Improved method for measuring low concentration radium and its application to the Super-Kamiokande Gadolinium project
S. Ito, K. Ichimura, Y. Takaku, K. Abe, M. Harada, M. Ikeda, H. Ito, Y. Kishimoto, Y. Nakajima, T. Okada, H. Sekiya
aa r X i v : . [ phy s i c s . i n s - d e t ] J un Prog. Theor. Exp. Phys. , 00000 (8 pages)DOI: 10.1093 / ptep/0000000000 Improved method for measuring lowconcentration radium and its application to theSuper-Kamiokande Gadolinium pro ject
S. Ito , K. Ichimura , Y. Takaku , K. Abe , M. Harada , M. Ikeda , H. Ito ,Y. Kishimoto , Y. Nakajima , T. Okada , and H. Sekiya Okayama University, Faculty of Science, Okayama 700-8530, Japan ∗ E-mail: [email protected] Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo,Kamioka, Gifu 506-1205, Japan ∗ E-mail: [email protected] Kavli Institute for the Physics and Mathematics of the Universe (WPI), theUniversity of Tokyo, Kashiwa, Chiba, 277-8582, Japan Present address: Research Center for Neutrino Science, Tohoku University, Sendai980-8578, Japan Institute for Environmental Sciences, Department of Radioecology, Aomori,039-3212, Japan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chemical extraction using a molecular recognition resin named “Empore Radium RadDisk” was developed to improve sensitivity for the low concentration of radium (Ra).Compared with the previous method, the extraction process speed was improved by afactor of three and the recovery rate for
Ra was also improved from 81 ±
4% to > − mBq level was achieved using a high purity germaniumdetector. This improved method was applied to determine Ra in Gd (SO ) · Owhich will be used in the Super-Kamiokande Gadolinium project. The improvement andmeasurement results are reported in this paper. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Subject Index XXX
1. Introduction
Detection of radium (Ra) is important not only for environmental studies, medical science,biology, but also for non-accelerator particle physics experiments (e.g. neutrino measure-ments and dark matter searches), which require a low background experimental environment.In general, short lifetime radioactive materials (e.g.
Ra) are measured using particle coun-ters such as α -spectrometers, liquid scintillators, and germanium detectors. However, thesensitivity is generally limited by the detection efficiency related to the sample size, i.e.self-shielding effect and geometrical acceptance. To minimize these problems and improvesensitivity, chemical extraction is often used. For example, Dulansk´a et al ., [1] have useda molecular recognition resin named “AnaLig-Sr01”, which is a product of IBC AdvancedTechnologies [2]. They determined the concentration of Ra included in rocks or buildingmaterials in the range of 5 . − . − . Other chemical extraction techniques have c (cid:13) The Author(s) 2012. Published by Oxford University Press on behalf of the Physical Society of Japan.This is an Open Access article distributed under the terms of the Creative Commons Attribution License(http://creativecommons.org/licenses/by-nc/3.0), which permits unrestricted use,distribution, and reproduction in any medium, provided the original work is properly cited. een developed using “Empore Radium Rad Disk” produced by 3M Corporation [3] (see thefollowing sections in more details) to determine
Ra in water at the 10 − mBq L − level [4–6]. However, the chemical extraction of Ra from a very high matrix sample witha much lower concentration has not been established.The Super-Kamiokande Gadolinium (SK-Gd) project is an upgrade of the Super-Kamiokande (SK) detector [7], with its final goal of dissolving gadolinium sulfate octahydrate(Gd (SO ) · O) into the SK detector up to the 0.2% concentration [8, 9]. One of the mainphysics targets of SK-Gd is to discover supernova relic neutrinos and study star formationof the universe [10]. The measurements of solar neutrinos with a low energy threshold of ∼ Ra,
U, and
Th) in Gd (SO ) · O should be minimized before loading into SK. Themaximum allowed level of these radio impurities in Gd (SO ) · O [12, 13] and the typicalexample of a commercially available product are summarized in Table 1. For the measure-ment of
U and
Th, the procedure was developed using inductively coupled plasma-massspectrometry (ICP-MS) with chemical extraction (see Ref. [14] for more details).As shown in Table 1, SK-Gd requires a method to determine low concentration
Ra.However, the sensitivity for
Ra of the previous method with the molecular recognitionresin “AnaLig-Ra01” was on the 1 mBq level [16]. In addition, the recovery rate of
Rawith the previous method was 81 ±
4% and the process time of the sample was 1 L per hour.To achieve the required sensitivity of SK-Gd, it was necessary to increase the concentrationrate with a higher recovery rate and a shorter processing time. In this study, the procedureof chemical extraction was improved to achieve the sensitivity of on the 10 − mBq kg − level by solving these problems: the improved method was applied to SK-Gd to determinethe concentration of Ra in Gd (SO ) · O. Table 1
Summary of the maximum allowed level for SK-Gd and the typical values ofcommercially available product [12, 13]. All units are mBq kg − (Gd (SO ) · O). Ra U ThRequirement for SK-Gd 0.5 5 0.05The typical concentration of 5 50 100commercially available product
2. Experimental equipment “Empore Radium Rad Disk” [3] was used for chemical extraction. The resin with the samechemical features as AnaLig-Ra01 was positioned on a filter (47 mm diameter and 0.5 µ mthickness) made of polytetrafluoroethylene fibrils . Figure 1 shows the experimental setup ofthe chemical extraction using a vacuum filtration system with the disk. The disk was placedon a holder with the volume of 800 mL (Advantech Toyo Ltd. [17], KP-47), and the holderwas connected to the vacuum container (Advantech Toyo Ltd. [17], VT-500). The solution This is generally called “disk” and simply called disk in this paper from now on.2/8 asses through the disk, and
Ra in the solution is adsorbed by the resin bedded to thedisk. The concentration of
Ra can be determined by measuring the disk directly using anHPGe detector.To produce solutions with low contamination, ultra-pure SK water [7] was used. Electronicgrade 70% nitric acid (HNO ) (Wako Pure Chemical Industries Ltd. [18]) was used to washthe disk and efficiently dissolve Gd (SO ) · O in the SK water.To easily check the concentration of
Ra in Gd (SO ) · O, Ra-rich hot springwater from the Kawakita hot spring in Ishikawa, Japan [19] was used as the calibrationsolution. The concentration of
Ra in the hot spring water was 112 +34 − mBq L − , whichwas determined by the HPGe detector measurement. The uncertainty was mainly due tothe systematic uncertainty of the HPGe detector (+30% or -10%) and statistics [16]. Thesampled hot spring water was filtrated by membrane filters with the pore size of 0.45 µ mand acidified to pH ≃ for preservation.Because barium (Ba) and Ra have similar chemical features and ionic radii, Ba is frequentlyused as a tracer for Ra analysis to estimate the recovery rate [20]. Thus, 1000 mg L − Baof standard solution (Merck Ltd. [21]) was used to estimate the recovery rate. The detailsof studies for the recovery rate are described in Sec. 3. The ICP-MS “Agilent 7900” [22]was used to measure the concentration of Ba to estimate the recovery rate of
Ra. Theperformance of this ICP-MS is described in Ref. [14]. (c)(a) (b)
Fig. 1 (A) Photograph of the entire experimental setup. (B) Top view of the setup. (C)Top view of the disk (47 mm diameter). .2. High purity germanium detector and its detection efficiency
The HPGe detector used for this measurement was a coaxial p-type HPGe crystal manu-factured by CANBERRA France [23]. The dimension of sample chamber was 23 × ×
48 cm . The details of the performance of the HPGe detector are described in Ref. [16]. Thesamples measured by the HPGe detector were covered by an ethylene vinyl alcohol bag tokeep radon from samples inside the bag (Fig. 2).The concentration of Ra was evaluated using the characteristic γ -lines of Pb(609 keV) and
Bi (352 keV and 1764 keV), which are daughter nuclei of
Ra, by consid-ering of their branching ratios and detection efficiencies. Figure 3 shows the typical observedenergy spectra for
Bi 352 keV measurements. The detection efficiency was evaluated bythe Monte Carlo simulation [24]. For example, the detection efficiency of 352 keV gammarays originating from
Bi, with the chemical extraction procedure using the disk was foundto be 15.9%. On the other hand, the detection efficiency of 352 keV gamma ray for
Rawas determined to be 0.8% for the direct measurement of 5 kg of Gd (SO ) · O withouta chemical extraction procedure. The detection efficiency was improved by a factor of 20owing to the smaller volume of the sample using the disk.
Fig. 2
Setup for the disk measurement with the HPGe detector.
3. Chemical extraction of
Ra from Gd (SO ) · O and its performance
The setup shown in Fig. 1 was connected to a vacuum pump: the solution loaded into theholder could pass through the disk. The disk was initially washed by loading 50 mL of3 mol L − HNO and 50 mL of the ultra-pure SK water into the holder. A total of 500 gGd (SO ) · O was dissolved in 10 L of a 0.2 mol L − HNO solution. Then, the samplesolution was loaded into the holder, and the vacuum pressure was adjusted to produce a flowrate of 50 mL min − (3 L per hour, which is a three times faster processing time than that nergy (keV)344 346 348 350 352 354 356 358 360 C oun t s / d ay Fig. 3
Energy spectra obtained by the HPGe detector around
Bi 352 keV. The black lineshows the background and red line indicates the extracted
Ra from 3 mL of hot springwater (corresponding to 0.33 mBq). The error bar at each bin represents only statisticaluncertainty.used in the previous method [16]). Then, the disk was directly placed on the HPGe detectorand measured.The blank amount of
Ra in the disk was evaluated by measuring 17 disks using the HPGedetector, the value of 1.9 +0 . − . mBq was obtained for 17 disks (corresponding to 0.11 +0 . − . mBqfor each disk). In the previous method [16], the value of procedure blank was 0.3 ± Ra-rich hot spring water.The volume of 3 mL and 100 mL of the hot spring water was added to 10 L of aGd (SO ) · O dissolved sample solution. Then, the concentration of
Ra adsorbed bythe disk was measured using the HPGe detector for 4.5 days. As shown in Table 2, the resultsof measurements were consistent with the expected amount of
Ra, and the achieved sen-sitivity was on the 10 − mBq level owing to the development of this chemical extractionprocedure.To estimate the recovery rate more accurately, Ba standard solution was added to thesample solution. The sample solution with the concentration of 4.0 × − g (Ba) mL − wasloaded into the holder, and the solution (which passed through the disk) was collected andmeasured using the ICP-MS. The concentration of remaining Ba in the solution was < . (SO ) · O) did not interfere withthe extraction of
Ra and the recovery rate obtained using the developed method wasestimated to be > ± ethod is on the 10 − mBq level and sufficient for measuring the experimentally allowedlevel of Ra for SK-Gd.
Table 2
Summary of the study for the recovery rate. The blank of the disk was alreadysubtracted. Hot spring water Expected amount Results(mL) of
Ra (mBq) (mBq)3 0.33 +0 . − . ± . +3 . − . +3 . − . Table 3
Summary of the performance in the previous and developed methods.Previous method Developed methodRecovery rate (%) 81 ± > ± +0 . − . Sensitivity of the test sample (mBq) 0.9 ± ±
4. Application to SK-Gd and results of measuring Gd (SO ) · O SK-Gd will be conducted in several experimental phases. For the first experimental phase,13 tons of Gd (SO ) · O will be dissolved in the SK tank corresponding to a 50% neu-tron tagging efficiency [9]. Gd (SO ) · O was produced with many lots: thus, all thelots should be measured before loading. Currently, 14 tons of Gd (SO ) · O are beingmeasured using the developed method to confirm that their radio impurities are below theexperimentally allowed levels (see Table 1). The concentration of
Ra in Gd (SO ) · O(unit: mBq kg − ) can be obtained from the amount of Ra measured in the disk dividedby the weight of Gd (SO ) · O. Table 4 shows the results of the measurement of
Rain Gd (SO ) · O for several production lots determined using the improved chemicalextraction method. The signals of
Pb and
Bi were not observed above the statisticaluncertainty. The concentrations of
Ra in the measured products were confirmed to bebelow the experimentally allowed level.On the basis of these studies and measurements, a highly sensitive method for mea-suring low concentration
Ra was established. This method can be applied to othernon-accelerator particle physics experiments as well. For example, the XENON-1T detectorwill be upgraded to the XENON-nT detector with a neutron veto system which is based ona high-purity Gd-loaded water Cherenkov detector [25].
5. Conclusion
The method for measuring
Ra using an HPGe detector with chemical extraction wasimproved to determine low concentration
Ra in a high matrix sample. More than 99.9%of
Ra was recovered from the high matrix sample with a shorter processing time of the able 4
Summary of the measurements of Gd (SO ) · O. The upper limits represent90% confidence level.Lot No. Concentration of
Ra Measurement time(mBq kg − ) (day)1 < . < . < . < . < . < . − mBq level. The improvedmethod is being applied to SK-Gd to determine Ra in Gd (SO ) · O. Currently, allthe measured Gd (SO ) · O products, which will actually be loaded into the SK tank,were confirmed to be below than the maximum allowed
Ra concentration level.
Acknowledgements
This work was supported by the JSPS KAKENHI Grants Grant-in-Aid for Scientific Research onInnovative Areas No. 26104004, 26104006, 19H05807, and 19H05808, Grant-in-Aid for Specially Pro-moted Research No. 26000003, Grant-in-Aid for Young Scientists No. 17K14290, and Grant-in-Aidfor JSPS Research Fellow No. 18J00049.
References al ., Applied Radiation and Isotopes 124, 2017, 83-89.[7] S. Fukuda et al., Nucl. Instrum. Methods. A501, (2003), 418.[8] John F. Beacom and Mark R. Vagins, Phys. Rev. Lett. 93, 171101 (2004).[9] Ll. Marti et al. , Nucl. Instr. Meth. A, et al ., Nucl. Instr. Meth. A, , (2003) 250-303.[25] The XENON Experiment (available at: https://science.purdue.edu/xenon1t/?p=1051), (2003) 250-303.[25] The XENON Experiment (available at: https://science.purdue.edu/xenon1t/?p=1051)