First search for 2ε and ε β + decay of 174 Hf
F.A. Danevich, M. Hult, D.V. Kasperovych, G.P. Kovtun, K.V. Kovtun, G. Lutter, G. Marissens, O.G. Polischuk, S.P. Stetsenko, V.I. Tretyak
aa r X i v : . [ nu c l - e x ] J a n First search for ε and εβ + decay of Hf F.A. Danevich a, , M. Hult b , D.V. Kasperovych a , G.P. Kovtun c,d , K.V. Kovtun e ,G. Lutter b , G. Marissens b , O.G. Polischuk a , S.P. Stetsenko c , V.I. Tretyak aa Institute for Nuclear Research, 03028 Kyiv, Ukraine b European Commission, Joint Research Centre, Retieseweg 111, 2440 Geel, Belgium c National Scientific Center “Kharkiv Institute of Physics and Technology”, 61108 Kharkiv,Ukraine d Karazin Kharkiv National University, 61022 Kharkiv, Ukraine e Public Enterprise “Scientific and Technological Center Beryllium”, 61108 Kharkiv, Ukraine
Abstract
The first ever search for 2 ε and εβ + decay of Hf was realized using a high-puresample of hafnium (with mass 179.8 g) and the ultra low-background HPGe-detectorsystem located 225 m underground. After 75 days of data taking no indication of thedouble beta decay transitions could be detected but lower limits for the half-lives of thedifferent channels and modes of the decays were set on the level of lim T / ∼ − a. Keywords : Double beta decay;
Hf, Low-background HPGe γ spectrometry A great interest to double beta (2 β ) decay, particularly to the neutrinoless mode of the process(0 ν β ), is related to unique possibilities to clarify properties and nature of neutrino and weakinteractions [1, 2, 3, 4, 5], and to test many other hypothetical scenarios of the 0 ν β decay[1, 3, 6, 7].The efforts of experimentalists are concentrated mainly on the searches for the 0 ν mode of2 β decay with electrons emission (see reviews [4, 8, 9, 10, 11, 12, 13, 14] and recent experimentalworks [15, 16, 17, 18, 19]). However, even the most sensitive experiments do not observe theeffect and only set half-life limits on the level of T ν β / > (10 − ) a, that lead to therestrictions on the effective Majorana mass of electron neutrino on the level of (0.1 – 0.7)eV, depending on the nuclei and the nuclear matrix elements calculations. The sensitivity tothe double beta plus processes: double electron capture (2 ε ), electron capture with positronemission ( εβ + ), and double positron emission (2 β + ) is much lower (we refer reader to the recentreviews [20, 21] and the references therein). At the same time, there is a strong motivation toimprove sensitivities in studies of the double beta plus decay processes related to a possibilityto clarify possible contribution of the right-handed currents to the 0 ν β decay rate in case ofits observation [22]. Corresponding author.
E-mail address: [email protected] (F.A. Danevich).
Hf is one of the potentially 2 ε , εβ + radioactive nuclides with the energy ofdecay Q β = 1100 . δ = 0 . . A simplifiedexpected decay scheme of Hf is shown in Fig. 1. While the double electron capture is possiblewith population of the ground state and the first 2 + excited level of the daughter nuclei, theelectron capture with positron emission is allowed with the population of the ground state only(at least, for captures from K and L atomic shells, for which this process is the most probable).All the expected decays should be accompanied by single or multiple γ (X-ray) quanta emissionthat opens a possibility to apply HPGe γ spectrometry to search for the decays. + Yb + (1) - Lu
71 174 Hf + e , eb + Q b = 1100.0(23) keV Figure 1: The simplified decay scheme of
Hf.In Section 2 we describe the high-purity hafnium sample production and the experimentaltechnique of ultra-low background HPGe γ spectrometry used in the present study. The dataanalysis and obtained limits on the 2 ε and εβ + processes in Hf are reported in Sect. 3. TheConclusions section contains a summary of the experiment and some discussion of possibilitiesto improve the experimental sensitivity.
A disc-shaped sample of metallic hafnium with sizes ⊘ . × . ≈ .
29 g of the isotope
Hf) was utilized in the experiment.The hafnium was produced by the former Soviet Union industry by reduction process fromhafnium tetrafluoride with metallic calcium. Then the material was purified by centrifugation ofgaseous Hf compound to reduce zirconium concentration that is typically the main contaminantof hafnium, which is very hard to separate by chemical and physical methods. Finally the It should be stressed that so low isotopic abundance is typical for the potentially double beta plus activeisotopes and is one of the practical reasons of the modest experimental sensitivity to this kind of nuclearinstability. ≃ . . The experiment was carried out with the help of two set-ups with three HPGe detectors (namedGe6, Ge7, and Ge10) at the HADES underground laboratory of the Joint Research Centre ofEuropean Commission (Geel, Belgium) located at 225 m depth below the ground. A schematicview of the both set-ups is presented in Fig. 2, while the main characteristics of the detectorsare given in Table 1, with some more details in [25, 26, 27].Table 1: Characteristics of the HPGe-detectors used in present experiment.Ge6 Ge7 Ge10Energy resolution (FWHM) at 84 keV 1.4 keV 1.3 keV 0.9 keVFWHM at 1332 keV 2.3 keV 2.2 keV 1.9 keVRelative efficiency 80% 90% 62%Ge crystal mass 2096 g 1778 g 1040 gWindow material and thickness LB Cu 1.0 mm HPAl 1.5 mm HPAl 1.5 mmTop dead layer thickness 0.9 mm 0.3 µ m 0.3 µ mLB Cu = Low Background CopperHPAl = High Purity AluminumThe Hf sample was stored 13 days underground before the low background measurementsto enable decay of short-lived cosmogenic radionuclides. In the first measurement the hafniumsample was installed directly on the endcap of the detector Ge10 (the “set-up I”, see Fig. 2). Themeasurements in the set-up were continued over 40.4 days with the Ge10 detector and 36.4 dayswith the detector Ge7. The Ge10 detector is developed for low-energy γ -rays measurementsand has a very high energy resolution and high detection efficiency to γ quanta in the energyregion ≈ (50 −
80) keV where most of the X-rays and γ quanta expected in the two-neutrinomode of the 2 ε process to the ground state and to the lowest excited level 2 + Ybshould be emitted. The Ge7 detector also has a rather high detection efficiency to low-energy γ quanta, despite a slightly worse energy resolution.After the first stage, the experiment was continued for 34.8 days with the Ge6 detectorinstead of Ge10 (the second stage of the experiment is named “set-up II”). The Ge6 detectorhas a comparatively high detection efficiency to middle and high-energy γ quanta, howeverits sensitivity to low-energy γ quanta is substantially lower than that of the Ge7 and Ge10detectors. Nevertheless, the detection efficiency of the detector Ge6 is high enough to detect γ quanta expected in the 0 ν ε and the εβ + processes with energies ∼ (0 . −
1) MeV (see Section3). Besides, the detector was useful to estimate radioactive contamination of the hafniumsample. The total exposure of the experiment was 42 g × d for the isotope Hf . The purity level of the sample is reported in detail in our previous work aimed to the first search for α decays of naturally occurring Hf nuclides with emission of γ quanta [25]. The exposure was calculated as a product of the isotope mass on the sum of measuring times of the four γ peaks in the data that can be ascribed to the naturally occurringprimordial radionuclides: K, daughters of the
Th,
U, and
U families. Some spe-cific activities of hafnium radioactive nuclides were observed too:
Hf [electron capturewith Q EC = 683 . T / = 70(2) days] and Hf [beta active with Q β = 1035 . T / = 42 . Hf and
Hf, re-spectively (both present in the Hf natural isotopic composition), and by interactions with highenergy cosmic neutrons on the Earth surface, and especially, during the sample transportationby air.Activities of the radionuclides in the hafnium sample were calculated with the followingformula: A = ( S sample /t sample − S bg /t bg ) / ( ξ · η ) (1)where S sample ( S bg ) is the area of a peak in the sample (background) spectrum; t sample ( t bg ) isthe time of the sample (background) measurement; ξ is the γ -ray yield for the correspondingtransition [28]; η is the full energy peak detection efficiency. The efficiencies were Monte Carlosimulated with the help of EGSnrc simulation package [29, 30]. The calculations were validatedby comparison with the experimental data obtained with Cd and Ba γ sources (in the detectors used in the experiment.
100 200 300 400 500 600
Pb, 46.5Hf X-rays54.1 - 65.3
U, 185.7
Pb, 238.6
Ac270.2
Pa300.1, 302.7
Ac, 338.7
Hf, 343.4
Pb, 351.9
Rn, 401.8
Ac463.0
Hf, 482.2511
Tl583.2
Bi609.3
I Ge7Energy (keV) C oun t s / k e V
100 200 300 400 500 600
Pb, 46.5Hf X-rays54.1 - 65.3
U, 185.7
Pb, 238.6
Ac270.2
Pa300.1, 302.7
Ac, 338.7
Hf, 343.4
Pb, 351.9
Rn, 401.8
Ac463.0
Hf, 482.2511
Tl583.2
Bi609.3
I Ge10Energy (keV) C oun t s / k e V Figure 3: (Color online) Energy spectra accumulated with the Hf sample (solid line) andwithout sample (dots) by ultra-low-background HPGe γ detectors Ge7 (over 38.4 d with thehafnium sample and over 38.5 d without sample), and Ge10 (over 40.4 d with hafnium and 38.5d background). The background energy spectra are normalized to the times of measurementswith the Hf sample. Energy of γ and X-ray quanta are in keV.set-up I), and Cd,
Ba,
Cs,
Eu, Am γ sources (set-up II). The standard deviationof the relative difference between the Monte Carlo simulations and the experiment is (5 − γ peaks in the energy interval (53 – 384) keV for the set-up I, and is 6% for γ peaks inthe energy interval (60 – 1408) keV for the set-up II. A summary of the estimated activities(limits) of radioactive impurities in the Hf sample is given in Table 2.A peculiarity of the radioactive contamination is a significant deviation of the U/ Uactivities ratio in the Hf sample . The excess of U can be explained by the application ofgas centrifugation method to remove zirconium in the hafnium production cycle (see Sec. 2).Despite the details of the production process are unknown, one could assume that the contam-ination by
U happened due to proximity between the industrial sites of the centrifugationfacilities to purify hafnium and to enrich uranium. A more detailed discussion of the Hf sampleradioactive contamination one can find in [25]. The observed ratio is 0 .
500 1000 1500 2000 2500
U, 185.7
Pb, 238.6
Ac, 338.7511
Ac, 911.2, 964.8
Pa, 1001.0 K, 1460.8
Bi, 1764.5
Tl, 2614.5
II Ge7Energy (keV) C oun t s / k e V
500 1000 1500 2000 2500
U, 185.7
Pb, 238.6
Ac, 338.7511
Ac, 911.2, 964.8
Pa, 1001.0 K, 1460.8
Bi, 1764.5
Tl, 2614.5
II Ge6Energy (keV) C oun t s / k e V Figure 4: (Color online) Energy spectra accumulated with the hafnium sample (solid line) andwithout sample (dots) by ultra-low-background HPGe γ detectors Ge7 (over 34.8 d with thehafnium sample and over 28.1 d without sample), and Ge6 (over 34.8 d with hafnium and 25.3d background). The background energy spectra are normalized to the times of measurementswith the Hf sample. Energy of γ quanta are in keV. No peculiarity was observed in the experimental data that could be ascribed to the 2 β decayprocesses in Hf. Thus, we set limits on different modes and channels of the decay by usingthe following formula: lim T / = N · η · t · ln 2 / lim S, (2)where N is the number of Hf nuclei in the sample (9 . × ), η is the detection efficiencyfor the effect searched for, t is the measuring time, and lim S is the number of events of theeffect which can be excluded at a given confidence level (C.L.). The detection efficiencies ofthe detectors to the γ (X-ray) quanta expected in different modes and channels of the doublebeta processes in Hf were simulated with the EGSnrc simulation package [29, 30], the decayevents were generated by the DECAY0 events generator [31].6able 2: Radioactive contamination of the Hf sample measured by HPGe γ -ray spectrometry.The activities of Hf and
Hf are given with reference date at the start of each measurementfor Set-up I and Set-up II (within brackets) separately. Upper limits are given at 90% C.L.,the reported uncertainties are the combined standard uncertainties.Chain Nuclide Activity in the sample (mBq) K ≤ . Co ≤ . Cs ≤ . Hf ≤ Hf 0 . ± .
05 (0 . ± . m Hf ≤ . Hf 1 . ± .
07 (0 . ± . Hf ≤ . Th Ra 3 . ± . Th 2 . ± . U U 3 . ± . Pa 11 ± Ac 2 . ± . U m Pa 11 ± Ra ≤ . Pb ≤ Hf In case of the 2 K and KL capture in Hf, a cascade of X-rays (and Auger electrons) of Ybatom with individual energies, in particular, in the energy interval (50 . − .
3) keV is expected,while energies of the 2 L capture X-ray quanta are ≈ (7 −
10) keV, that are below the detectors’energy thresholds. We took into account only the most intense X-rays of ytterbium [28]: 51.4keV (the intensity of the X-ray quanta is 27.2%), 52.4 keV (47.4%), 59.2 keV (5.2%), 59.4 keV(10.0%), and 61.0 keV (3.4%). The energy spectra accumulated with the Hf sample were fittedby the sum of several Gaussian functions: five peaks of 2 ν K decay of Hf with energies(51 . − .
0) keV, a peak of
Pb with energy 46.5 keV, Gaussian functions to describe theX-ray peaks of Hf, the 63.3 keV peak of
Th, 67.7 keV peak of
Th and a straight line todescribe the continuous background. A highest sensitivity to the effect was achieved by analysisof a sum spectrum of the detectors Ge7, Ge10 in the set-up I and of the Ge7 detector in theset-up II. The individual energy dependencies of the detectors energy resolutions were takeninto account in the model. The data of the Ge6 detector was not used taking into account arather low detection efficiency of the detector to low-energy γ (X-ray) quanta. The best fit wasachieved in the energy interval (39 −
71) keV with χ / n.d.f. = 41 . /
47 = 0 .
89, where n.d.f.is number of degrees of freedom. The fit gives an area of the effect − ±
26 counts. By using It should be noted that also two X-ray quanta with a total energy up to ≈
122 keV could be detected incoincidence in the 2 ν K process, however, the detection efficiency to the events is substantially lower than thatto one K X-ray quanta. S with 90% confidence level (C.L.) . Thesum energy spectrum in the vicinity of the effect and the model of background are presented inFig. 5 together with the excluded effect. It should be noted that here and below the estimationsof the lim S values include only the statistical errors coming from the data fluctuations, andthat systematic contributions have not been considered. However, the statistical errors alreadydo include correlations to the background model. The detection efficiency for the effect wascalculated by the following formula: η = X η i × t i / X t i , (3)where η i are the individual detection efficiencies and t i are the measuring times of the detectorsused in the analysis. The detection efficiency is estimated to be 1.24% for the whole X-raysdistribution, and the following limit on the 2 ν K decay of Hf to the ground state of
Ybwas set: T ν K / ≥ . × a. The detection efficiency to the X-ray quanta in the energyinterval (51 . − .
0) keV in the case of the 2 νKL decay in
Hf to the ground state of
Ybis lower (0.73%) that results in the following half-life limit: T νKL / ≥ . × a. The resultsare presented in Table 3 together with the values of the lim S and the detection efficiencies. Pb, 46.5 H f , n K , K L Hf X-rays, 54.1 - 65.3
Th, 63.3
Th, 67.7Tl, Pb, Bi, PoX rays } Energy (keV) C oun t s / . k e V Figure 5: (Color online) The sum energy spectrum collected with the detectors Ge7, Ge10 inthe set-up I, plus the Ge7 detector in the set-up II (solid histogram) in the energy region where K X-ray quanta are expected for the 2 ν K and 2 νKL decays of Hf. The fit of the data bythe background model is shown by solid line, while the excluded effect is presented by dashedline. The background data accumulated with the detectors, normalized on the measuring time,are shown by dots. Energy of γ and X-ray quanta are in keV. All the limits in the present work were set with 90% C.L. by using the recommendations [32]. ε and εβ + processes in Hf. The energies of the γ quanta ( E γ ),which were used to set the T / limits, are listed with their corresponding detection efficiencies( η ) and values of lim S .Channel Decay Level of E γ η lim S Experimental limitof decay mode daughter (keV) (%) (counts) T / (a) at 90% C.L.nucleus (keV) at 90% C.L.2 K ν g.s. 51 . − . ≥ . × KL ν g.s. 51 . − . ≥ . × K ν + . − . ≥ . × KL ν + . − . ≥ . × L ν + . ≥ . × K ν g.s. 977.4 4.53 10.0 ≥ . × KL ν g.s. 1028.9 4.46 3.0 ≥ . × L ν g.s. 1080.4 4.39 7.2 ≥ . × K ν + ≥ . × KL ν + ≥ . × L ν + ≥ . × Kβ + (2 ν +0 ν ) g.s. 511 10.6 202 ≥ . × Lβ + (2 ν +0 ν ) g.s. 511 10.7 202 ≥ . × A similar group of Yb K X-ray quanta is expected also in the 2 ν K and 2 νKL decays of Hf to the first 2 + Yb. The sensitivity with the (51 . − . γ quanta. The reasons are:1) a higher detection efficiency to the X-ray quanta (in particular, because in deexcitation ofthe 76.5 keV level mostly electrons are emitted, conversion coefficient is equal 9.43 [33], whilethe set-up is not sensitive to these electrons); 2) a rather high counting rate in the vicinity ofthe energy 76.5 keV due to the background caused by X-rays of Tl, Pb, Bi and Po. Thus, thelimits on the 2 ν K and 2 νKL decays to the 76.5 keV level were derived from the analysis ofthe energy region where the group of (51 . − .
0) keV X-ray quanta is expected (see Fig. 5).The detection efficiencies, excluded effects’ areas and obtained half-life limits are presented inTable 3.However, the limit for 2 ν L process was obtained by analysis of the 76.5 keV peak in theexperimental data, taking into account that L X-ray quanta are below the acquisition energythreshold of our experimental set-ups and cannot be detected in the present experiment. Theanalysis of the data near 76.5 keV is rather complicated, since there are many X-ray peaks inthe energy region. Thus, we have fitted the experimental sum spectrum of the three detectorsin the energy interval around the energy 76.5 keV by sum of the most intensive K X-rayquanta of Tl (72.9 keV), Pb (75.0 keV), Bi (77.1 keV and 87.3 keV), Po (76.9 keV, 79.3 keV)and Rn (81.1 keV, 83.8 keV). The model describes the experimental data rather well withthe χ / n.d.f. = 41 . /
47 = 0 .
72, giving the 76.5 keV peak area S = 5 . ± . S = 20 . −
100 keV is presentedin Fig. 6 together with the background model and the excluded peak with energy 76.5 keV.9he obtained half-life limit on the 2 ν L decay of Hf to the first 2 + Yb is given in Table 3. T h , . Tl, Pb, Bi, Po, Rn X rays
Hf 2 n Yb, 76.5 L Th92.4 + 92.8
Energy (keV) C oun t s / . k e V Figure 6: (Color online) Part of the sum energy spectrum collected with the detectors Ge7,Ge10 in the set-up I, and with the Ge7 detector in the set-up II (solid histogram) in the energyregion where 76.5 keV γ quanta emitted in the 2 ν L decay of Hf to the first 2 + Yb are expected together with the model of background (solid line). Theexcluded 76.5 keV peak with area 20.4 counts is shown by dashed line. The background energyspectrum accumulated with the detectors, normalized on the measuring time, is shown by dots.Energy of γ and X-ray quanta are in keV.In case of the 0 ν double electron capture in Hf from K and L shells to the ground stateof the daughter nuclei, we suppose that the energy excess in the process is taken away bybremsstrahlung γ quanta with an energy: E γ = Q β − E b − E b , where E bi are the bindingenergies of the captured electrons on the K and L atomic shells of the daughter Yb nuclide.The energy spectrum measured with the Hf sample by the detectors Ge7 and Ge6 in the set-upII was fitted by a model constructed from a peak searched for and a 1st degree polynomialfunction to describe the continuous background (see Fig. 7). In the case of the 0 ν L decay,the γ peak of Bi with energy 1078.8 keV was also included in the background model. Theenergy of the Gaussian function used to describe the effect was varied taking into account theuncertainty of the Q β value ( ± . . ± . − . ± .
2) counts, and (3 . ± .
4) counts for the 0 ν K , 0 νKL , and 0 ν L peaks,respectively. The corresponding lim S values are 10.0, 3.0 and 7.2 counts. The excluded peaksare also shown in Fig. 7. The data gathered in the set-up I were not used in the analysis due to the restricted at ∼ . Ac 968.8
Ac 964.4 0 n Pa 1001.0
Energy (keV) n KL Energy (keV) C oun t s / . k e V Bi 1078.8 0 n Energy (keV)
Figure 7: (Color online) Parts of the sum energy spectrum accumulated with the Hf sampleby the detectors Ge7 and Ge6 in the set-up II, where the γ peaks from the 0 ν K (upperpanel), 0 νKL (middle panel), and 0 ν L (lower panel) captures in Hf to the ground state of
Yb are expected. The fit of the data are shown by solid lines, while the excluded peaks arepresented by dashed lines. The horizontal lines (above the arrows labelling the energy of thepeaks searched for) show the energy interval ± . Q β value of Hf. The background data accumulated with the detectors, normalized on themeasuring time, are shown by dots. The energy of the background γ peaks are in keV.11 similar analysis was performed also for the 0 ν double electron capture transitions to the2 + Yb. The results of the analysis are shown in Fig. 8. The obtainedlower half-life limits for the 0 ν ε decays of Hf to the ground and the 2 + Yb are given in Table 3.
Ac 911.20 n
2K 76.5
Energy (keV) n KL 76.5
Ac 968.8
Ac 964.4
Energy (keV) C oun t s / . k e V Pa 1001.0 0 n
2L 76.5
Energy (keV)
Figure 8: (Color online) Parts of the sum energy spectrum accumulated with the Hf sample bythe Ge7 and Ge6 detectors in the set-up II, where the γ peaks from the 0 ν K (upper panel),0 νKL (middle panel), and 0 ν L (lower panel) captures in Hf to the 2 + Yb are expected. The fits of the data are shown by solid lines, while the excluded peaksare presented by dashed lines. The horizontal lines (above the arrows labelling the energy ofthe peaks searched for) show the energy interval ± . Q β value of Hf. The background data accumulated with the detectors, normalized onthe measuring time, are shown by dots. The energy of the background γ quanta are in keV. Hf One positron with an energy up to 78 ± . εβ + decay of Hf.The annihilation of the positron should give two 511 keV γ ’s leading to an extra counting ratein the annihilation peak. The sum of all four detectors’ energy spectra was fitted in the energyinterval (495 − ±
33 counts in the peak in the data accumulated with the hafniumsample, and 508 ±
32 counts in the background data. The Monte Carlo simulations show that12he main part of the excess (153 ±
17 counts) can be explained by decays of
Ac and
Tl inthe Hf sample. The residual peak area of 122 ±
49 counts, despite showing excess of more thantwo sigma, cannot be accepted as effect of electron capture with positron emission in
Hf. Thedifference indicates presence of some systematics, that needs more careful investigations, e.g.,by using isotopically enriched sample. Thus, assuming that the 511 keV peak excess providesno evidence of the effect searched for, 202 counts should be accepted as lim S . Taking intoaccount the detection efficiency 10.6% (10.7%) for Kβ + ( Lβ + ) decay mode, we have obtainedthe same limit on the half-life of Hf relatively to the electron capture from the K and L shells of daughter atom with positron emission in Hf: T / ≥ . × a. The limits arepresented in Table 3. The limits are valid for both the 2 ν and 0 ν modes of the decay, since themodes cannot be distinguished by the γ spectrometry method. Ac, 463.0
Hf, 482.2 Annihilation, 511
Energy (keV) C oun t s / . k e V Figure 9: (Color online) Part of the energy spectra measured with the Hf sample by the detectorsGe7, Ge10 (set-up I) and Ge7, Ge6 (set-up II) in the vicinity of the 511 keV annihilation peak.The background data accumulated with the detectors, normalized on the measuring time withthe Hf sample, are shown by dots. The fits of the data are shown by solid lines. The energy ofthe background γ peaks are in keV. A highly purified hafnium-disc with mass 179.8 g and the dimensions ⊘ . × . γ -ray spectrometry systemlocated 225 m underground at the HADES laboratory with an aim to search (at the first time)for 2 ε and εβ + decay of Hf. No effect was observed after 75 days of data taking but lowerlimits on the half-lives for the different channels and modes of the decays were set on the levelof lim T / ∼ − a.The sensitivity of the experiment could be advanced by using even more highly purified fromtrace radioactive impurities hafnium enriched in the isotope Hf, increasing the exposure,13nd detection efficiency by application of thinner samples and multi-crystal system of HPGedetectors. It should be stressed that such an experiment looks practically realizable thanks toa general possibility to apply gas centrifugation for Hf isotopical enrichment, for the momentonly the viable technology to produce large enough amount of isotopically enriched materials.
This project received support from the EC-JRC open access project EUFRAT under Hori-zon 2020. The group from the Institute for Nuclear Research (Kyiv, Ukraine) was supportedin part by the program of the National Academy of Sciences of Ukraine “Fundamental researchon high-energy physics and nuclear physics (international cooperation)”. D.V.K. and O.G.P.were supported in part by the project “Investigations of rare nuclear processes” of the programof the National Academy of Sciences of Ukraine “Laboratory of young scientists” (Grant No.0118U002328).
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