARIS-II. The desired EVRs are separated from projectile and target-like fragments by GARIS-II. The gas cell, located 20 cm from the GARIS-II exit window (0.5 lm mylar), is a reentrant chamber within the GARIS-II focal plane vacuum chamber and explicitly designed for cryocooling. Between the GARIS-II exit window and the gas cell’s 2.5 lm mylar entrance window (supported by a 0.6 mm thick stainless steel honeycomb pattern) is a retractable Si PIN diode array for measuring the incoming beam (particle identification and beam profile determination) and a 160 mm 130 mm 4 lm rotatable mylar degrader. The gas cell is mounted inside the GARIS-II focal plane chamber. It is supported from above by the large copper block of a cryocooler cold head. The focal chamber vacuum provides thermal insulation. Inside the gas cell chamber, a Kapton printed circuit board is rolled into a 100 mm diameter tube. The circuit board, consisting of 46 copper strips connected by a series of surface-mount resistors, provides a drift field to transport ions to the RF carpet located at the far end of the chamber. The RF carpet consists of a series of concentric rings (gold surface) of 80 lm width with 80 lm spacing, with a maximum diameter of 80 mm; beyond the last ring is a 10 mm wide annular shield ring. It is printed on 45 lm Kapton. At the center of the circuit board is a 0.36 mm diameter hole for extracting ions. The RF carpet is affixed by TorrSealÒ to an Aluminum Nitride block, mounted on the back wall of the gas cell chamber. A carbon-fiber RF sextupole ion guide (SPIG) is mounted after the RF carpet. The SPIG is supported from the back wall of the focal plane chamber to minimize thermal load on the gas cell. A recess in the back of the Aluminum Nitride block allows the SPIG to be inserted to within 1 mm of the carpet. The presently reported results were made using a Si PIN diode following the top trap triplet (see Fig. 1). This Si PIN diode is on a ladder which includes, additionally, an offline Cs thermal ion source and a CEM.
3. Method
Fig. 1. Schematic overview of the SlowSHE facility for low-energy SHE ions.
A 1 plA 40 Ar11+ primary beam with energy 4.825 MeV/A, accelerated by the heavy ion linac RILAC, impinged thin target windows on a rotating wheel upstream of GARIS-II. The target wheel contained 4 windows of Ta foils (1 lm thickness) and 12 windows of 0.29 mg/cm2 169 Tm sputtered onto 3 lm Ti foils. A rotating shadow wheel was used to ensure that only the desired target windows were irradiated at any given time [14]. When the projectile beam interacts with the target, the projectile and target nuclei can fuse to form a compound nucleus in an excited state. The compound nucleus will then rapidly de-excite by particle emission, typically multiple neutrons (xn), a proton and multiple neutrons (pxn), or an a-particle and multiple neutrons (axn) are emitted. The remaining evaporation residue (EVR), in this case, will pass out from the target with an energy of 30 MeV and an energy spread of several percent. In the case of 169 Tm(40 Ar), the beam at center of target had an energy of 143 MeV and the 4n-channel reaction cross-section should have been 545 lb [15]. For 181 Ta(40 Ar) the beam at center of target had an energy of 180 MeV and the 4n-channel reaction crosssection should have been 1.1 lb [15]. The products were separated in-flight from projectiles and target-like particles using GARIS-II. The separator was filled with Helium gas at 73 Pa. Magnetic rigidity for each reaction product was set to be 1.669 Tm. The ions passed through an exit window of 0.5 lm thick mylar upon leaving GARIS-II.
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To tune GARIS-II, perform initial particle identification, and determine the incoming rate for later use in efficiency determinations, an array of Si PIN diodes (see Fig. 2) was inserted between the rotatable degrader and the GARIS-II exit window. The Si PIN diodes were calibrated using the a-decays of 205 Fr and its daughter 201 At. By examining the observed a-decay spectrum the various nuclei delivered by GARIS-II and the rate of each can be determined. An elastic scattering monitor near the target provides a constant non-invasive monitor of the incoming beam rate. By simultaneously measuring the a-decay spectrum and recording the elastic scattering rate, we could determine normalization factors for each species delivered by GARIS-II. After tuning GARIS-II and identifying the incoming EVR ions via a-decay, and carefully determining the elastic scattering normalization factor for each species, the detector array was removed and ions were allowed to enter the gas cell. The gas cell was initially pressurized to 10 kPa at room temperature. Just prior to the online studies, a Cs thermal ion source installed in the gas cell was used to optimize the transport from the gas cell to the Si PIN diode following the trap triplet; no trapping conditions were applied. The RF amplitudes of the various RF multipole transport components were initially scaled to achieve first-order optimal conditions for the evaporation products. The degrader was initially set rotated to the optimal value as calculated with SRIM [16]. The ladder at the end of the first trap triplet was set to place the Si PIN diode directly in front of the trap assembly. Once an a-decay signal was observed on the downstream detector, the amplitudes of each of the RF multipole transport components were adjusted to optimize the transport. With optimal conditions present, various parameters were systematically adjusted to observe their effect. The most prominent of these are described below. As seen in Fig. 2, the upstream detector array does not ideally overlap with the gas cell. The beam shape seen on the upstream detector array was typically much wider along the x-axis than along the y-axis, with the top and bottom row registering less than 10% of the counts as the central row while the outer columns still registered >70% as many counts as the central column. This makes assigning gas cell efficiency a little tricky. We have chosen to calculate the efficiency based on the ions which actually could enter the gas cell, by prorating the counts registered on the outermost Si PIN diodes based on the fractional area which overlaps with the gas cell entrance window while ions impinging on the dead region between detectors are taken into account by means of 2D Gaussian fitting.
Fig. 2. Drawing of the upstream Si PIN array. The array is larger than the gas cell drift tube, represented by the solid circle. The dashed circle shows the area of the RF carpet. A smoothed color relief plot showing the beam profile gated on a-decays of 205 Fr is superimposed on the image.
4. Results When using the 169 Tm target, the 4n channel EVR 205 Fr was the most intense isotope observed in the upstream Si PIN diode array. The second-most intense isotope was the 3n channel EVR 206 Fr with 1% of the total intensity. Using 205 Fr we investigated the efficiency as a function of the degrader thickness. The results, shown in Fig. 3, indicate a peak efficiency of 28(3)% where the uncertainty is based on counting statistics, which is lower than the 100% observed offline for Cs thermal ions. In addition, the optimal thickness was found to be 1 lm thicker than SRIM calculations indicated. The results in Fig. 3 were performed with attenuated primary beam to limit the incoming rate to 100 cps. To investigate possible space charge effects, the degrader was set to the optimal thickness and the efficiency was measured as a function of incoming ion rate by adjusting the primary beam attenuator; incoming rate was determined from the elastic scattering monitor. It was observed, at room temperature, that the efficiency decreased slightly as the rate increased toward incoming beam rates of 104 cps. After this, the cryocooler was turned on until the system cooled below 0 °C and then the measurement was repeated. The results, shown in Fig. 4, indicate that the cooling slightly improved efficiency while mitigating the possible observed space charge effects. After investigating the space charge effect with the gas cell temperature near 0 °C, the cryocooler was turned back on and the gas cell was allowed to cool to 45 °C to investigate possible further increases in efficiency with reduced temperature. No such increase was observed. The efficiency was measured again as a function of degrader thickness (Fig. 3, red filled points) and found to agree completely with the curve observed at room temperature. When using the Ta foil target, several EVR channels were observed. The highest intensity was seen in the a4n and an channel EVRs 213 Ac (T1/2 = 738 ms) and 214 Ac (T1/2 = 8.2 s) and the p4n channel EVR 216 Th (T1/2 = 26 ms). Also observed were the p2n channel EVR 215 Ac (T1/2 = 170 ms) and the 4n channel EVR 217 Pa (T1/2 = 3.5 ms). The observed a-decay spectrum can be seen in Fig. 5; lighter species are presumed to be decay products. Due to the rich spectrum, and to better ensure calibration, two measurements were made at the upstream detector array. First the primary beam was cycled on and off with a 40 s period and 50% duty cycle for 12 min, the data is shown in green on Fig. 5(top).
Fig. 3. Efficiency as a function of degrader thickness measured for
205
Fr.
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With the beam being pulsed with a 40 ms cycle, the a-decay spectrum downstream was also measured for 30 min, as shown in Fig. 5(bottom). The resolution of the downstream detector was not as good as the upstream detector. Nonetheless, we could clearly observe a similar spectrum to that seen upstream. Such measurements were made for various degrader thicknesses and, unlike in the case of Fig. 3, a flat response curve was seen for the extraction efficiency in each case. This may be due to the use of a much thicker target material, which would reduce the average energy to well-below that of 205 Fr for these isotopes, while also broadening the energy distribution. Despite this, we could place lower limits on the efficiency for these shorter-lived species. In the case of 213—215 Ac and 216 Th we can set a lower efficiency limit of >10%. For 217 Pa we can set a lower limit of >3% — which is quite good considering the extremely short half-life. In all likelihood, a thinner degrader would have increased these efficiencies. 5. Conclusions and outlook Fig. 4. Efficiency as a function of incoming beam rate measured for
(Coutns/Elastic) / 2.653 keV
10
Fr.
-3 213Ra (2.7 m) 209Fr (50 s)
10
205
-4
209Rn
208Rn
212Ra 214Ac (13 s) (8.2 s)
211Fr
214Ra
(3.1 m)
(2.4 s)
213Ac (738 ms) 216Th (26 ms)
(28 m) (24 m)
205At (26 m)
215Ac (170 ms)
10-5
217Pa ? (3.5 ms)
10-6
Acknowledgements
102
209Fr:
6646 (50 s) 7215 (8.2 s)
Counts / 1.496 keV
6537 (3.1 m)
214Ac:
7081 7137 (2.4 s)
201At:
6342 (85 s)
214Ra:
1
We wish to express gratitude to the Nishina Center for Accelerator Research and the Center for Nuclear Science at Tokyo University for their support of online measurements. This work was supported by the Japan Society for the Promotion of Science KAKENHI (Grant Nos. 2200823, 24224008, 24740142, and 15K05116).
214Ac:
211Fr:
10
213Ac:
7364 (738 ms)
212Ra:
6899 (13 s)
References
215Ac:
7,600 (170 ms)
216Th:
7,922 (26 ms)
217Pa:
8,336 (3.5 ms)
100 5000
We have tested a small gas cell located after the gas-filled recoil ion separator GARIS-II, using a isotopes of a variety of elements. We observed an efficiency of 30% for 205 Fr and more than 10% for 213215 Ac, and 216 Th. For the extremely short-lived isotope 217 Pa we observed and efficiency of at least 3%, providing evidence of the rapid extraction from the gas cell. These efficiencies will be remeasured using thinner target materials along with a wider range of degrader thicknesses in summer 2016. Space charge effects were also investigated. At room temperature, a slight reduction in efficiency was observed for incoming ion rates approaching 104 cps, but the reduction was not observed when the gas cell was cooled to 0 °C. The next stage of development will be to transport ions from the gas cell to the MRTOF and perform mass measurements. Mass measurements will initially be performed with the beams presented here. After demonstrating success with these subUranium beams, we will move on to No, Lr, and Rf.
5500
6000
6500
7000
7500
8000
8500
Eα (keV) Fig. 5. (top) The a-decay spectra observed on the upstream detector array. Green data comes from 40 s primary beam pulse cycle, while red curve used 40 ms primary beam pulse cycle. (bottom) The a-decay spectra observed on the downstream detector array with 40 ms primary beam pulse cycle. Half-lives of noted isotopes are given parenthetically. Characteristic a-decay energies are given in the bottom panel.
Then, the primary beam was similarly pulsed with a 40 ms cycle and 50% duty cycle (red data) for 60 min. In this way, short-lived species could easily be distinguished.
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