Low-background HPGe Research Articles

New half-life limits on double-βdecays of110Pd and102Pd into excited states

A search for excited state transitions of double beta decay in Pd110 and double electron capture in Pd102 has been performed in the HADES underground laboratory with two low background HPGe detectors in sandwich configuration. After an exposure of 35.92 kg*d, no signal was found. The frequentist spectral analysis resulted in lower half-life limits of 1.98e20 yr and 1.72e20 yr (95% CL) for the first 0+ and 2+ excited state in Pd110 respectively. This is an improvement by more than a factor of 3 with respect to previous measurements. In Pd102, the lower half-life limit could be improved to 5.95e18 yr (95% CL) for the first 0+ excited state. Furthermore, first experimental lower half-live limits are found for all possible excited states in the Pd110 and Pd102 systems.

Measurement of fallout with rain in Hiroshima and several sites in Japan from the Fukushima reactor accident

Fallout with rain from the Fukushima reactor accident was monitored for about two months in Higashi-Hiroshima City, Hiroshima, Japan, from March 20 to May 23, 2011. Gamma-ray (γ-ray) spectra measured using a low background HPGe spectrometer showed clear evidence of fission products—131I, 137Cs, and 134Cs. The 131I was observed on March 27 and April 8, while 137Cs and 134Cs were observed on March 27, April 18, and April 22. The 131I, 137Cs, and 134Cs activity concentrations in rainwater collected in Hiroshima reached 0.44 ± 0.09 on April 8, 0.17 ± 0.01 on April 18, and 0.15 ± 0.01 Bq/L on April 18, 2011, respectively. These activity levels were compared with global results collected from the Northern Hemisphere. Several samples of rainwater that were collected in Chiba (Kashiwa) on March 21, April 11, and May 12; Tokyo (Nerima) on March 21 and April 11; Osaka (Hirano) on April 8; Nara (Kitakatsuragi) on April 9; and Fukushima (Fukushima) on April 19, were also measured by our spectrometer and compared. Among these samples of rainwater collected at different locations in Japan, the one collected at Kashiwa City, Chiba Prefecture, on March 21 showed the highest activity concentrations of 6072 ± 1, 632 ± 4, 766 ± 3, 637 ± 1, 97.6 ± 0.8, and 752 ± 2 Bq/L for 131I, 132I, 132Te, 134Cs, 136Cs, and 137Cs, respectively. The health risks due to these high activity levels were considered in terms of several regulations. The ratio of the activites for the isotopes 131I to 137Cs and 134Cs to 137Cs were compared with the other measurements and discussed.

Search for 7Li solar axions using resonant absorption in LiF crystal: Final results

The resonance excitation of the 7Li nuclei in a LiF crystal with mass of 553 g by hypothetical axions emitted in the deexcitation of the 7Li nuclei in the Sun was searched for deep underground at the Gran Sasso National Laboratories (LNGS) of INFN (3600 m w.e.). The data collected with a low background HP Ge detector 244 cm3 during 4044h have allowed us to set the limit on the axion mass: ma<8.6 keV (90% C.L.) which is the best one for the 7Li solar axions to-date.

First search for double β decay of dysprosium

A search for double $\beta$ decay of dysprosium was realized for the first time with the help of an ultra low-background HP Ge $\gamma$ detector. After 2512 h of data taking with a 322 g sample of dysprosium oxide limits on double beta processes in $^{156}$Dy and $^{158}$Dy have been established on the level of $T_{1/2}\geq 10^{14}-10^{16}$ yr. Possible resonant double electron captures in $^{156}$Dy and $^{158}$Dy were restricted on a similar level. As a by-product of the experiment we have measured the radioactive contamination of the Dy$_2$O$_3$ sample and set limits on the $\alpha$ decay of dysprosium isotopes to the excited levels of daughter nuclei as $T_{1/2}\geq 10^{15} - 10^{17}$ yr.

First observation ofαdecay ofPt190to the first excited level (Eexc=137.2keV) ofOs186

The $\ensuremath{\alpha}$ decays of naturally occurring platinum isotopes, which are accompanied by the emission of $\ensuremath{\gamma}$ quanta, have been searched for deep underground (3600 m water equivalent) at the Gran Sasso National Laboratories of the INFN (Italy). A sample of Pt with a mass of 42.5 g and a natural isotopic composition has been measured with a low background HP Ge detector (468 cm${}^{3}$) during 1815 h. The $\ensuremath{\alpha}$ decay of $^{190}\mathrm{Pt}$ to the first excited level of $^{186}\mathrm{Os}$ (${J}^{\ensuremath{\pi}}={2}^{+}$, ${E}_{\mathrm{exc}}=137.2$ keV) has been observed for the first time, with the half-life determined as ${T}_{1/2}=2.{6}_{\ensuremath{-}0.3}^{+0.4}(\mathrm{stat}.)\ifmmode\pm\else\textpm\fi{}0.6(\mathrm{syst}.)\ifmmode\times\else\texttimes\fi{}{10}^{14}$ yr. The ${T}_{1/2}$ limits for the $\ensuremath{\alpha}$ decays of other Pt isotopes have been determined at the level of ${T}_{1/2}\ensuremath{\simeq}{10}^{16}$--10${}^{20}$ yr. These limits have been set for the first time or they are better than those known from earlier experiments.

Use of silicon pixel detectors in double electron capture experiments

A novel experimental approach to search for double electron capture (EC/EC) is discussed in this article. R&D for a new generation EC/EC spectrometer based on silicon pixel detectors (SPDs) has been conducted since 2009 for an upgrade of the TGV experiment. SPDs built on Timepix technology with a spectroscopic readout from each individual pixel are an effective tool to detect the 2νEC/EC signature of the two low energy X-rays hitting two separate pixels. The ability of SPDs to indentify α/β/γ particles and localize them precisely leads to effective background discrimination and thus considerable improvement of the signal-to-background ratio (S/B). A multi-SPD system, called a Silicon Pixel Telescope (SPT), is planned based on the experimental approach of the TGV calorimeter which measures thin foils of enriched EC/EC-isotope sandwiched between HPGe detectors working in coincidence mode. The sources of SPD internal background have been identified by measuring SPD radiopurity with a low-background HPGe detector as well as by long-term SPD background runs in the Modane underground laboratory (LSM, France), and results of these studies are presented.

Background capabilities of pixel detectors for double beta decay measurements

We discuss the possible use of a progressive detection technique based on pixel detectors for the study of double beta decay (ββ) processes. A series of background measurements in various environments (surface laboratory, underground laboratory, with and without Pb shielding) was performed using the TimePix silicon hybrid pixel device. The pixel detector response to the natural background and intrinsic background properties measured by a low-background HPGe detector are presented.

Influence of the physico-chemical properties of Selangor soil series on the distribution coefficient (K d-value) of 226Ra

Ra is a member of the 238 U natural decay series and is one of the most important isotope to be determined among the naturally occurring nuclides in environmental samples. In order to evaluate the radiation dose from 226 Ra, it is important to know its mobility in different types of soils. The aim of the present study is to quantify the influence of physico-chemical soil properties on 226 Ra adsorption. The distribution coefficients (Kd-value) of 226 Ra in Selangor soil series samples were measured in one core, at three depth levels to evaluate the adsorbability of 226 Ra. The soil samples were spiked with 226 Ra tracer and the activities of 226 Ra in the separated phase from batch sorption test were measured by a low background but high efficiency well-type HPGe detector. Several physico-chemical soil properties were also char- acterised for each soil samples. Pearson's correlation and stepwise multiple regression test were applied at the 0.05 level of significance throughout all analysis to determine the relationships and influences between distribution coefficients (Kd-value) of 226 Ra with physicochemical soil properties for the Selangor soil series. The observed Kd value was in the range of 50.55-172.28 mL g -1 (mean: 93.20 ± 46.99 mL g -1 ). The regression showed that the highest positive correlations were observed for organic matter (OM) and cation exchange capacity (CEC) (r = 0.96**, 0.81**, respectively) with Kd-values. The results indicate that the stepwise multiple regression model incorporating the soil's OM and CEC accounts for 98% of the variability in distribution coefficients of 226 Ra.

Investigation of ββ decay in 150Nd and 148Nd to the excited states of 150Sm and 148Sm

Double beta decay of 150Nd and 148Nd to the excited states of daughter nuclei have been studied using a 400 cm3 low-background HPGe detector and an external source consisting of 3046 g of natural Nd2O3 powder. The half-life for the two-neutrino double beta decay of 150Nd to the excited 0+1 state in 150Sm is measured to be T1/2 = [1.33+0.36−0.23(stat)+0.27−0.13(syst)] · 1020 y. For other (0ν + 2ν) transitions to the 2+1, 2+2, 2+3, and 0+2 levels in 150Sm, limits are obtained at the level of ~ (2 − 8) · 1020 y. In the case of 148Nd only limits for the (0ν + 2ν) transitions to the 2+1, 0+1, and 2+2 excited states in 148Sm were obtained and are at the level of ~ (4 − 8) · 1020 y.

Cosmogenic production as a background in searching for rare physics processes

We revisit calculations of the cosmogenic production rates for several long-lived isotopes that are potential sources of background in searching for rare physics processes such as the detection of dark matter and neutrinoless double-beta decay. Using updated cosmic-ray neutron flux measurements, we use TALYS 1.0 to investigate the cosmogenic activation of stable isotopes of several detector targets and find that the cosmogenic isotopes produced inside the target materials and cryostat can result in large backgrounds for dark matter searches and neutrinoless double-beta decay. We use previously published low-background HPGe data to constrain the production of 3H on the surface and the upper limit is consistent with our calculation. We note that cosmogenic production of several isotopes in various targets can generate potential backgrounds for dark matter detection and neutrinoless double-beta decay with a massive detector, thus great care should be taken to limit and/or deal with the cosmogenic activation of the targets.

Investigation ofββdecay inNd150andNd148to the excited states of daughter nuclei

Double beta decay of $^{150}\mathrm{Nd}$ and $^{148}\mathrm{Nd}$ to the excited states of daughter nuclei have been studied using a $400 {\mathrm{cm}}^{3}$ low-background HPGe detector and an external source consisting of 3046 g of natural ${\mathrm{Nd}}_{2}{\mathrm{O}}_{3}$ powder. The half-life for the two-neutrino double beta decay of $^{150}\mathrm{Nd}$ to the excited ${0}_{1}^{+}$ state in $^{150}\mathrm{Sm}$ is measured to be ${T}_{1/2}=[1.33{}_{\ensuremath{-}0.23}^{+0.36}(\mathrm{stat}){}_{\ensuremath{-}0.13}^{+0.27}(\mathrm{syst})]\ifmmode\times\else\texttimes\fi{}{10}^{20}$ yr. For other $(0\ensuremath{\nu}+2\ensuremath{\nu})$ transitions to the ${2}_{1}^{+}$, ${2}_{2}^{+}$, ${2}_{3}^{+}$, and ${0}_{2}^{+}$ levels in $^{150}\mathrm{Sm}$, limits are obtained at the level of $~(2\text{\ensuremath{-}}8)\ifmmode\times\else\texttimes\fi{}{10}^{20}$ yr. In the case of $^{148}\mathrm{Nd}$ only limits for the $(0\ensuremath{\nu}+2\ensuremath{\nu})$ transitions to the ${2}_{1}^{+}$, ${0}_{1}^{+}$, and ${2}_{2}^{+}$ excited states in $^{148}\mathrm{Sm}$ were obtained and are at the level of $~(4\text{\ensuremath{-}}8)\ifmmode\times\else\texttimes\fi{}{10}^{20}$ yr.

First limits on neutrinoless resonant 2 ε captures in 136Ce and new limits for other 2 β processes in 136Ce and 138Ce isotopes

A small CeCl 3 crystal (6.9 g) was measured deep underground (3600 m w.e.) at the Gran Sasso National Laboratories of the INFN (Italy) with a low background HP Ge detector (244 cm 3) during 1280 h. The crystal is not polluted by 232Th, 40K, 60Co, 137Cs (limits were set on the activity at a level of 0.05–1 Bq/kg), but contains radioactive 138La (0.68 Bq/kg) and, probably, is polluted by 235U and 238U (0.3–0.7 Bq/kg). The half life limits for different double β processes in 136Ce and in 138Ce were set in the range of ( 1 – 6 ) × 10 15 yr . In particular, the 2 ε captures in 136Ce to various excited levels of 136Ba were considered for the first time. The limits for the resonant neutrinoless transitions to the 2392.1 keV and 2399.9 keV levels of 136Ba are: T 1 / 2 > 2.4 × 10 15 yr and T 1 / 2 > 4.1 × 10 15 yr at 90% C.L., respectively.

Temporal variations of 7Be, 210Pb and 137Cs in moss samples over 14 month period

Concentrations of 210Pb, 7Be and 137Cs in moss samples were continuously measured, using low-background HPGe spectrometer at Novi Sad, Serbia (45°14′45″N, 19°51′35″E). Weekly data collected over 14 month period from January 2007 to March 2008 are presented and discussed. Measured values of 7Be activity concentrations in dry moss samples are ranged from 201 to 920 Bq/kg showing prominent increase in summer and autumn season. Data for 210Pb, ranged from 347 to 885 Bq/kg do not show such trend. The average concentration of 137Cs is 8.9 Bq/kg. It is shown that yield of 7Be can be estimated using average values of activity concentrations. Precipitation amount and duration of precipitation are measured and their possible influence on activity concentrations measured in moss samples was considered.

7Li solar axions: Preliminary results and feasibility studies

Feasibility studies on a search for 7Li solar axion have been carried out. In particular, the data collected with a LiF powder of 243 g measured during 722 h with a 408 cm 3 low background HP Ge detector (GSOR) at the Gran Sasso National Laboratories (LNGS) of INFN has allowed to set limit on the 7Li solar axion mass of 13.9 keV, better than the previously available one: 16.0 keV [A.V. Derbin, et al., JETP Lett. 81 (2005) 365]. It has also been shown that improved results would be achieved with LiF(W) crystal which—in contrast with the LiF powder—does not show a relatively high contamination by U/Th chains. The potential backgrounds and the reachable sensitivity of an experiment are discussed.

On the use of mercury as a means of locating background sources in ultra low-background HPGe-detector systems

In low-level gamma-ray spectrometry, it is common to measure large samples in order to obtain low detection limits for the massic activity (in mBq/kg). These samples have significant shielding effects. In order to study whether the background sources in three ultra low-background HPGe detectors were located in the detector or in the shield, Marinelli beakers filled with hyperpure mercury were measured. Although the measurements were hampered by the presence of cosmogenically produced 194Hg, information regarding the major background location of 40K, 60Co, 137Cs, 210Pb, 226Ra, 228Ra and 228Th could be obtained.

INVESTIGATION OF INDUCED RADIOACTIVITY IN THE LINAC-ADONE ACCELERATOR COMPLEX FOR THE MANAGEMENT OF THE DECOMMISSIONED MATERIAL

The LINAC-ADONE accelerator complex of the INFN-LNF Frascati National Laboratories, operating for 27 y prior to the commissioning of DAPhiNE, was dismantled in 1993. The scraps resulting from the decommissioning of LINAC-ADPhiNE have been temporarily stored in the same Frascati laboratory, waiting for definitive disposal. Relying on recommendations of the IAEA, European Commission and Italian committees, an experimental characterization study of the LNF repository was performed. The main objective was a classification of the scraps on the basis of internationally recognized "clearance levels," which are 0.1 Bq g(-1) for the isotopes of interest for this work. Secondly, a measurement of the materials suspected to be above 0.1 Bq g(-1) was planned. Activation isotopes were expected from the aluminum, copper, steel, and iron of the LINAC and the ADONE ring sections. For screening purposes, the repository area has been divided into zones, where in-situ measurements with a portable HP-Ge detector have been performed. In addition, small samples have been cut from a representative number of pieces, and accurate laboratory measurements have been made with a low background HP-Ge spectrometer. The experimental results are in good agreement with other studies and show that a large part of the material is below the mentioned specific activity level.

Radioactivity concentrations in soils of the Xiazhuang granite area, China

The natural radioactivity of soils at the Xiazhuang granite massif of Southern China has been studied. The radioactivities of 55 samples have been measured with a low-background HPGe detector. The radioactivity concentrations of 238U and 40K ranged from 40.2 to 442 and from 442 to 913 Bq/kg, respectively, while the radioactivity concentration of 232Th varied only slightly. In order to evaluate the radiological hazard of the natural radioactivity, the radium equivalent activity ( R a eq ) , the absorbed dose rate ( D ˙ ) , the annual effective dose rate and the external hazard index ( H ex ) have been calculated and compared with the internationally approved values. The study provides background radioactivity concentrations in a granite area, specifically, the area in the vicinity of a uranium mine in Southern China. The data can be used in exploring granite-type uranium deposits.

A Germanium spectrometer for routine characterisation of samples with the sensitivity of ββ-decay spectrometers

The status of a new ultra low background HPGe spectrometer is described. The aim is to allow measurement of samples with the sensitivity of ββ -experiments, to support ongoing research in chemistry and astrophysics.

A method to determine 238U activity in environmental soil samples by using 63.3-keV-photopeak-gamma HPGe spectrometer

The paper presents a method of measuring 238U activity in environmental soil samples by a low background HPGe spectrometer at the 63.3 keV gamma photopeak. The low 238U activity requires a large size soil sample with a mass of about 100 g. The geometrical and self-absorption effects as well as the density dependence of soil samples were then investigated. A procedure of analyzing environmental soil samples was established with a relative error of about 10%. Two soil samples of known activity were tested and 106 surface soil samples collected from the Southern part of Vietnam were analyzed, and then these results were compared by instrumental neutron activation analytical method.

Double-beta decay of 150Nd to the first 0+ excited state of 150 Sm: Current state

Two-neutrino double-beta decay of 150Nd to the first 0+ excited state in 150Sm is investigated with the 400-cm3 low-background HPGe detector. Preliminary data analysis for 6843 h shows an excess of events at 333.9 and 406.5 keV. If this excess is assigned to the investigated transition, then its half-life can be estimated at [1.2−0.3+0.5±0.4(syst.)]×1020 yr.