arXiv:2002.06834v2 [physics.ins-det] 10 Sep 2020 π aorp ihaqarpl assetoee [24]. spectrometer chro- gas mass a H to quadrupole by up Pure analyzed a is purified with and is [23], gas matograph distiller HD a the by 99.99% 96%, approximately is gas by [20–22]. 2005 [19] in (JLAB) University Osaka Lab at Jefferson target the experiments and physics [18] for Sandorfi BNL used the was the and at at at and 17], [13], 15], [16, Syracuse [14, ORSAY (BNL) at Laboratory The developed National been Brookhaven [12]. had mechanism target relaxation the HD the after on target work polarized frozen-spin important a as (HD) exotic of existence the and particles. mechanism, structure, hadron production the of its understanding to precious the realized provide to be that knowledge can asymmetries ex- experiments double-spin LEPS of measure the type a for new If a introduced periments, is targets. target nucleon or polarized liquid unpolarized e n oiie ylqi eimadtesldH is HD solid lique- the is and gas helium HD liquid by The gas solidified HD times. and purified the relaxation fied to long added achieve is and to catalyst, the as used ino aiu eosadbros uhas such baryons, Photoproduc- and mesons [2]. various electrons are GeV of ul- GeV 8 tion an 1.5–3.0 from of of scattering laser Compton range traviolet backward polarized energy the circularly by the produced or in Linearly beams SPring- [1]. photon at 2000 beamline since Photon Electron 8/LEPS Laser the at ments oioigtebidu fhdoe oaiainfrpolar for polarization hydrogen of build-up the Monitoring ie htteprt fcmecal vial HD available commercially of purity the that Given hydrogen–deuteride the of use the suggested Honig ehv encryn u htpouto experi- photoproduction out carrying been have We [5], K .Ohta, T. 2 3 ∗ ainlIsiue o unu n ailgclSinea Science Radiological and Quantum for Institutes National tal et a iha muto prxmtl .%is 0.1% approximately of amount an with gas 6,adhprn 71] a tde with studied was [7–11], hyperons and [6], esotnd ehv eeoe e ula antcreson magnetic nuclear new ( a times developed relaxation have We shortened. be ocnrto fteortho-H the of para-H concentration to converted were ihmgei edsrnt o ots h polarizatio was the months, T) 3 (17 H for field the strength of magnetic aging field the A high-magnetic After produ a method. the refrigerator. static of dilution the with temperature a polarization The with mK helium. 30 liquid by solidified and f76ad11Mz n ucee ntemntrn ftepola enables technique the This of T. monitoring 17 the at o-H mK in the 30 succeeded of to and 600 MHz, from 111 temperatures and 726 of xeiet tSrn-/ES ueH a ihasalamoun small a with gas HD Pure SPring-8/LEPS. at experiments esatdtedvlpeto h HD the of development the started We . erpr ntefoe-pnplrzdhydrogen–deuteride polarized frozen-spin the on report We ,2 1, .INTRODUCTION I. 2 eerhCne o ula hsc,OaaUiest,Iba University, Osaka Physics, Nuclear for Center Research 1 eateto ailg,TeUiest fTkoHospital, Tokyo of University The Radiology, of Department .Fujiwara, M. 2 n oecetyplrz h Dtre ihnasotndag shortened a within target HD the polarize efficiently to and 4 aoaUiest,Ciuak,Ngy,Aci 6-62 J 464-8602, Aichi, Nagoya, Chikusa-ku, University, Nagoya T 5 nttt fPyis cdmaSnc,Tie 12,Taiwa 11529, Taipei Sinica, Academia Physics, of Institute 1 fthe of ) ,3 2, H)treswt M t1 Tesla 17 at NMR with targets (HD) .Hotta, T. 2 1 oeue htehbtdwa pnitrcin ihteHD the with interactions spin weak exhibited that 2 and H a eue ttebgnigo h gn rcs,teaigt aging the process, aging the of beginning the at reduced was 2 Dtd etme 1 2020) 11, September (Dated: 2 uliwt w rqec wesa h epciefrequen respective the at sweeps frequency two with nuclei H .Ide, I. φ 3 4], [3, 4 .Ishizaki, K. n para-H and 1 = in h ouainrtoo o-H of ratio population the tion, eprtr.Teo-H The temperature. rto n reigo h Dsi oaiain The polarization. spin HD gen- the the of to H leads freezing nuclear period and static three-month eration the a high-magnetic a of for at strength use and field The build- temperatures low to at polarization. applied polarization is HD generated and the is solenoid, up field superconducting magnetic a T 17 by A refrigerator. dilution olddw oapoiaey2-0m iha with mK 20-30 approximately to down cooled n rcs.A h n fteaigpro,ams all almost period, aging the of end o-H the the At process. ing generate and 2 temperatures, approximately low at week 1 approximately tatmeaueo . tamgei edo for T 1 of field magnetic a at K SPring-8 experiments. 1.5 to photoproduction of transported temperature is a target can HD at field the The magnetic University, the decreased. Osaka and be increased at Once be frozen can [25]. is temperature SPring-8 polarization at spin experiments the planned the of duct n naei eksi neatoswt h D We HD. 8 the approximately the with for of time interactions relaxation spin a weak obtained in engage and osmto flqi eima aeo approximately of the rate and a long at very helium is liquid calibration period of the of consumption aging that three-month of to A signal ratio NMR the signal. polariza- final of the H estimation of the the area and on the based T the obtained months, 1 3 was to for tion decreased T 17 was [26]. at field K target 4.2 HD magnetic at the MHz of 40 aging of the frequency ap- After a at Resonance with sweeps T Magnetic 1 field proximately Nuclear magnetic with on measurements based (NMR) out carried was 2 nteps,teclbaino the of calibration the past, the In a scmoe fortho-H of composed is gas dTcnlg,Tki brk 1-15 Japan 319-1195, Ibaraki Tokai, Technology, nd 4 1 tlwtmeaue ntepeec of presence the in temperatures low at D rz.Ams l ortho-H all Almost froze. n 2 ulu,wihwsaeutl ogfrtecon- the for long adequately was which nucleus, H .Kohri, H. oeue r ovre otep-H the to converted are molecules e oi Dwsrdcdt about to reduced was HD solid ced ne(M)sse omauethe measure to system (NMR) ance fortho-H of t H)tresfrphotoproduction for targets (HD) ai sk 6-07 Japan 567-0047, Osaka raki, st piieteconcentration the optimize to us ple oteH ogo the grow to HD the to applied iainbidu tdecreasing at build-up rization 2 oy 1-65 Japan 113-8655, Tokyo µ (p-H ,4 5 4, 2, eta h einn fteag- the of beginning the at heat W zdHydrogen–Deuteride ized n time. ing 2 2 2 .Yanai, Y. ihasi of spin a with ) ( ∼ oeue aeadcytm of time decay a have molecules apan n .% a liquefied was 0.1%) 2 2 (o-H 2 2 op-H to n .Yosoi M. and molecules m can ime fthe If . 2 ihasi of spin a with ) J 1 cies 2 polarization H s31a room at 3:1 is .I addi- In 0. = 2 ± molecules months 2 3 2 He- 4 He J 2

24 L/day for the operation of the dilution refrigerator and B. New NMR system using frequency sweeps at the superconducting solenoid is costly. Most of the liquid high frequencies helium can be supplied by the Low Temperature Center of Osaka University. To ensure smooth operations, we We developed a new NMR system that operated in a purchased commercial liquid helium for use during long wide frequency range up to 726 MHz. Given that the po- holiday periods. Evaporated helium gas was returned to larization measurement was performed during the aging the Low Temperature Center for recycling. of the HD target at 17 T, the superconducting solenoid If the concentration of the o-H2 is decreased at the be- was operated with a persistent current mode, and a fre- ginning of the aging, the aging time can be shortened. quency sweep method was applied for the polarization The relaxation time of the solid HD depends on the con- measurements. centration of the o-H2, temperature, magnetic field and so on. The relaxation time of the solid HD was mea- Laptop PC PXI system Input Attenuator LabVIEW Signal generator Splitter sured in the temperature range of 1.2–4.2 K at various -20 dbm PXIe-5650 concentrations of the o-H2 [13, 29–32]. However, no mea- Coaxial Oscilloscope 0~1.3 GHz cable(2 m) surements were conducted at temperatures lower than 1 Preamp Coaxial K and at magnetic fields higher than 10 T. Although R&K cable(2 m) LA110 Network it was necessary to measure the relaxation time of the +35 dbm HD polarization within the temperature range below 1 analyzer ADC K at 17 T for the optimization of the concentration of PXIe-5162 Reference Coaxial Lock-in cable the o-H2, there were technical difficulties in monitoring 5 G Sample/s (2.5 m) amplifier Crossed the build-up of the polarization by NMR measurements Output coil at high-frequencies of approximately 700 MHz. To over- come these difficulties, we developed a new NMR system DRS2500 which could be operated within a broad frequency range C = 4.2 pF up to 726 MHz. Some brief explanations have been re- Output cable Input cable ported elsewhere [33]. Output coil Input coil R = 1.0 W R = 1.0 W C = 238 pFL = 0.893 mH L = 0.876 mH C = 238 pF

M = 4 nH II. NMR SYSTEMS

DRS2500 dilution refrigerator A. Portable NMR system with PCI eXtensions for instrumentation FIG. 1. Schematic of the new NMR system. The crossed-coil method and frequency sweeps were applied. We measure the polarization of the 1H and 2H nuclei at Osaka University and at SPring-8. In order to make reliable calibration for the polarization, it was necessary The signal generator PXI-5404 was replaced with to use the same NMR system at both sites. However, PXIe-5650 which generated radiofrequency (RF) signals the weight of the conventional NMR system was 80 kg, at frequencies up to 1.3 GHz. The analog-to-digital con- and frequent transportation of the system (which was verter (ADC) PXI-5142 was also replaced with PXIe- mounted on a rack with a height of 2 m) between Osaka 5162 which sampled the data at different rates up to 5 G University and SPring-8 was not easy. We constructed a Sample/s. Instead of the single-coil method used in the portable NMR system with an operating software system previous NMR system [26], the crossed-coil method was with PCI eXtensions for Instrumentation (PXI) [26]. The applied, as shown in Fig. 1. Given that the tuning circuit weight of the portable NMR system was only 7 kg and in the previous NMR system specified the frequency and the cost was reduced to 25%. required manual operations for each nucleus, the tuning The portable NMR system consisted of PXI-1036 circuit was not used. Although the measurements with- (chassis), PXI-8360 (connection between laptop PC out the tuning circuit resulted in poor S/N ratios, auto- and PXI), PXI-5404 (signal generator), and PXI-5142 matic NMR measurements within wide frequency ranges (ADC), which were developed by the National Instru- were performed. ments Company. This system was controlled by a Lab- VIEW program on the laptop. The frequency range of the PXI-5404 ranged from 0 to 100 MHz, and was suit- III. EXPERIMENTS able for NMR measurements of the 1H nucleus with mag- netic field sweeps at a field strength of approximately 1 A. Dilution refrigerator, target cell and NMR coils T. The signal-to-noise (S/N) ratio of the portable NMR system depended on the performance of the laptop which We used a 3He-4He dilution refrigerator (DRS2500) was used to operate it. produced by Leiden Cryogenics B.V. [27] in the Nether- 3

(a) Mixing Chamber (b) (c) Cold Input-coil Output-coil finger Vacuum Chamber Thin Al wires 500 mm 500 Coil Support H0 H H frame of 1 0 NMR 87 mm coils Coil Target cell

38 mm Support frame of NMR coils Nb3 Sn Superconducting NbTi solenoid

FIG. 2. (a) Mixing chamber of the DRS2500 dilution refrigerator with the cold finger surrounded by the superconducting solenoid made of NbTi and Nb3Sn. (b) Cross-section of the target cell and support frame of the NMR coils. (c) Structure of the support frame of the NMR coils with the directions of the magnetic field H0 of the superconducting solenoid and the applied radiofrequency (RF) field H1.

lands to cool the HD target. The DRS2500 refrigerator H2 impurity of 0.3%, was liquefied at approximately 20 has a lowest temperature of 6 mK and a cooling power K and solidified at 4.2 K in the target cell. The super- of 2500 µW at 120 mK. A strong magnetic field was conducting solenoid was excited with a current of 270 A produced by the superconducting solenoid (NbTi and to produce a magnetic field strength of 17 T, and the Nb3Sn) produced by JASTEC Co., Ltd. [28] in Japan. operation was changed to the persistent current mode. The target cell was attached to a cold finger made of NMR measurements with frequency sweeps for 1H, 2H, pure copper (99.99%) with a length of 500 mm. In turn, and 19F nuclei were initiated. The 1H and 2H nuclei were the cold finger was attached to the mixing chamber with the main components of the HD target, and the 19F nu- a lowest temperature, as shown in Fig. 2(a). A carbon cleus was contained in the target cell and support frame resistance thermo sensor was used to measure the tem- of the NMR coils. The NMR frequencies for the 1H, perature of the mixing chamber. 2H, and 19F nuclei were 726, 111, and 683 MHz, respec- The HD target cell and the support frame of the tively, at 17 T. The speed of the frequency sweeps was NMR coils, shown in Fig. 2(b, c), were made of Kel-F 0.544 MHz/s. We accumulated 100 k data points and (Poly-Chloro-Tri-Fluoro- (PCTFE)) which did estimated the average values at each frequency point. not contain any hydrogen. A Teflon-coated silver wire The temperature of the target decreased to 600 mK with a diameter of 0.3 mm was used for input signals. when the 1K pot of the DRS2500 was pumped and 3He It was wound to form a single-turn saddle coil on the gas was liquefied into the mixing chamber. When 4He support frame. The other wire was also wound on the gas was also liquefied into the mixing chamber, the tem- support frame in the perpendicular direction and served perature of the HD target became lower than 100 mK as the crossed-coil for picking up output signals. The RF and gradually dropped down to 30 mK. A heater (power field of H was produced by the coil for input signals, and 1 of 0.09 W) was applied to increase the flow of the circu- the direction of H was perpendicular to that of the mag- 1 lating 3He gas. netic field H0 produced in the superconducting solenoid. Thin aluminum wires (20% in weight of the HD target) with a purity higher than 99.999% were soldered on the target cell to insure the cooling of the solid HD.

C. Polarization B. Cooling HD target by dilution refrigerator

The DRS2500 refrigerator and superconducting The nuclear spins of 1H, 2H, and 19F nuclei are 1/2, solenoid were precooled to 77 K by liquid . After 1, and 1/2, respectively. If the population distribution the liquid nitrogen was blown out, liquid helium cooled of the spin system obeys the Boltzmann statistics, the them to 4.2 K. The HD gas (1 mol), which had an o- polarization of the 1H or 19F nucleus with the spin 1/2 4 can be calculated according to, Before the aging After the aging 7 14 N+ − N− 6 12 1 19 P H/ F = V) 5 10 N+ + N− m 4 (1) 8 µH 3 6 0 2

= tanh( ), ( Output 4 1 (mV) Output kBTTE 2 0 0 -1 where N+ and N− are the numbers of substates m=+1/2 0.87 0.871 0.872 0.873 0.874 0.875 0.871 0.872 0.873 0.874 0.875 Magnetic field (T) Magnetic field (T) and -1/2, respectively. H0 is the magnetic field, µ is the 1 19 magnetic moment of the H (2.793µN ) or F (2.629µN ) 1 −8 FIG. 3. NMR signals of the H nucleus measured with the nucleus, µN =3.152×10 eV/T, kB is the Boltzmann −5 portable NMR system before and after the aging process. constant (8.617×10 eV/K), and TTE is the temper- ature of the thermal equilibrium state. In the case of the 2H nucleus with the spin 1, the vector polarization is calculated to be respectively, when the temperature decreased from 4.2 K 19 N+ − N− to 30 mK. The intensities of the F signals increased by P 2H = a factor of approximately 3 at 30 mK. It was considered N+ + N0 + N− that the deterioration of the 1H and 19F signals at 30 4tanh( µD H0 ) (2) = 2kB TTE , mK was caused by the high-frequency signal detection 2 µD H0 difficulties. The 1H and 19F signals were clearly observed 3 + tanh ( 2kB TTE ) at magnetic fields below 7 T, however these deteriorated where N+, N0, and N− are the numbers of substates at field strengths above 7 T. m=+1, 0, and -1, respectively, and µD is the magnetic 2 moment of the H nucleus (0.857µN ). 4.2 K 30 mK 1 2 19 The polarizations of the H, H, and F nuclei at the 0.06 4 thermal equilibrium state at 17 T are calculated to be 0.04 1 0.41%, 0.08%, and 0.39% at 4.2 K, and 52%, 12%, and H 0.02 2 50% at 30 mK, respectively. Given that the magnetic 0 0 Output (V) Output Output (mV) Output 19 0.02- moment of F is close to and slightly smaller than that 725.8 725.9 726 725.8 725.9 726 726.1 of 1H, these two nuclei have similar polarizations. The 2H 0.02 nucleus has a smaller magnetic moment than those of the 0.4 1 19 2 0.01 H and F nuclei, and yields a smaller H polarization. 0 2 0.2 H -0.01 -0.02 0 IV. RESULTS 111.4 111.42 111.44 111.46 111.4 111.42 111.44 111.46

0.2 0.6 A. NMR signals 0.4 19 F 0.1 0.2

Output (mV)0 (mV) Output 1 0 Figure 3 shows NMR signals of the H nucleus mea- Output (mV) (V) Output sured before and after the aging process based on the use 682.6 682.8 683 683.2 683.4 682.6 682.8 683 683.2 683.4 of the portable NMR system. The NMR signals before Frequency (MHz) the aging were measured at 4.2 K and 1 T. The NMR sig- 1 2 19 nals of the 1H nucleus were measured again at 0.3 K and FIG. 4. NMR signals of the H, H, and F nuclei measured 1 T for comparison after the aging process at tempera- at 4.2 K (left) and 30 mK (right) at 17 T with the new NMR system (Fig. 1). tures of about 20 mK at 17 T over a three-month period. The NMR signals measured after the aging were approxi- mately 2000 times larger than those measured before the aging process. NMR signals measured with frequency sweeps at 4.2 K and 30 mK at 17 T based on the use of the new NMR B. Monitoring the build-up of the polarization system (Fig. 1) are shown in Fig. 4. The signals of 1H and 2H nuclei were small and the S/N ratios were poor Only the central regions of the NMR imaginary signals at 4.2 K. Given that the number of 19F nuclei was much in Fig. 4 were fitted with a Gaussian function, and the larger than those of the 1H and 2H nuclei, and given that signal heights were obtained. The temperature of the the 19F nuclei were located near the NMR coils where mixing chamber and the build-up of the polarization of the coil sensitivity was high, the 19F signals were clearly the 1H, 2H, and 19F nuclei are shown in Fig. 5. We observed even at 4.2 K. The intensities of the 1H and 2H succeeded in monitoring the build-up of the polarization signals became larger by approximately 100 and 30 times, during the aging process of the HD target at 17 T. 5

4He condensation finished. be approximately equal to 1.5 K. The thermal conduc- tivity of Kel-F at 500 mK was approximately 2×10−5 W/cm·K, and was adequately large for cooling the NMR 0.6 (a) support frame [34]. Insufficient cooling of the NMR sup- port frame was inferred owing to the poor thermal con- 0.4 ductivity between the support frame and the cold finger. Grease should be added to the screws for better thermal 0.2 conductivity in the next cooling attempt. The NMR signal heights within the range of 9 to 25 h

Temperature (K) Temperature were fitted by the function, 0 1 (b) H t − t0 P = P0(1 − exp(− )), (3) 4 T1

where P0, t0, and T1 are free parameters. The T1 values 2 of the 1H and 2H nuclei at 30–600 mK at 17 T were 2.96±0.03 and 7.72±0.72 h, respectively. By fitting the 1 data with narrow regions, the T1 value of the H nucleus 0 increased as time elapsed. Given that the concentration (c) 2H - of o-H2 did not decrease so fast, it was considered that the prolonged value of T1 was inferred to be caused by 0.4 the low temperature. We also carried out data analyses based on the areas 0.2 of the NMR peak regions which were estimated by inte- gration. As a result of this analysis, the relaxation times of the 1H and 2H nuclei were 3.69±0.03 and 7.90±0.12 Signal height (mV) height Signal 0 (d)19 F h, respectively. The uncertainties due to the selection of 0.8 the integration range were about 0.22 and 0.03 h for the 1 2 0.6 H and H nuclei, respectively. The 2H relaxation time was longer compared with the 0.4 1H relaxation time. In previous studies, the same results 0.2 were obtained at 1.8 K and 0.85 T [31]. At the beginning of the aging process, we used H2 as a catalyst but did not 0 use D in the HD gas, which may have led to a longer 2H 0 5 10 15 20 25 30 35 2 Elapsed Time (h) relaxation time. The NMR data were measured 12 days after the liquefi- cation and solidification of the HD. The o-H2 concentra- FIG. 5. (a) Temperature of the mixing chamber measured by tion was estimated to decrease from 0.3% to 0.05% during a carbon resistance thermometer. The NMR signal heights of 1 2 19 the measurements. Given that the relaxation times of the (b) H, (c) H, and (d) F nuclei at the beginning of the aging 1 2 of the HD target at 17 T. The solid curves are the results of H and H nuclei were found to be short enough, the o- the fits obtained with the use of function (3). H2 concentration of 0.05% should be reduced to shorten the aging time of the HD target.

The 3He condensation was completed at 0 h, while the V. SUMMARY AND FUTURE OUTLOOK condensation 4He was initiated at 2.5 h and was com- pleted at 9 h. The temperature of the mixing chamber In order to optimize the amount of o-H2 in the HD decreased from 600 to 30 mK during the 4He condensa- target and the aging time, we developed a new NMR tion. The 1H polarization started to grow at 9 h. The system, and succeeded in the monitoring of the build-up polarization grew up to its maximum value within 1 day. of the polarizations of the 1H and 2H nuclei at 17 T. The The 2H polarization also started to grow at 9 h. The polarizations were found to grow within 1 day when the speed of the growth of the 2H polarization was slower temperature decreased from 600 to 30 mK. The relax- than that of the 1H polarization. Both the 1H and 2H ation times of the 1H and 2H nuclei at 30–600 mK and polarizations became approximately 10 times larger at 17 T were obtained as 2.96±0.03(stat)±0.73(syst) and 30 mK than those at 600 mK. The 19F polarization be- 7.72±0.72(stat)±0.18(syst) h, respectively, where the dif- came larger by only 10% when the temperature decreased ferences between the results of two different analyses from 600 mK to 30 mK. Given that the NMR signals of are considered as the systematic uncertainties. The o- 19 F at 30 mK are larger than those at 4.2 K by approxi- H2 concentration of 0.05% was excessively large for the mately 3 times, the actual temperature was estimated to build-up of the polarizations. Accordingly, in future 6 work, we will optimize the concentration of o-H2, and ACKNOWLEDGMENTS shorten the traditionally used three-month aging period. In addition, the present frequency sweep method will The presented study involved the conduct of experi- be useful for the monitoring of the polarization of the ments at the BL33LEP of SPring-8 with the approval HD target during the photoproduction experiments at of the Japanese Synchrotron Radiation Research Insti- SPring-8. tute (JASRI) as the contract beam line (Proposal No. In these experiments, we also observed the NMR sig- BL33LEP/6001). We are grateful to the staff of the Low nals of the 27Al and 35Cl nuclei. Recently, we started Temperature Center of Osaka University for supplying developing a polarized 139La target for a T-violation ex- us with the required liquid helium. We thank Dr. J.-P. periment with polarized neutron beams at J-PARC. The Dideletz, Dr. S. Bouchigny, Dr. G. Rouille, Professor present NMR system can be used for a broad range of G. Frossati, Dr. N. R. Hoovinakatte, Dr. A. M. San- frequencies, including the frequency of 102 MHz for the dorfi, Dr. X. Wei, Dr. M. M. Lowry, and Dr. T. Kageya 139La nucleus at 17 T. Although the maximum frequency for their important advice. We also thank Professor K. in this experiment was 726 MHz, the present NMR sys- Fukuda, Dr. T. Kunimatsu, and Professor M. Tanaka, tem can generate and observe signals at higher frequen- for the construction of the primary NMR system and for cies up to 1.3 GHz. The present technique would play the provision of some additional modules. The present important roles not only for the target development of work was supported in part by the Ministry of Educa- HD but also for various other polarized nuclear target tion, Science, Sports, and Culture of Japan, and by the developments. National Science Council of the Republic of China.

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