238U Chains Research Articles

Development of ultralow-background cryogenic calorimeters for the measurement of surface [formula omitted] contamination

Next-generation experiments searching for rare events must satisfy increasingly stringent requirements on the bulk and surface radioactive contamination of their active and structural materials. The measurement of surface contamination is particularly challenging, as no existing technology is capable of separately measuring parts of the 232Th and 238U decay chains that are commonly found to be out of secular equilibrium. We will present the results obtained with a detector prototype consisting of 8 silicon wafers of 150 mm diameter instrumented as bolometers and operated in a low-background dilution refrigerator at the Gran Sasso Underground Laboratory of INFN, Italy. The prototype was characterized by a baseline energy resolution of few keV and a background <100 nBq/cm2 in the full range of α energies, obtained with simple procedures for cleaning of all employed materials and no specific measures to prevent recontamination. Such performance, together with the modularity of the detector design, demonstrate the possibility to realize an alpha detector capable of separately measuring all alpha emitters of the 232Th and 238U chains, possibly reaching a sensitivity of few nBq/cm2.

Characterization of Uranium Ore Samples by HPGe Gamma-Ray Spectroscopy

Gamma logging for uranium exploration is currently based on total counting with the Geiger–Muller gas detectors or NaI(Tl) scintillators. However, the total count rate interpretation in terms of uranium concentration may be impaired in case of roll fronts when the radioactive equilibrium of the natural 238U radioactive chain is modified by the differential leaching of uranium and its daughter radioisotopes radium, radon, and so on. Indeed, in the case of secular equilibrium, more than 95% of gamma rays emitted by uranium ores come from 214Pb and 214Bi isotopes that are in the back end of the 238U decay chain. These two isotopes might produce an intense gamma signal even when uranium is not present, or with a much smaller activity, in the ore. Therefore, gamma spectroscopy measurements of core samples are performed on the surface with HPGe detectors to directly characterize uranium activity from the 1001-keV gamma ray of 234mPa that is at the beginning of the 238U chain. However, due to the low intensity of this gamma ray, acquisitions of several hours are needed. In view to characterize uranium concentration within a few minutes, we propose a method using both the 92 keV gamma rays of 234Th and 235U and the 98.4-keV uranium X-ray. This last is due to uranium self-induced fluorescence caused by gamma rays of the uranium chain, mainly 214Pb and 214 Bi. The gamma rays of 214Pb and 214Bi undergo scatterings in the sample leading to photons with energy in the 100-keV range that favor photoelectric interactions and, thus, fluorescence. The comparison of the uranium activity obtained with the 92- and 98.4-keV lines allows detecting a uranium heterogeneity in the ore. Indeed, in the case of uranium nugget, the 92-keV peak leads to underestimated uranium concentration due to the gamma self-absorption, but on the contrary, the 98.4-keV peak leads to an overestimation because of increased fluorescence. In order to test this new approach, tens of uranium ore samples have been measured with a handheld HPGe FALCON 5000 detector.

EVALUATION OF RADIOACTIVITY IN THE PHOSPHOGYPSUM STOCKPILE OF “HIV” VELES, THE REPUBLIC OF NORTH MACEDONIA

The production of phosphoric acid from natural phosphate ore generates an industrial waste product named phosphogypsum. Phosphogypsum contains considerable amounts of natural radionuclides from the 238U chain, originating from the ore but enriched during the technological process. In order to perform radiological characterization of the “HIV” (Chemical Industry Veles) phosphogypsum stockpile, five phosphogypsum samples were collected and analyzed. The mean values of gross alpha and beta specific activities ± standard deviation values were: (950±104) Bq/kg and (1694±220) Bq/kg, respectively. Further analysis showed increased gross activities of radionuclides of the 238U chain, while the radionuclides of the 232Th chain and 40K were below the detection limit. The mean values of the specific activities of 238U and 226Ra were (360±55) Bq/kg and (280±84) Bq/kg, respectively. The estimated annual outdoor effective dose, at 1m received by adults was 0.25 mSv/y, which is below a dose limit of 1 mSv/y for members of general public. The results obtained in this study show that radionuclides, although present in relatively high concentrations in the phosphogypsum pile, do not imply an increased external radiation risk for members of the population. The possible use of phosphogypsum in civil construction and agriculture may not be excluded if conditions of prior good planning taking into account the radionuclides activities exist.

Integral transform analysis of radionuclide transport in variably saturated media using a physical non-equilibrium model: application to solid waste leaching at a uranium mining installation.

The Generalized Integral Transform Technique (GITT) was employed to simulate transient one-dimensional flow in variably saturated porous media, as well as radioactive waste transport within different layers (a solid waste pile, nearby soil, and a granular aquifer) towards the edge of a uranium mining installation under institutional control. Computational codes, written using the Mathematica software system, were implemented and tested for solution of the coupled advection-dispersion equations for an arbitrary number of daughter products within a radioactive chain migrating in saturated and unsaturated soil layers. The computer simulations were verified in great detail against results obtained using the built-in routine NDSolve of the Mathematica platform and the HYDRUS-1D software system. The present work reports the main results for 238U chain radionuclide transport using data extracted from a safety assessment of solid waste repositories at a uranium mining and milling installation in Caetité, state of Bahia, Brazil, operated by INB (Indústrias Nucleares do Brasil). Concentration evolutions of the various radionuclides obtained with the simulations were analyzed for five different cases to explore variations in the infiltration and recharge rates, the effect of assuming physical equilibrium or non-equilibrium transport conditions, and of different initial concentrations of some of the radionuclides.

The use of self-induced X-ray fluorescence in gamma-ray spectroscopy of uranium ore samples

Gamma logging for uranium exploration are currently based on total counting with Geiger Müller gas detectors or NaI (TI) scintillators. However, the total count rate interpretation in terms of uranium concentration may be impaired in case of roll fronts, when the radioactive equilibrium of the natural 238U radioactive chain is modified by differential leaching of uranium and its daughter radioisotopes of thorium, radium, radon, etc. Indeed, in case of secular equilibrium, more than 95 % of gamma rays emitted by uranium ores come from 214Pb and 214Bi isotopes, which are in the back-end of 238U chain. Consequently, these last might produce an intense gamma signal even when uranium is not present, or with a much smaller activity, in the ore. Therefore, gamma spectroscopy measurements of core samples are performed in surface with high-resolution hyper-pure germanium HPGe detectors to directly characterize uranium activity from the 1001 keV gamma ray of 234mPa, which is in the beginning of 238U chain. However, due to the low intensity of this gamma ray, i.e. 0.84 %, acquisitions of several hours are needed. In view to characterize uranium concentration within a few minutes, we propose here a method using both the 92 keV gamma ray of 234Th and the 98.4 keV uranium X-ray. This last is due to uranium self-induced fluorescence caused by gamma radiations of 214Pb and 214Bi, which create a significant Compton scattering continuum acting as a fluorescence source and resulting in the emission of uranium fluorescence X-rays. The comparison of the uranium activity obtained with the 92 keV and 98.4 keV lines allows detecting a uranium heterogeneity in the ore. Indeed, in case of uranium nugget, the 92 keV line leads to underestimated uranium concentration due to gamma self-absorption, but on the contrary the 98.4 keV line leads to an overestimation because of increased fluorescence. In order to test this new approach, several tens of uranium ore samples have been measured with a handheld HPGe FALCON 5000 detector.

Measurement of the ion fraction and mobility of 218Po produced in 222Rn decays in liquid argon

We report measurements of the charged daughter fraction of 218Po as a result of the 222Rn alpha decay, and the mobility of 218Po+ ions, using radon-polonium coincidences from the 238U chain identified in 532 live-days of DarkSide-50 WIMP-search data. The fraction of 218Po that is charged is found to be 0.37 ± 0.03 and the mobility of 218Po+ is (8.6 ± 0.1) × 10−4 cm2/Vs.

Estimation of Radionuclide Concentrations and Average Annual Committed Effective Dose due to Ingestion for the Population in the Red River Delta, Vietnam

Radioactivity concentrations of nuclides of the 232Th and 238U radioactive chains and 40K, 90Sr, 137Cs, and 239+240Pu were surveyed for raw and cooked food of the population in the Red River delta region, Vietnam, using α-, γ-spectrometry, and liquid scintillation counting techniques. The concentration of 40K in the cooked food was the highest compared to those of other radionuclides ranging from (23 ± 5) (rice) to (347 ± 50) Bq kg−1 dw (tofu). The 210Po concentration in the cooked food ranged from its limit of detection (LOD) of 5 mBq kg−1 dw (rice) to (4.0 ± 1.6) Bq kg−1 dw (marine bivalves). The concentrations of other nuclides of the 232Th and 238U chains in the food were low, ranging from LOD of 0.02 Bq kg−1 dw to (1.1 ± 0.3) Bq kg−1 dw. The activity concentrations of 90Sr, 137Cs, and 239+240Pu in the food were minor compared to that of the natural radionuclides. The average annual committed effective dose to adults in the study region was estimated and it ranged from 0.24 to 0.42 mSv a−1 with an average of 0.32 mSv a−1, out of which rice, leafy vegetable, and tofu contributed up to 16.2%, 24.4%, and 21.3%, respectively. The committed effective doses to adults due to ingestion of regular diet in the Red River delta region, Vietnam are within the range determined in other countries worldwide. This finding suggests that Vietnamese food is safe for human consumption with respect to radiation exposure.

Low-background gamma spectroscopy at the Boulby Underground Laboratory

The Boulby Underground Germanium Suite (BUGS) comprises three low-background, high-purity germanium detectors operating in the Boulby Underground Laboratory, located 1.1 km underground in the north-east of England, UK. BUGS utilises three types of detector to facilitate a high-sensitivity, high-throughput radio-assay programme to support the development of rare-event search experiments. A Broad Energy Germanium (BEGe) detector delivers sensitivity to low-energy gamma-rays such as those emitted by 210Pb and 234Th. A Small Anode Germanium (SAGe) well-type detector is employed for efficient screening of small samples. Finally, a standard p-type coaxial detector provides fast screening of standard samples. This paper presents the steps used to characterise the performance of these detectors for a variety of sample geometries, including the corrections applied to account for cascade summing effects. For low-density materials, BUGS is able to radio-assay to specific activities down to 3.6mBqkg−1 for 234Th and 6.6mBqkg−1 for 210Pb both of which have uncovered some significant equilibrium breaks in the 238U chain. In denser materials, where gamma-ray self-absorption increases, sensitivity is demonstrated to specific activities of 0.9mBqkg−1 for 226Ra, 1.1mBqkg−1 for 228Ra, 0.3mBqkg−1 for 224Ra, and 8.6mBqkg−1 for 40K with all upper limits at a 90% confidence level. These meet the requirements of most screening campaigns presently under way for rare-event search experiments, such as the LUX-ZEPLIN (LZ) dark matter experiment. We also highlight the ability of the BEGe detector to probe the X-ray fluorescence region which can be important to identify the presence of radioisotopes associated with neutron production; this is of particular relevance in experiments sensitive to nuclear recoils.

The BiPo-3 detector for the measurement of ultra low natural radioactivities of thin materials

The BiPo-3 detector, running at the Canfranc Underground Laboratory (Laboratorio Subterr'aneo de Canfranc, LSC, Spain) since 2013, is a low-radioactivity detector dedicated to measuring ultra low natural radionuclide contaminations of 208Tl (232Th chain) and 214Bi (238U chain) in thin materials. The total sensitive surface area of the detector is 3.6 m2. The detector has been developed to measure the radiopurity of the selenium double β-decay source foils of the SuperNEMO experiment. In this paper the design and performance of the detector, and results of the background measurements in 208Tl and 214Bi, are presented, and the validation of the BiPo-3 measurement with a calibrated aluminium foil is discussed. Results of the 208Tl and 214Bi activity measurements of the first enriched 82Se foils of the double β-decay SuperNEMO experiment are reported. The sensitivity of the BiPo-3 detector for the measurement of the SuperNEMO 82Se foils is \U0001d49c(208Tl) <2 μBq/kg (90% C.L.) and \U0001d49c(214Bi) <140 μBq/kg (90% C.L.) after 6 months of measurement.

A review of the evaluation of TENORM levels at the produced water lagoon of the Minagish oil field using high-resolution gamma-ray spectrometry

An evaluation of the specific activity concentrations associated with technologically enhanced naturally occurring radioactive materials (TENORM) and anthropogenic radionuclides has been undertaken as part of a systematic study to provide a radiological map of the outer boundary of the produced water lagoon located in the Minagish oil field in the south west of the State of Kuwait. The lagoon contains material from the discharge of produced water which is a by-product of oil production in the region. The lagoon samples were prepared and placed into sealed, marinelli beakers for a full gamma-ray spectrometric analysis using a high-resolution, low-background, high-purity germanium detection systems at the University of Surrey Environmental Radioactivity Laboratory. Of particular interest are the calculation of the activity concentrations associated with members of the decay chains following decays of the primordial radionuclides of the 238U chain (226Ra, 214Pb, 214Bi) and the 232Th chain (228Ra, 228Ac, 212Pb, 212Bi, 208Tl), and the enhanced concentrations of radium isotopes. This conference paper presents an overview summary of the experimental samples which have been measured and the analysis techniques applied, including isotopic correlation plots across the sample region. The result shows the expected significant increase in 226Ra (and progeny) concentrations compared to the NORM values previously reported by our group for the overall terrain in Kuwait.

ANAIS: Status and prospects

ANAIS (Annual modulation with NAI Scintillators) experiment aims to look for dark matter annual modulation with 250 kg of ultra-pure NaI(Tl) scintillators at the Canfranc Underground Laboratory (LSC), in order to confirm the DAMA/LIBRA positive signal in a model-independent way. The detector will consists in an array of close-packed single modules, each of them coupled to two high efficiency Hamamatsu photomultipliers. Two 12.5 kg each NaI(Tl) crystals provided by Alpha Spectra are currently taking data at the LSC. These modules have shown an outstanding light collection efficiency (12-16 phe/keV), about the double of that from DAMA/LIBRA phase 1 detectors, which could enable reducing the energy threshold down to 1 keVee. ANAIS crystal radiopurity goals are fulfilled for 232Th and 238U chains, assuming equilibrium, and in the case of 40K, present crystals activity (although not at the required 20 ppb level) could be acceptable. However, a 210Pb contamination out-of-equilibrium has been identified and its origin traced back, so we expect it will be avoided in next prototypes. Finally, current status and prospects of the experiment considering several exposure and background scenarios are presented.

Measurement of the total activity concentrations of Libyan oil scale

Twenty-three oil scale samples obtained from the Libyan oil and gas industry production facilities onshore have been measured using high-resolution gamma-ray spectrometry with a shielded HPGe detector, the work being carried out within the Environmental Radioactivity Laboratory at the University of Surrey. The main objectives of this work were to determine the extent to which the predominant radionuclides associated with the uranium and thorium natural decay chains were in secular equilibrium with their decay progeny, also to compare differences between the total activity concentrations (TAC) in secular equilibrium and disequilibrium and to evaluate the measured activities for the predominant gamma-ray emitting decay radionuclides within the 232Th and 238U chains. The oil scale NORM samples did not exhibit radioactive equilibrium between the decay progeny and longer-lived parent radionuclides of the 238U and 232Th series.

Intrinsic radiopurity of a [formula omitted] crystal

The radiopurity of a lithium molybdate ( Li 2 MoO 4 ) single crystal, a potential detector for the search for the neutrinoless double beta decay of 100Mo, was studied deep underground at the Laboratori Nazionali del Gran Sasso (LNGS) (Italy). Data were collected with a crystal sample with a mass of 33.95 g, using a high purity germanium (HPGe) detector with a total measurement time of 1240.3 h. With the exception of slight pollution with 40K ( ( 170 ± 80 ) mBq / kg ), no other radioactive contaminants were found; the derived upper limits are: < 20 mBq / kg for 238U chain, < 30 mBq / kg for 232Th chain, < 4 mBq / kg for 137Cs and < 8 mBq / kg for 60Co.

Ancient Greek lead findings in Ukraine

In June–August 2006 an expedition with the aim to look for archaeological lead with low levels of 210Pb was organised by a Korean–Ukrainian collaboration on the shelf of the Black Sea, near the Crimean Peninsula. The first samples with ∼0.2 ton of total mass were found at a depth of 28 m among the relics of an ancient Greek ship. Their age has been dated to the first century BC. The element composition of the samples was measured by means of X-ray fluorescence and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) analyses. The radiopurity of the lead was tested using low-level and ultra-low-level γ-spectrometry at a surface laboratory in Kyiv, at the Solotvina Underground Laboratory (Ukraine), and deep underground at the Laboratori Nazionali del Gran Sasso (LNGS, Italy). The samples have been assessed at the LNGS also by means of α-spectroscopy. For all investigated radionuclides, only upper limits could be obtained. Limits on activities of radionuclides in the lead after melting were set at the level of <(0.2–0.3) mBq kg −1 ( 60Co), <(0.2–0.7) mBq kg −1 ( 137Cs), <(0.2–0.9) mBq kg −1 ( 226Ra), <(0.1–0.9) mBq kg −1 ( 228Th), <(5–7) mBq kg −1 ( 40K), <(0.3–1.4) Bq kg −1 ( 210Po), and <(12–13) Bq kg −1 ( 210Pb). Any 210Pb present in the lead after it was produced ca. 2000 years ago has decayed away. Assuming secular equilibrium in the 238U chain in the lead, the activity of 210Pb due to 238U can be restricted to <(5–17) mBq kg −1 before melting, and <(0.2–0.9) mBq kg −1 after melting.

Use of the 1001 keV peak of 234mPa daughter of 238U in measurement of uranium concentration by HPGe gamma-ray spectrometry

For the direct gamma-ray spectrometric measurements of uranium concentrations in the samples, the use of 1001 keV peak of 234mPa, second daughter of 238U is emphasized. This “clean” peak is well resolved by HPGe detectors and gives accurate indication of uranium concentration in the samples without any self-absorption correction. The 1001 keV peak of 234mPa in the 238U chain is selected because it does not include any contribution from any other gamma emissions and does not have any interference with other peaks in the high-energy region even if the samples contain high amounts of thorium. The activity of 234mPa is determined by calibrating a HpGe detector with uranium standards. The measurement of 1001 keV gamma-ray emission from 234mPa by high-resolution gamma-ray spectrometry provides the basis for a reliable determination of 238U in the samples. The results obtained from the 63.3 keV peak of 234Th and those of the 1001 keV peak from 234mPa in the 238U chain for the uranium standards are compared with the certified values of the same samples. The results obtained from the measurements of 1001 keV peak from 234mPa agreed to within 2–5% with the certified activity values of 238U in the samples with uranium content ranging from 0.014 to 1.02 wt%. The results indicate that the uranium concentrations in the samples can be determined to within 5% error by about 14 h counting in a HpGe detector system.

Measurements of natural radioactivity in phosphate fertilizers

The natural radioactivity, mainly due to radium ( 226Ra), in phosphate fertilizers used in north-western Greece has been measured by γ-spectroscopy. Also radioactivity measurements were performed in soil samples and were compared to samples from undisturbed soils. 226Ra belongs to the 238U chain and is the precursor of radon gas ( 222Rn). The radon concentrations in warehouses, where large quantities of fertilizers are kept, were measured with CR-39 SSNTDs. The radium concentrations in the fertilizers ranged from 0 to 4584 Bq kg −1 and the radon concentrations in warchouses were measured 540–3320 Bq m −3. The results are discussed from the radiation protection point of view.

Natural radionuclide emission from a coal power plant and the population exposure to external radiation in its vicinity

Investigations carried out in the vicinity of four coal-fired power plants showed that the average annual emission of natural radionuclides for each MWe of produced electric power is an average of 0.200 MBq for each component of 238U chain, and of 0.130 MBq for each component of 232Th chain, respectively, and of 1.027 MBq for 40K. The average annual absorbed dose of about 1 mGy was found on five locations studied. The results of specific activity measurements on the samples taken from several locations studied showed that there is a concentration of natural radionuclides in ash and slag of up to about five times. The absorbed dose levels found on depots of ash and slag were close to the values recommended by the International Commission on Radiological Protection.

Very low level α-particle detection by means of pulse shape discrimination

A charge comparison pulse shape discriminator has been shown to be capable of low level alpha particle detection in a NE213 scintillator by discriminating against a high rate of gamma background. The clearly observed alpha peaks were due to 212Po in 232Th chain and 214Po in 238U chain with rates of 13.5/h and 15.6/h, and energies of 8.95 and 7.83 MeV respectively. For the 8.95 MeV alpha, the gamma background rate was reduced by a factor of 2.1 × 10 4 to a level of 22.5/h. The measurements were made in a low muon flux environment underground (1230 m deep). The detection limit for the alphas could be lowered to as low as 0.1–1 event per day deep underground by use of high purity PSD scintillators.

Time: Measured responses

I was dismayed by Raphael G . Kazmann's conclusion in his review of a symposium on ‘Cosmochronology, geochronology, and the neutrino crisis’ (Time: In Full Measure, Eos Trans. AGU, 60(2), pp. 21–22, January 9, 1979) that essentially casts in doubt the entire science of geochronology, on the basis of an absurd interpretation of the origin of ‘polonium’ halos in minerals observed by Robert Gentry. I have no doubt that Gentry's halos exist, but to ascribe the halos to ‘primordial’ 218Po unsupported by 222Rn can be quickly reduced to an absurdity by the following argument: (1) 218Po has a half‐life of 3 minutes; (2) more than one 218Po atom would be required to produce a halo; (3) the probability of 1 atom of 218Po surviving after 490 minutes is 1 out of 16.5 × 1048 based upon the exponential decay law; (4) 16.5 × 1048 atoms of 218Po are equal to the mass of the earth. Therefore, in order to accept the assertion that Gentry's halos are caused by unsupported primordial 218Po, either the earth must be composed entirely of 206Pb (the stable isotope in the 238U chain following 218Po) or the earth must have been less than 490 minutes (8.2 hours) old at the time the mineral was formed so that a n amount of 218Po less than the mass of the earth was originally present. Clearly the earth is not composed entirely of 206Pb. It is no less credible to suggest that the earth and its strata were formed and intruded by granitic magma that subsequently cooled to the point where both the mineral and the halo were stable all in less than 8.2 hours. ‘Curiouser and curiouser!’ said Alice in Wonderland.