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Nuclear Instruments and Methods in Physics Research A 606 (2009) 484–489

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/nima

Qb-measurements with a total absorption detector composed of through-holeHPGe detector and anti-Compton BGO detector

Hiroaki Hayashi a, Michihiro Shibata b,�, Itaru Miyazaki a, Osamu Suematsu a, Yasuaki Kojima c,Kiyoshi Kawade a, Akihiro Taniguchi d

a Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japanb Radioisotope Research Center, Nagoya University, Nagoya 464-8602, Japanc Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japand Research Reactor Institute, Kyoto University, Kumatori 590-0494, Japan

a r t i c l e i n f o

Article history:

Received 9 February 2009

Received in revised form

20 April 2009

Accepted 23 April 2009Available online 9 May 2009

Keywords:

Qb

Total absorption detector

HPGe detector

Anti-Compton BGO scintillator235U(n,F)147La148La

On-line mass separator

02/$ - see front matter & 2009 Elsevier B.V. A

016/j.nima.2009.04.047

esponding author. Fax: +8152 789 2567.

ail address: i45329a@nucc.cc.nagoya-u.ac.jp (

a b s t r a c t

A total absorption detector, which is composed of a through-hole type HPGe detector coupled with a

surrounding annular anti-Compton BGO detector, has been developed for b�-decay energy (Qb)

measurements of nuclei far off from the b-stability line. This detector can measure radiations with an

almost 4p solid angle by putting radioactive sources in the middle of the HPGe detector and also can

suppress the scattered photons with the anti-Compton BGO scintillation detector. The systematic

uncertainty was determined to be 30 keV by measuring 19 nuclei having Qbs between 3 and 8 MeV by

means of conventional square-root plot analysis. The Qb’s of mass-separated fission products 147La and148La were successfully determined to be 5366(40) and 7732(70) keV, respectively.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

Atomic masses of unstable nuclei are fundamental andimportant physical quantities related to nuclear structure andnucleosynthesis in astrophysics. Moreover, the masses of fissionproducts are important for the evaluation of decay heat in nuclearpower plants. Nevertheless, most b�-decay energies (Qb) of nucleifar off from the b-stability line have not been determinedexperimentally due to the low intensities of available beams.Available theoretical mass values and the systematics haveuncertainties of at least 500 keV. It is meaningful to determinethe values experimentally with uncertainties smaller than100 keV. Presently, ion-trap methods coupled with on-line massseparators (ISOL) are used at various facilities. Alternately, Qb

measurement is also considered to be a precise method. However,in Qb measurement by applying b–g coincidence method,information on a decay scheme is necessary. In the nucleiof interest, and especially in newly observed isotopes, mostof their decay schemes are not known, making b–g coincidence

ll rights reserved.

M. Shibata).

measurement quite difficult. On the contrary, a total absorptiondetector can be extremely effective because the Qb can bedetermined independent of the decay scheme information.This is because the energies of the b and g rays can be summedup to the Qb, and the end-point of the measured spectrumcorresponds to the QbU It means the Qb can be determinedindependent of the decay scheme information. Hence, a totalabsorption detector composed of twin large-volume BGO scintil-lators has been developed and installed in two ISOL facilities(KUR-ISOL and Tokai-ISOL). The Qb’s of fission products of 235U(n,f)or 238U(p,f) including new isotopes have been successfullydetermined independent of the decay scheme information [1–4].This total absorption detector has high efficiency but less energyresolution in comparison with the Ge detector, so the deducedQb’s include uncertainties of 60–100 keV.

In order to determine Qb’s with higher accuracy and reliability,another total absorption detector composed of HPGe and BGOscintillation detectors has been recently developed. The efficiencyof this total absorption detector is less than that of the BGO totalabsorption detector [1], but the energy resolution is much higher.Moreover, energy calibration can be carried out up to the high-energy region by the observed g-rays (e.g. prompt g-rays).Consequently, the deduced Qb’s are much more reliable.

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H. Hayashi et al. / Nuclear Instruments and Methods in Physics Research A 606 (2009) 484–489 485

The performance of the detector, namely that it could measureQb’s with systematic uncertainties within 30 keV under goodstatistics, was confirmed by measuring the well-evaluated Qb’sbetween 3 and 8 MeV using conventional square-root plotanalysis. Finally, the Qb of 147La and 148La that were separatedfrom fission products using an on-line separator were proposedto be 5366(40) and 7732(70) keV, respectively.

2. Experimental

As shown in Fig. 1, the newly developed detector is composedof two detectors. One is a custom-made HPGe detector in whichthe large-volume single crystal (84 mmf

�90 mmt) has a 20 mmf

diameter through-hole in the center. This allows radiation to be

movable tape

Fig. 1. Schematic view of the total absorption detector and th

measured with an almost 4p solid angle and to be measured asenergy sum of b-ray and g-ray following the b-decay. The Gecrystal is covered with aluminum housing and the thickness of thewell is 0.4 mm in order to reduce the energy loss and absorptionas low as possible. The other detector is 25-mm-thick anti-Compton BGO detector which surrounds the HPGe detector.The energy resolution of the HPGe detector was approximately2.5 keV at the 1332 keV g-ray. Measurements were takenas follows: a singles spectrum using the HPGe detectorand a coincidence spectrum with the BGO detector weremeasured simultaneously. Here, coincidence events correspondto incomplete energy-absorption events, namely Compton com-ponents of g-rays or bremsstrahlung photons associated withb-particles. By subtracting the coincidence spectrum fromthe singles spectrum, total absorption events can be extracted.

GeBGO

DifferentialPumping

32

90

25

150

0.4 mmt Al

Coldfinger

RI beamfrom ISOL

PhotomultiplierTube

e experimental setup. The size is indicated in mm scale.

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H. Hayashi et al. / Nuclear Instruments and Methods in Physics Research A 606 (2009) 484–489486

Therefore, the deduced spectrum principally corresponds to thesuperimposed spectrum of a fully absorbed b-ray, and the end-point of the spectrum corresponds to the Qb.

In practice, b-ray spectra are distorted by the energy distribu-tion resulting from energy straggling by the Al housing, a deadlayer including some materials on the surface of the crystals.Hence, in order to determine the effective energy loss experi-mentally, many nuclei having well-determined Qb’s were mea-sured, and the measured end-point energies were compared to

101

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2000 40

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Fig. 2. Measured spectra of mass-separated 93Sr(a) and 92Rb(b). The solid and broken lin

absorption spectrum which was obtained by subtracting the coincidence spectrum from

the total absorption spectrum. The region between two dotted lines indicates the analyz

square-root plots show putting the minus sign when the counts become negative after s

in high-energy region. It means the spectra were subtracted properly.

their evaluated values. Nine radioisotopes 27Mg, 38Cl, 42K, 52V,56Mn, 72Ga, 90Y, 139Ba and 142Pr were prepared by the (n,g)reactions at the pneumatic-tube facility in the Kyoto UniversityReactor (KUR) and ten fission products of 91�94Rb, 93,95Sr and139�142Cs were provided by the on-line mass separator at the KUR(KUR-ISOL) [e.g. 5,6]. On the (n,g) experiment, radioactive sourcesin liquid form were dropped on thin filter paper. On the ISOLexperiment, the mass-separated radioactive beams were im-planted into a thin Mylar tape, and the sources on the tape were

SinglesCoincidenceTotal absorption

rgy (keV)

SinglesCoincidenceTotal absorption

rgy (keV)

92Rb

00 3000 4000

4000

00 6000 8000

8000

93Sr

es indicate the singles and coincidence spectra, respectively. Dots indicate the total

the singles one multiplied by a factor of 1.4. The inset shows the square-root plot of

ed region. It shows almost a straight line near the end-point. The negative counts in

ubtracting the coincidence spectra. The counts scatter after subtracting around zero

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H. Hayashi et al. / Nuclear Instruments and Methods in Physics Research A 606 (2009) 484–489 487

moved periodically with predetermined time interval from thecollecting port in a vacuum chamber to the measuring position inthe air using differential pumping. The tape was computer-controlled and the reproducibility of the source position waswithin 1 mm. On the ISOL experiment, the detector was shieldedfrom the background neutrons with 5 cm lead blocks, 5-mm-thick40% boron-doped rubber sheets and 20-cm-thick paraffin blocks.These shields reduced the background from 830 to 120 cps,approximately. This background rate was nearly constant duringthe measurements. In both experiments, the source preparationsand measurements were iterated many times in order to obtainsufficient counting statistics. The total counting rate was keptbelow 1.5 kcps to reduce pulse pile-up. Energy calibration wascarried out with 24Na and capture g-rays of the surroundingmaterials such as Fe, H and N.

3. Results

In this paper, the results obtained through conventionalanalyzing method, namely square-root plots method, weredescribed. The total absorption spectrum was deduced from thesingles spectrum by subtracting the coincidence spectrum multi-plied by a factor of 1.4 for each nucleus. This factor corresponds tothe inverse of the 70% solid angle of the BGO scintillation detectorand it was consistent with not only the measurements, butalso the simulations by the Monte Carlo code GEANT4 [7] formono-energetic g-rays. The background spectra in singles andcoincidences were also subtracted from each measured spectrum.Two typical total absorption spectra of 93Sr (Qb ¼ 4.2 MeV) [8]and 92Rb (Qb ¼ 8.2 MeV) [8] are shown in Fig. 2(a) and (b),respectively, together with singles and coincidence spectra. Here,the spectrum was analyzed with the energy bin of 20 keVwith convenience. Generally, in energy spectra measured withGe detector, the energy can be determined within the uncertaintyof 1/5 channel or much better, in this case 4 keV, under the good-statistics condition. The energy broadening represented as theFWHM of response for 3–8 MeV mono-energetic electrons, whichoccurs mainly by the housing of aluminum well, is evaluatedto be approximately 70 keV by the Monte Carlo simula-tion GEANT4. Moreover, as mentioned later, the uncertainty ofthe analysis is evaluated to be 30 keV practically. Therefore,the uncertainty originating from the bin does not have influenceon the analysis, and the energy bin of this 20 keV is enough toanalyze the spectra with much better precision than that of theBGO detector.

142Pr

-300

-200

-100

2000

Res

idua

ls (E

xp.-R

ef.)

(keV

)

90Y42K

56Mn52V

72Ga

139Cs

93Sr

38Cl

141

27Mg

4000Qβ in

Fig. 3. Effective energy loss of the detector. The differences between the determined en

detector.

The square-root plot method is often the preferred method ofQb analysis for measurements with a plastic scintillation detectorand can also be applied to the spectra measured with the totalabsorption BGO detector [1,2]. As shown in the inset of Fig. 2, thesquare-root plot of the spectrum shows an almost straight linenear the end-point. It suggests the distortion of the response forthe electrons does not have much influence near the end-pointenergy at least 500 keV, and the root plot analysis is enough toanalyze the measured total absorption spectrum in this detector.When the net counts become negative after subtracting thebackground counts at some channel, then the minus sign is putthe absolute value of the net counts. The fact that the countsscatter around zero in high-energy background region aftersubtracting means the spectra were subtracted properly. Thespectrum also shows tailing to the high-energy side at the end-point owing to energy broadening. In the root plot analysis, theregion of interest for analysis was chosen to be about 500 keVbelow 100–200 keV from the end-point of each spectrum in orderto exclude this tailing effect. This method was employed for allspectra independent of each decay scheme.

The comparison between the deduced end-point energies(Eb�max) obtained through energy calibration carried out withg-ray peaks and the literature Qb values [7] is shown in Fig. 3. Theeffective energy loss by this analysis method was experimentallydetermined to be 180 keV between 2 and 8 MeV with anuncertainty of 30 keV. The Qb of newly measured nuclei can bededuced by adding this effective energy loss to the Eb-max of thespectra. The origin of the energy loss was evaluated as follows.According to the Monte Carlo simulation for mono-energeticelectrons, the energy loss by Mylar tape was evaluated to beapproximately 20 keV. So that by 0.4 mmt Al housing and deadlayer including some materials on the crystal well, it was deducedto be approximately 160 keV. On the other hand, that by only0.4 mmt Al housing was evaluated to be approximately 130 keV,then, by dead layer including surface materials was evaluated tobe 30 keV. This effective energy loss for the (n,g) experiment wasalmost the same as that for the ISOL experiment. The uncertaintyobtained through the conventional analysis is comparable with orbetter than that of the BGO total absorption detector [3].

The Qb of some nuclei around A ¼ 150 were measured with thedetector at the KUR-ISOL. The periods for collection–measurementof the tape transport system for 147La (T1/2 ¼ 4.0 s [9]) and 148La(T1/2 ¼ 1.4 s [9]) were set at 8–8 and 3.4–3.4 s, respectively, to reduceeach daughter activity. The total measurement period for eachnucleus was longer than 15 hours. As shown in Fig. 4(a) and (b), theEb-max’s of 147La and 148La were analyzed to be 5186 and 7552 keV,

91Rb

Cs

95Sr

140Cs

142Cs

93Rb

92Rb

-180 keV

6000 8000 ref. (keV)

d-point energies and the evaluated Qb’s show the effective energy loss of the HPGe

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6400 6800 7200 7600

148La

Fig. 4. The square-root plots and the deduced Eb-max of 147La(a) and 148La(b). Each region between dotted lines is adopted for analysis.

H. Hayashi et al. / Nuclear Instruments and Methods in Physics Research A 606 (2009) 484–489488

respectively, by means of the square-root plot as described above.Finally, after adding the effective energy loss of 180 keV, the Qb’swere deduced to be 5366(40) and 7732(70) keV for 147La and 148La,respectively. The uncertainties were derived from the effectiveenergy loss and each statistic. The deduced value for 148La has arelatively larger uncertainty which consists in the statistical one of60 keV and the systematic one of 30 keV, approximately.

4. Discussion

The results and the previously proposed values are summar-ized in Table 1. The value determined for 147La is not in agreement

with the previously reported values of 4945(55) keV [10] and5150(40) keV [11] with their uncertainties. However, it is close tothe value 5150(40) keV. The value of 4945(55) [10] was adoptedby Audi et al. in 1995. The preliminary proposed value of5370(100) keV [12], which was measured with the BGO totalabsorption detector, is in good agreement with the present result.The last evaluation in 2003 [7] and the value was found to be5180(40) keV, which seems to take Ref. [11] into accountintensively, but not Ref. [12]. Anyhow, the value of 4945(55) keVis quite small compared to the present result.

Similarly, the result for 148La is also in agreement with theresult that was measured with the BGO total absorption detector[12]. Previously, three experimental values of 45862(100) [13],

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Table 1The comparison of the experimental and systematics Qb for 147La and 148La.

Nuclide Qb

Experimental Systematics

Present Previous 1995a 2003b

147La 5366(40) 4945(55)c 5150(40)d 5370(100)g 4945(55) 5180(40)148La 7732(70) 7310(150)e 7255(55)f 7650(100)g 7262(50) 7260(50)

a Taken from Ref. [15].b Taken from Ref. [7].c Taken from Ref.[10].d Taken from Ref. [11].e Taken from Ref. [13].f Taken from Ref. [14].g Taken from Ref. [12].

H. Hayashi et al. / Nuclear Instruments and Methods in Physics Research A 606 (2009) 484–489 489

7255(55) [14] and 7650(100) keV [12] were proposed. As de-scribed in Ref. [14], the value of 5862 keV [13] was re-evaluatedto be 7310(150) keV from the information of decay study. Inthe previous systematics in 1995 [15], the value 7262(50) keVwas proposed. It seems to stand on Ref. [14] only. The lastsystematics of 7260(50) keV for 148La by Audi and Wapstra [7]seems to exclude the value of 7650(100) keV [12]. In the caseof the neutron-rich La isotopes, it had been proposed that thereis disagreement in the two neutron separation energies (S2n)between the experimental values [12] and the systematicsdetermined by Audi and coworkers [15], [16]. Recently, Clarket al. [17] also proposed that the experimentally determinedatomic masses of 147,148La by Canadian ion trap were more than400 keV larger than those in [15]. These results are consistent withthe present results. Such large differences might indicate somenuclear effects. Spectroscopic study is necessary for furtherunderstanding of the nuclear structure. As mention above, thesystematics strongly depends upon the experimental values. Itmeans the experimental values are important for the propositionof reliable systematics. In further analysis taking into account theenergy broadening owing to energy straggling, the tailing regionin high-energy side could be included in the analyzing region, andthen the uncertainties will be expected to be reduced. The resultswill be described in detail in a forthcoming paper.

5. Conclusion

A novel total absorption detector for Qb determination wasdeveloped. It is composed of a large-volume HPGe detector havinga through-hole and a BGO anti-Compton detector. The ability ofthe detector to determine Qb with high accuracy and independentof decay scheme information was demonstrated. The Qb’s of the147La and 148La were determined to be 5366(40) and 7732(70) keV,respectively.

Acknowledgements

This work was carried out under the Research CollaborationProgram of the Research Reactor Institute, Kyoto University.This work was partially supported by the Ministry of Education,Science, Sports and Culture, Grant-in-Aid for Scientific Research(B), no. 15360504.

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