Rn• removal for ultra low background liquid xenon base detector
Masaki• Yamashita
Kamioka• Observatory, ICRR, the University of Tokyo
April/05/2017 XsSAT2017, Khon Kaen, Thailand
Masaki Yamashita 1 Motivation –Rn is one of the major background for ,especially, the future dark matter experiments.
Masaki Yamashita 2 Motivation –Rn is one of the major background for ,especially, the future dark matter experiments. –Currently it has X 10 or more than pp solar neutrino and the goal for future experiments is to reduce 1/100 or more. ~10 μBq/kg (=> ~1x10-4 dru)
Masaki Yamashita 3 Motivation –Rn is one of the major background for ,especially, the future dark matter experiments. –Currently it has X 10 or more than pp solar neutrino and the goal for future experiments is to reduce 1/100 or more. ~10 μBq/kg (=> ~1x10-4 dru) –Dark matter detector can be used for not only WIMP search but also –neutrino physics such as pp/7Be, double electron capture, double beta decay –axion or axion like particle –model independent search by annual modulation and so on. –It is very important to reduce Rn background in the future experiment.
Masaki Yamashita 4 μ v μ v ° v ° °v v v ° vy × × z ° w w p μ × v v × q ° × r ×μ × μ v ° × %( 5]&WT × v ×w × / )WT J 4EE < x ° v ( 5]&WT × °v () WRH v × ×w — × ×w p ° × × × v × w ° ° v/ × ×w v w ° × v v)( ×w Background Budged for low background LXe detector J 4EE / )WT v ~ 1/100 ° BG in the fducial volume before any kind of PID • pp solar neutrino (ν+e -> ν+e ) ×wμ ° x • 2νββ 136Xe ° × × I< C ± x –it can be reduce if you have depleted Xe. L( ,Mw • 85 × ()0JR × Kr Ea R I< C ° –The goal is less than 0.1 ppt. v () WRH– Less than a few ppt Kr in Xe was achieved by some groups. (e.g. distillation) × × • 222Rn background v –214Pb daughter beta decay v /%)h %,Y5]&WT .Y5] –Currently, X 10 or more than pp Solar neutrino w v ( J 4EE • vUltimate background will be neutrino coherent scattering(solar atmospheric) Masaki Yamashita 5 ) × (1 v v v × (&( × w L M ° v ) ×w74 4 AN< FX ( ° × vI< C J 4EE L,M GJ vJ8ABA × × — ×w ;6 vEGEK v EGEK I< C v x v w v ° J 4EE × °v μ × ×w ( wJ 4EE × × v × v v × v x (&( ×w ×w Comparison of background rate
■ Background rate in the fiducial volume before separation of nuclear recoils from e/γ.
-4 ■ XMASS achieved O(10 ) event/ XMASS day/kg/keVee at a few 10’s keV. XMASSXMASS ■ Even modest background at low energy, XMASS has good sensitivity with a large mass (832 kg) and low energy threshold. (~ 1keVee) by annual modulation search. Added to D.C.Malling thesis (2014) Fig.
Masaki Yamashita PTEP 2014,063C01 H.Uchidaetal.
Table 1. Signal efficiencies with their systematic errors for deriving the limit shown in Figs. 4 and 7.Therowstartingfrom(a)isbasedonRef.[29], and the one starting from (b) on Ref. [33]. WIMP mass (GeV) 20 50 100 300 1000 3000 5000 (a) signal efficiency (%) 23 7 29 4 26 2 19 1 16 1 15 1 15 1 ±6 ±5 ±4 ±3 ±3 ±3 ±3 (b) signal efficiency (%) 24 7 30 2 29 2 26 2 25 2 25 2 25 2 ±6 ±5 ±4 ±5 ±5 ±5 ±5
used in Ref. [29], it can be seen in Fig. 1 that the shape of this distribution for a 50 GeV WIMP does not change much with the use of the more modern form factors. These cut values and the signal Downloaded from window optimized for the 50 GeV WIMPs were also used to obtain the limits for the other WIMP masses. Our signal efficiency is defined as the ratio between the number of simulated events remain- ing after all cuts in the 36–48 keV signal region and the number of simulated events generated within
K. Abe et al. / Physics Letters B 719 (2013) 78–82 the fiducial volume81 (radius less than 15cm, containing 41kg of LXe). As shown in Table 1, sig- nal efficiency ranges from 29% for 50 GeV WIMPs to 15% for 5 TeV WIMPs for the nuclear form http://ptep.oxfordjournals.org/ factors given in Ref. [29].
5. Results and discussion As clearly visible in Fig. 3,thecutsdiscussedintheprevioussectionalmosteliminateallbackground in and around the signal window. After all cuts, 5 events remain in our 36–48 keV signal region. The main contribution to the remaining background in this energy region stems from the 222Rn daughter at Kokusai Hoken Keikakugaku (UNIV OF TOKYO) on June 6, 2014 214Pb. From our simulation we estimate this background alone to contribute 2.0 0.6 events. As ± other background contributions are smaller but less certain, we do not subtract background when calculating our limits. Our detector’s low background allows us to directly use the event count in Fig. 6. WIMP signal acceptance efficiency after data reduction for the analysis. the signal region to extract our limit on the inelastic scattering cross section of WIMPs on 129Xe select these events a time-of-flight correction is made to the tim- nuclei. Using Eq. 6 and taking into account the nuclear form factor and our signal efficiency we ing distribution of each event assuming the event vertex is on derive a 90% C.L. upper limit for this cross section, which in Fig. 4 is compared to the result from the surface of the PMT closest to the charge-weighted center of gravity of the event. After this correction the timing distribution Fig. 7. Simulated WIMP energy spectra in the XMASS3.7 detectorRefs. assuming [ Annual12, the13 maxi-]. The gray modulation band reflects our systematic uncertainties. The systematic uncertainty on of Cherenkov-like events is found to be narrower than that for mum cross section that provides a signal rate no larger than the observation in any scintillation-like events. Events with more than 60% of their PMT bin above 0.3 keVee. Search by XMASS hits occurring within the first 20 ns of the event window are re- The annual modulation of] the WIMP rate on a target detector is induced by the Earth’s moved as Cherenkov-like. The ratio of the number of PMT hits b within the first 20 ns relative to the total number of hits in the motion around the Sun[47].) [p 2 The expected129 nuclear+ 129Xe* recoil energy spectrum depends on the as I 10 χ + Xe à χ Solar axion event window for all events (head-to-total ratio) is shown in Fig. 4. σ WIMP velocity distribution and on the Earth’s velocity129 in129 the galactic frame, v (t). It Each step of the data reduction is shown in Fig. 5. Xe* à Xe + γ r The expected WIMP acceptance efficiency of these cuts was varies along the year due to the expression, estimated with the detector simulation. In the simulation WIMP recoil energy spectra were generated for each WIMP mass and MC events were distributed uniformly throughout the detector volume vE(t)=Vsun +Vearth cosγ cosω(t t0), (3.39) using a liquid scintillation decay constant of 25 ns [16]. Fig. 6 10 − shows the resulting signal acceptance efficiency at energies be- low 1 keVee. The size of the error bars comes primarily from the Low Mass where Vsun = 232 km/s is the Sun’s velocity with respect to the halo, Vearth = 30 km/s is systematic uncertainty in the xenon scintillation decay constant, 25 1ns,whichisestimatedbasedonthedifferencebetweenthe the Earth’s orbital velocity around the Sun on a plane with inclination γ = 60◦ respect to ± Asymptotic cross section ( XMASS model [16] and the NEST model [17] based on [18].Asys- the galactic one, ω = 2π/T with T = 1 year and t 2nd June. Fig.3.5 illustrates the tematic error on the selection efficiency is determined based on 0 1 the error resulting from a linear fit to the points in the figure. At motion of the Earth relative to the Galactic10 coordinates.2 10 The3 annual modulation signature the 0.3 keVee analysis threshold this error is 6.1%. is the one of the most strong evidence of WIMPs, butWIMP the total mass event[GeV] variation is only Phys. Lett. B 724 (2013) 463 ∼ 4. Results and discussion %, a large mass detector is needed for statistics. as Fig. 4. The black solid line isInelastic our 90% C.L. upper limit scattering on the asymptotic cross section σI for inelastic scat- 129 Fig. 8. Spin-independent elastic WIMP-nucleon cross sectionteringFig.3.6 as a function on shows ofXe WIMP using the the expected same form factorsspectrum as DAMA. in Jun The 2nd, gray Dec band 4th covers and its their variation rate with difference. our systematic In Fig. 7 shows simulated WIMPs energy spectra overlaid on the light mass WIMP mass. All systematic uncertainties except that from eff are taken into account in observed spectrum after the data reduction was applied. WIMPs L uncertainty. The dotted line is the limit obtained by the DAMA group [12,13]. It was derived after statisti- the XMASS 90% C.L. limit line. The effect of the thiseff uncertainty calculation, on the limit is the following value were used. L PTEP 2014, 063C01 are assumed to be distributed in an isothermal halo with vo shown in the band. Limits from other experiments andcally favored subtracting regions are also background. Our low background allows us to derive this limit without such background = Phys. Lett. B 719 (2013) 78 220 km/s, a galactic escape velocity of v 650 km/s, and an shown [4–9]. coherent ν-n scattering esc = subtraction. 5 average density of 0.3GeV/cm3.Inordertosetaconservative Cross section to WIMP for SI case is 1.0 10− pb. upper bound on the spin-independent WIMP-nucleon cross sec- nuclear-recoil and electronic events, and many• events remain in × tion, the cross section is adjusted until the expected event rate the analysis sample, the present result excludes part of the param- in XMASS does not exceed the observed one in any energy bin eter space favored by other measurements [4–6] whenWIMP those datamass is 50 GeV. 7/11 above 0.3 keVee. Implementing the systematic errors discussed in are interpreted as a signal for light mass WIMPs.• Finally, we are the text above, the resulting 90% confidence level (C.L.) limit de- working on modifications to the inner surface of XMASS, especially VE in Jun 2nd is 247 km/s. rived from this procedure is shown in Fig. 8.Theimpactofthe around the PMTs, to improve the detector performance.• uncertainty from is large in this analysis, so its effect on the Leff super-WIMPs(ALPs) limit is shown separately in the figure. Acknowledgements VE in Dec 4th is 217 km/s. Supernova After careful study of the events surviving the analysis cuts, • their origins are not completely understood. Contamination of 14C We gratefully acknowledge the cooperation of Kamioka Mining in the GORE-TEX® sheets between the PMTs and the support struc- and Smelting Company. This work was supported by the Japanese ture may explain a fraction of the events. Light leaks through this Ministry of Education, Culture, Sports, Science and Technology, material are also suspect. Nonetheless, the possible existence of a Grant-in-Aid for Scientific Research, and partially by the National Rare decay search WIMP signal hidden under these and other backgrounds cannot Research Foundation of Korea Grant funded by the Korean Govern- be excluded. Although no discrimination has been made between ment (NRF-2011-220-C00006). Double electron capture ν K-shell X-ray
Dec. 4th o 60 o 232 km/s
Sun Jun. 2nd Earth
ν K-shell X-ray Phys. Rev. Lett. 113 (2014) 121301 anual modulation arXiv:1511.04807v1 arXiv:1510.00754Masaki Yamashita, ICRR, Univ of Tokyo
Figure 3.5: The motion of the Earth relative to the Galactic coordinates.
27 Rn background in the dark matter community
Experiment LXe Mass[kg] 222Rn[μBq/kg]
XMASS 832 9.8±0.6 XENON100 62 33.4±1.3 LUX 270 20 PandaX-II 500 8.6±4.6
XMASS: K. Abe et al. NIM A 716 (2013) 78 1μBq/kg ~ pp solar XENON100: E. Aprile et al. arXiv:1702.06942v1 LUX:arXiv: D. S. Akerib et al. PRL 112, 091303 (2014) PandaX-II: A. Tan et al. PhysRevLett.117.121303 (2016)
Masaki Yamashita 8 Rn removal from gaseous xenon with activated charcoal
Nuclear Instruments and Methods in Physics Research A 661 (2012) 50–57
Contents lists available at SciVerse ScienceDirect Nuclear Instruments and Methods in Physics Research A
journal homepage: www.elsevier.com/locate/nima
Radon removal from gaseous xenon with activated charcoal
K. Abe a, K. Hieda a, K. Hiraide a, S. Hirano a, Y. Kishimoto a, K. Kobayashi a, Y. Koshio a, J. Liu b, K. Martens b, S. Moriyama a, M. Nakahata a,b, H. Nishiie a, H. Ogawa a, H. Sekiya a, A. Shinozaki a, Y. Suzuki a,b, O. Takachio a, A. Takeda a, K. Ueshima a, D. Umemoto a, M. Yamashita a, K. Hosokawa c, A. Murata c, K. Otsuka c, Y. Takeuchi c, F. Kusaba e, D. Motoki d,n, K. Nishijima e, S. Tasaka f, K. Fujii g, I. Murayama g, S. Nakamura g, Y. Fukuda h, Y. Itow i, K. Masuda i, Y. Nishitani i, H. Takiya i, H. Uchida i, Y.D. Kim j, Y.H. Kim k, K.B. Lee k, M.K. Lee k, J.S. Lee k
a Kamioka Observatory, Institute for Cosmic Ray Research, The University of Tokyo, Kamioka, Hida, Gifu 506-1205, Japan b Institute for the Physics and Mathematics of the Universe, The University of Tokyo, Kashiwa, Chiba 277-8582, Japan c Department of Physics, Kobe University, Kobe, Hyogo 657-8501, Japan d School of Science and Technology, Tokai University, Hiratsuka, Kanagawa 25 9-1292, Japan e Department of Physics, Tokai University, Hiratsuka, Kanagawa 25 9-1292, Japan f Department of Physics, Gifu University, Gifu, Gifu 501-1193, Japan g Department of Physics, Faculty of Engineering, Yokohama National University, Yokohama, Kanagawa 240-8501, Japan h Department of Physics, Miyagi University of Education, Sendai, Miyagi 980-0845, Japan i Solar Terrestrial Environment Laboratory, Nagoya University, Nagoya, Aichi 464-8602, Japan j Department of Physics, Sejong University, Seoul 143-747, South Korea k Korea Research Institute of Standards and Science, Daejeon 305-340, South Korea
The XMASS Collaboration
Masaki Yamashita 9 article info abstract
Article history: Many low background experiments using xenon need to remove radioactive radon to improve their Received 29 August 2011 sensitivities. However, no method of continually removing radon from xenon has been described in the Received in revised form literature. We studied a method to remove radon from xenon gas through an activated charcoal trap. 23 September 2011 From our measurements we infer a linear relationship between the mean propagation velocity vRn of Accepted 28 September 2011 3 radon and vXe of xenon in the trap with vRn=vXe 0:9670:10 10À at 85 1C. As the mechanism for Available online 8 October 2011 ¼ð Þ À radon removal in this charcoal trap is its decay, knowledge of this parameter allows us to design an Keywords: efficient radon removal system for the XMASS experiment. The verification of this system found that it Radon reduces radon by a factor of 0.07, which is in line with its expected average retention time of 14.8 days Xenon for radon. Charcoal & 2011 Elsevier B.V. All rights reserved.
1. Introduction scintillation light from nuclear and other recoils in its LXe volume. The high scintillation light yield of LXe (comparable to that of Liquid noble gases are increasingly used in low background NaI(Tl)) allows for good energy resolution and a low detection experiments due to both their high purity and good performance threshold. Its high density and lack of radioactive isotopes provide as a radiation detector. Xenon (Xe) is a particularly attractive excellent shielding against background radiation from without as candidate, as it has the highest mass number, produces scintilla- well as within the detector [4].ThesensitivityoftheXMASS 45 2 tion light in the range of typical photomultiplier (PMT) sensitiv- experiment is expected to be better than 10À cm for a dark ities, and has no long lived radioactive isotopes of its own [1–3]. matter particle mass of 100 GeV/c2 with 100 kg effective volume The XMASS experiment at the Kamioka Observatory in Japan uses and 5 years exposure time. As the power dispersed by the PMTs liquid Xe (LXe) to search for possible WIMP Dark Matter recoils. It submerged in the LXe naturally leads to continuous evaporation of uses especially developed low radioactivity PMTs to measure the liquid from above the detector itself, both liquid and gas phase circulation are part of the XMASS detector design and can be used to maintain the purity of the detector medium, the LXe. n Corresponding author. Thanks to the short half-life times of all its radioactive isotopes E-mail address: [email protected] (D. Motoki). chemically pure Xe does not have any intrinsic radioactivity.
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.09.051 Rn removal by charcoal
• It is well known that Rn in Air/Ar can be effectively removed34 RADIOISOTOPES Vol．５９，No．１
by the activated charcoal. (e.g. RADIOISOTOPES,59, 29-36 キャリア気体中のラドン濃度 Bq/m３ である。
(2010) ) したがって，吸着開始から（Nabs（０）＝０），t 秒 後の吸着されたラドン数 （ ）Bq は • In 2009, for the frst time, we reported about Rn removal in Nabs t const・ AQF －λ t gaseous xenon with activated charcoal. Nabs（t）＝ （１－e ） （６） λ • Our fnding was that Rn actually was not absorbed by the となる。次に，脱離ランの循環時において，ラ charcoal (Shirasagi G2X 4/6) but it only slow down the ドン検出器のカウント数がラドンの崩壊定数に propagation speed in low temperature charcoal in gaseous 沿って減少していく時（Fig．５），その崩壊曲
３ xenon. 線から得られた初期ラドン濃度 Cre Bq/m は， トラップによって吸収されたラドン数 Nabs に • From our measurements we infer a linear relationship Fig．５Radon concentration inargongassince the 脱離率 R をかけ，検出器体積 V で割ったもの beginning of source run. Data points show between the mean propagation velocity v of radon and v なので，式（６）より Rn Xe the radon concentration averaged over１ hour, and dotted line shows the fitting re- of xenon in the trap with V /V ~ 1/1000 at -85℃. Cre＝R ･Nabs/V Rn Xe ２２２ sult with Rn decay curve（T１/２＝３．８days）. const・ RAQF －λ t The average concentrationsMasaki Yamashita during the10 ＝ （１－e ） （７） source run and the collection run, and the λ V initial concentration before the release run areshown in this figure. である。これと実際に得られたラドン数との割 合から脱離率を求める。Table２に異なる流量 のラドン濃度 Bq/m３（２１４Po の CPM１０より求 設 定（Flow）の 時 の 吸 着 率（Collection），脱 めた）を示す。線源ランと吸着ランでの平衡状 離率（Release％），露点（Dew），絶対湿度（Abs. 態での計数率の比より吸着率を求めることがで Hum）をまとめた。なお，このときの温度は きる。 ２２℃，系内の圧力は大気圧であった。 次に脱離率は以下のようにして求める。吸着 ラン開始からの時間が t のとき，活性炭に吸着 ３・３ バックグラウンド測定 しているラドン数 N の増加は，単位時間あた 系を純アルゴンで充填したあと（系内の圧力 りに， はゲージ 圧 で０．０１MPa），三方弁V１，V２を バイパス側にして流量１．０L/min で循環を行 dN ＝－λ N ＋const･A･Q ･F （５） い，バックグラウンドの測定を行った。 dt a．活性炭を通さない 測定時間１．９日
ここで，λ はラドンの崩壊定数（ln２/ T1/2）， 気温２２℃，露 点－３２℃，絶 対 湿 度０．３１g/ A は吸着率で時間によらず一定として，Q は m３，のとき約６．２±２．５CPD。
Table２ Summary of collection and release efficiency with different flow rate. The three numbers of dew point and absolute humidity are during the source run, the collection run, and the release run, respectively. The uncertainties of the collection and release show only statistical uncer- tainties
（３６） 52 K. Abe et al. / Nuclear Instruments and Methods in Physics Research A 661 (2012) 50–57
54 K. Abe et al. / Nuclear Instruments and Methods in Physics Research A 661 (2012) 50–57
200 the XMASS detector. From the pertinent measurements on the serial measurements relevant materials we estimate that less than 12 mBq=700 l are 180 52 K. Abe et al. / Nuclear Instruments and Methods in Physics Research A 661 (2012) 50–57emanating directly into the gas volume above the detector, originat- 160 ing mainly from the signal and high voltage cables of the PMTs. 12 mBq is also our upper limit estimate for emanation into the 140 liquid, mainly coming from the PMT bases. If we achieve Rn reduction by a factor 10 or better in the gas, there should therefore 120 be no noticeable addition to the activity in the detector from that 100 source any more. At this level of 10% of background from the PMTs [hour] 5 Rn 0.25L/min the remaining Rn would contribute of order 10À dru to the overall Rn/Xe velocity in the charcoal T 80 background level in our fiducial volume. 0.50L/min 222 60 With the half-life of Rn being 3.8 d, the TRn for Rn to stay in the trap must be longer than 12.7 days. Assuming a flow rate 1.00L/min 5 3 40 f 1:00 l=min 1:7 10À m =s of gaseous Xe to be liquefied, a • Rn injected in the circulation system with Rn Xe ¼ ¼ Â trade off between L and A can be made for this system and 20 1.50L/min trap trap detector our choice of charcoal (Shirasagi G2X 4/6), but the volume of the 0 system has to be 0 5 10 15 20 25 vRn 3 • We observed ‘oscillation’ of Rn concentration and path length [m] LtrapAtrap f Xe TRn Z17 560 cm ¼ vXe Fig. 1. Layout of the Xe circulation and Rn concentration measurement system. The charcoal traps are connected at points A and B. The right side of the figure shows the Fig. 4. TRn as a function of the length of the trap. Dashed lines are fits to the data layout of the newly designed charcoal trap for XMASS with the location of its temperaturefrom sensors, which arrival was also time tested in theinformation, left side system. we measured a using our measured value from above for vRn=vXe. This implies a points for traps arranged in series at different flow rates, the solid line is for data points derived with two traps in parallel. Up to three traps could be arranged in total charcoal mass of more than 4.8 kg. velocity of Rn in the charcoal. series, and two in parallel. Entries for path lengths longer than the sum of real trap until the pressure in the cold trap became too high to maintain 104 lengths are obtained by exploiting the oscillatory pattern to evaluate multiple 3.2. System implementation Rn concentration passes through the existing traps. the desired flow rate; at this point the outlet side of the trap was decay curve As the trap needs to be cooled, constraints on the dimensions opened too to allow continuous circulation through the trap. Fig. 1. Layout of the Xe circulation and Rn concentration measurement system. The charcoal traps are connected at points A and B. The right side of the figure shows the decay curve -3 of the system mainly come from the bath for the refrigerant and Circulating through the charcoal trap the system ultimately layout of the newly designed10 charcoal trap for XMASS with the location of its temperature sensors, which was also tested in the left side system. 1.50L/min its insulation requirements. With an eye also to the flexibility a 3 1.00L/min settles into a new and lower equilibrium concentration for Rn in 10 modular design offers, we chose to arrange two tubes of modest 0.50L/min until the pressure in the cold trap became too high to maintain 104 the Xe carrier gas. Once this new equilibrium concentration is Rn concentration 0.25L/min the desired flow rate; at this point the outlet side of the trap was diameter in series, with enough room in the bath to add two more decay curve reached and our measurements traced the Rn decay curve again opened too to allow continuous circulation through the trap. such tubes if needed.decay curve Each of these tubes is made from stainless Circulating through the charcoal trap the system ultimately 2
3 steel, is 90 cm long, has an inner cross-sectional area of 113 cm , for a suitable amount of time at the new lower level, cooling is -4 1.50L/min settles into a new10 and lower equilibrium concentration for Rn in 103 1.00L/min 0.50L/minand contains 2750 g of charcoal. Thus the two tubes amount to removed from the trap and the trap is baked once again to restore 2 the Xe carrier gas. Once this new equilibrium concentration is 10 0.25L/min 5.5 kg of charcoal in the trap. To contain the charcoal and its dust the trapped Rn to the system. That the ensuing rise in the Bq/m reached and our measurements traced the Rn decay curve again
for a suitable amount of time at the new lower level, cooling is 3 and avoid contamination of the detector, the entrance and exit of measured Rn concentration always recovered the decay curve [m/sec] removed from the trap and the trap is baked once again to restore 102 each tube are sealed with a 0:003 mm metal filter mesh. Rn the trapped Rn to the system. That the ensuing rise in the Bq/m originally measured right after Rn filling is proof that the Rn that v A schematic view of the system is shown on the right hand measured Rn concentration always recovered the decay curve had been removed while operating the trap at low temperature 10-5 10 originally measured right after Rn filling is proof that the Rn that side of Fig. 1. This figure also shows the locations of temperature really had been adsorbed in the charcoal. Circulation through the had been removed while operating the trap at low temperaturemeasurements of test10 trap (serial) sensors in and around one of the charcoal tubes in the trap. One trap was continuing throughout this radon release phase of the really had been adsorbed in the charcoal. Circulation through the measures the gas temperature at the inlet of the tube, while the trap was continuing throughout this radon release phasemeasurements of the of test trap (parallel) other three measure its surface temperature from the outside. measurement. A new measurement cycle may have been started measurement. A new measurement cycle may have beenmeasurement started on XMASS trap Around each tube a heating wire is wrapped to allow for baking of without adding new Rn by simply isolating the baking trap again without adding new Rn by simply isolating the baking trap again 1 1 to cool it down during10-6 bypass circulation. One measurement cycle 100 150 200the 250 tube 300 and the 350 charcoal 400 contained in it. Fig. 6 shows different to cool it down during bypass circulation. One measurement cycle 100 150 200 250 300 350 400 -3 -2 -1 therefore consists of10 three different phases:10 bypass circulation, 10 1 viewstime [hour] of the system. therefore consists of three different phases: bypass circulation, time [hour] trapping circulation, and release circulation. Masaki Yamashita As mentioned before the parameters we want tov controlXe [m/sec] in our Fig. 2. Rn concentration measurements forGiven a sequence11 above of Rn trapping system experiments parameters and f Xe 1:00 l=min we can with three charcoal traps installed in series. The data points for four Rn release and 3 ¼ trapping circulation, and release circulation. À system are the overall flow rate fXe as well as length Ltrap and trapping sequences at different Xe flowcalculate rates are shown that togethervRn withwill222Rn be decay 1:47 10 m=s in the charcoal hous- Fig. 2. Rn concentration measurements for a sequence of Rn trapping experiments Fig. 5. Mean propagation speeds for Rn vs. Xe. Circular data points come from the  As mentioned before the parameters we want to control in our cross-sectional area Atrap of the trap itself. Control of Ltrap and Atrap curves fitted to the Rn released andings trapped and equilibrium for our states system of the system.TRn becomes 14:7471:60 days. This with three charcoal traps installed in series. The data points for four Rn release and Data points are derived from the count rates for 218Po accumulated over 10 min. ð Þ was achievedfits shown by simply in Fig. preparing 4, the star three comes identical from the sections verification of measurement on the trap corresponds to 3.8 half-lives of 222Rn or an expected reduction by system are the overall flow rate fXe as well as length Ltrap and trapping sequences at different Xe flow rates are shown together with 222Rn decay charcoal trapdesigned that infor a XMASS sequence (Fig. of 7). experimentsThe line is constrained were then to go through the origin and a factor of 1/14.6.2 cross-sectional area A of the trap itself. Control of L and A installed eitherfitted in to sequence the data points or in parallel, measured doubling before the or triplingtrap was designedseries, resulting for XMASS. in Ltrap 522 cm and Atrap 0:94 cm with a total trap trap trap curves fitted to the Rn released and trapped equilibrium states of the system. ¼ ¼ was achieved by simply preparing three identical sections of Data points are derived from the count rates for 218Po accumulated over 10 min. either the length (sequence) or the area (parallel) of the trap. One of 175.8 g of charcoal for these measurements. Note that between such charcoalAlso section shown had a – length but of not 174 used cm and in a cross-section the previousmeasurement fit – is the cycles result bypass3.3. circulation System is always performance necessary test in charcoal trap that in a sequence of experiments were then of 0.94 cm2, containing 58.6 g of activated charcoal. The tube order to allow the cooling down of the charcoal trap without 2 itself was cutobtained from standard with 1/2 the in. Rn copper removal tubing. system designedstarting for use to trapin XMASS radon from the reservoir in the 70 l Rn detector installed either in sequence or in parallel, doubling or tripling series, resulting in Ltrap 522 cm and Atrap 0:94 cm with a total ¼ ¼ on the basis of above measurements, which willprematurely. be discussed in To verify the system performance the newly designed trap was either the length (sequence) or the area (parallel) of the trap. One of 175.8 g of charcoal for these measurements. Note that between 2.2. Extractingthe the remainder Rn propagation of the velocity paper. in the trap The data points in this figureinstalled show the into222Rn the concentration circulation in system as in Fig. 1, and Rn was 3 218 such charcoal section had a length of 174 cm and a cross-section measurement cycles bypass circulation is always necessary in [Bq/m ] as measured in the Rnintroduced detector by intoPo the counts. XefXe again.was As usual, the trap had been isolated 2 Fig. 2 shows the results of a sequence of measurement cycles different for each measurement cycle and is indicated in the of 0.94 cm , containing 58.6 g of activated charcoal. The tube order to allow the cooling down of the charcoal trap without under baking conditions, but then was cooled to 90 1C) for this made after one Rn injection and with all three traps arranged in figure. The figure also shows two decay curves with the proper À itself was cut from standard 1/2 in. copper tubing. starting to trap radon from the reservoir in the 70 l Rn detector 3. Radon removal system design for the XMASS LXe detector test. While its intended operating temperature in XMASS detector is 100 1C, 90 1C is closer to the 85 1C previously used to prematurely. À À À 2.2. Extracting the Rn propagation velocity in the trap The data points in this figure show the 222Rn concentration in 3.1. System requirements establish vRn=vXe. For those previous measurements the tempera- ture was limited by the cooling system. During the test the [Bq/m3] as measured in the Rn detector by 218Po counts. f was Xe The XMASS detector in its current incarnation has a 700 l gas temperature sensors indicated in Fig. 1 showed uniform readings Fig. 2 shows the results of a sequence of measurement cycles different for each measurement cycle and is indicated in the volume above and in equilibrium with its 1100 kg ( 380 l LXe) within 70:3 1C even if warm Xe gas was circulated in the trap. made after one Rn injection and with all three traps arranged in figure. The figure also shows two decay curves with the proper of liquid that fill the whole of the detector volume in this single- The results of this performance test are shown in Fig. 7. phase experiment. Despite a very high initial Rn concentration of 1000 Bq/m3 at Great care was taken in choosing detector materials with respect the start of the measurement cycle and integrating the Rn to controlling the radioactivity emanating from all materials used in detector count rates over 12 h intervals the measurement was Set up (Abe et al. NIM A 661(2012) 50) nano-flter • Based on that study , we built a removal system with 5.5 kg (Shirasagi G2X 4/6) .
• Xe 1 L/min with VRn will be 1.47x10 3 m/s in the charcoal housings
• TRn becomes about 14.7 days.
• This corresponds to 3.8 half-lives of 222Rn or an expected reduction by a factor of 1 - (1/2)^(14.7/3.8) ~0.93
6.7: Photographs of the Rn removal system for the XMASS. (1) is charcoal housings 図 6.8: Performance test diagram of Rn図 removal system. Masaki Yamashita 12 with heating wires. (2) is cryogenic system, which use pulse tube refrigerator and HFE as coolant.
102
103 K. Abe et al. / Nuclear Instruments and Methods in Physics Research A 661 (2012) 50–57 55
first and second oscillation maximum. These allowed us to extract T again, which for this system evaluated to 14:7570:50 days, Rn ð Þ which is in line with expectation—as evident in Fig. 5, where a star marks the measurement.
3.4. Measurement of radon emanation from system
Adding 5.5 kg of charcoal to a dedicated low background system, one concern has to be Rn emanation from the charcoal itself. However, the signal of Rn emanation from charcoal at low temperature is probably below the detection limit due to adsorp- tion by charcoal itself. In order to make a conservative measure- ment of Rn emanation from the charcoal it has to be kept at baking temperature. For the high Rn concentrations artificially introduced for our previous measurements all Rn seems to be recovered from being trapped in the charcoal when the charcoal temperature is raised to 120 1C. The emanation measurement was made after the new trap was installed to the circulation system of Fig. 1,andbeforeanyRnwas injected. To get a realistic result relevant for the intended operating conditions of the trap Xe was used as carrier gas for the measure- ment and circulated at a rate of 1:00 l=min. Since for this measure- ment we cannot artificially raise the Rn level, the integration time for each bin was increased from 10 min to 1 day, and instead of quoting Rn concentration we use the number of Rn decays directly. Fig. 8 shows the result of this measurement. To interpret it we use the following equation: dN t ð Þ lN ac adet 1 dt ¼À þ þ ð Þ where N(t) is the number of 222Rn atoms in the Rn detector, l is 222 the Rn decay constant, and ac and adet describe the emanation of 222Rn from the charcoal and the Rn detector respectively. Here the term for the Rn detector is meant to include all system components other than the trap. The solution to this simple differential equation is Fig. 6. Photographs of the Rn removal system for the XMASS. (1) is charcoal atot lt lt N t 1 eÀ N eÀ 2 housings with heating wires. (2) is cryogenic system, which use pulse tube ð Þ¼ ð À Þþ 0 ð Þ refrigerator and HFE as coolant. l
222 lt lt Rn decay atot 1 eÀ lN0eÀ 3 5.5 kg charcoal test ¼ ð À Þþ ð Þ 7 10 where N0 is the number of Rn atoms at the start of the measurement and atot ac adet .ThecontributionfromtheN0 nuclei already ¼ þ 106 Po-218 Po-214 14 Decay curve 105 14.75+-0.5 days 12
3 104 10
mBq/m 103 8
2 Po-214
10 mBq 6 Po-218 10 estimated emanation Emanation α (1 - e-λ)t 4 total -λt <3.1mBq Decay of initial radon N0 e 1 0 10 20 30 40 50 estimated2 from 120℃ sum time [day] temperature. 0 Masaki Yamashita 13 Fig. 7. The result of the Rn removal system test with 180 cm path length, 113 cm2 cross-section. Data points are averaged over 12 h. Dashed spaced line is radon 0 2 4 6 8 10 decay curve expected from initial radon value. elapsed time [day]
Fig. 8. The result of charcoal emanation measurement at baking temperature. clearly limited by the sensitivity of our 70 l Rn detector. It cannot Data points are averaged over 24 h. Dashed spaced line indicates decay of initial 3 measure Rn concentrations less than 10 mBq/m . Clearly visible radon in the Rn detector. Dotted line shows emanated radon from charcoal and are the initial fast reduction of Rn in the system, as well as the detector background. Solid line is sum of them. Rn source
•Rn source current set up •It is called ‘radium ceramic ball’ to have Onsen(hot spring ) at home !! 図 6.8: Performance test diagram of Rn removal system. •buy at Amazon
pump
ceramic ball 103 700g, ~100Bq Masaki Yamashita 14 Charcoal A At same time we are looking for new material Xe nano- as well. flter HPGe couing (U-chain） NIM(2011) Shirasagi 67+/-15 mBq/kg Charcoal A < 11.9 mBq/kg (90%CL) Tested with Rn source. (next page)
12 40 ~ 1/10 reduction 9 Temp 0
6 with 1L/min -40
Rn Inner Temperature(deg.C) 3 -80 Radon ConcentraBon(Bq/m3)
0 -120 50cm 6-Sep 9-Sep 12-Sep 15-Sep 18-Sep 21-Sep 24-Sep 27-Sep 30-Sep 3-Oct 6-Oct 9-Oct 12-Oct charcoal
date(2016)
12 2.0
9 1.5 Rn Flow
6 1.0 Xe Flow(L/min) 3 0.5 Radon ConcentraEon(Bq/m3) 140g 6cm 0 0.0 6-Sep 9-Sep 12-Sep 15-Sep 18-Sep 21-Sep 24-Sep 27-Sep 30-Sep 3-Oct 6-Oct 9-Oct 12-Oct ICF114 date(2016) Masaki Yamashita 15 Plan for Testing in XMASS
Masaki Yamashita 16 XMASS experiment M
832kg LXe Kamioka mine Gifu, Hida city, Ikenoyama
Kamland super Kamiokande
ICRR, UTokyo
Masaki Yamashita 17 Water Shield
10 m
-φ10m x 10m ultra pure water shield with 20 inch x 70 PMTs for muon veto
Masaki Yamashita 18 XMASS-I Rn removal Plan for XMASS Getter
冷 Mass flow control
-9 多孔質ガラス 冷 多孔質ガラス
PTR -100℃ LXe
PMT Water shield
Masaki Yamashita 19 Summary
•Rn will be the one of the main background for future dark matter experiment with liquid xenon.
•The activated charcoal was tested for this purpose and we found that Rn atom moves more slower than Xe atom in the cold charcoal.
•Prototype Rn removal test show ~1/10 reduction @1L/min and will be test in XMASS detector.
Masaki Yamashita 20