Naturally occurring low-level radioactivity measurements play an essential role during the setting up of nuclear physics experiments searching for rare events like neutrino-less double-beta decay, neutrino interactions and investigations about the nature of dark matter. The rare event processes are so-called because they have a low probability of happening or the particles involved have an extremely low interaction cross-section with the matter. All these experiments have in common the need for ultrapure materials to build extremely high sensitivity detectors often represented by crystals [1
Some of the inorganic crystals are intrinsic scintillators in which the luminescence is produced by a part of the crystal lattice itself. However, other crystals require the addition of doping ions (e.g., thallium or cerium), which are responsible for producing the scintillation light [2
In 1948, NaI(Tl) scintillation crystals came into use and provided better conditions for γ-ray detection even if the history of the scintillating materials used for radiation detection goes back to work by Röntgen and his discovery of X-rays in 1895. Over the years, numerous useful scintillators have been discovered and developed. Scintillation detection has been extended to many other inorganic crystals (e.g., CsI(Tl), ). In the early 1960s, Freck and Wakefield reported the use of germanium crystals as detectors to measure ionizing radiation, but only 10 years later, high purity Ge detectors will revolutionize the nuclear sector because of their best energy resolution. Over the past few decades, the great interest in rare event searches has involved a broadening of the scintillators’ fields of application. Relevant technological developments have been achieved with many improvements in terms of performance of detector materials, photomultipliers, electronic components, computing and data transmission. Several examples of the development of new scintillators as components of modern detector systems have been reported in the literature.
Among the experiments located in the underground laboratories of Gran Sasso National Laboratories (LNGS) investigating the nature of the neutrino or the nature of dark matter, some examples must be highlighted within this framework: the cesium hafnium chloride (
) crystal scintillator used to investigate rare naturally occurring α decays of 174
Hf isotope [7
] (extensively discussed in Section 2
) or the high purity enriched Zn82
Se crystals for the study of the neutrino-less double-β-decay of 82
Se and the determination of the two-neutrino double-β-decay half-life of 82
Se in the framework of LUCIFER project and CUPID-0 experiment respectively [1
]. Neutrino-less double-β-decay of the isotope 130
Te has been investigated by the cryogenic underground observatory for rare events (CUORE) experiment in which the detector consists of an array of 988
crystal bolometers operated at cryogenic temperatures.
Moreover, scintillating NaI(Tl) crystals are widely used as dark matter (DM) detectors: for instance, 25 highly radiopure NaI(Tl) detectors are successfully used by DAMA/LIBRA experiment to investigate the presence of DM particles in the galactic halo by exploiting the DM annual modulation signature. In addition, these detectors are suitable for various other rare process studies like searches for solar axions and cluster decay, possible processes of Pauli exclusion principle violation in 127
I and 23
Na isotopes and electron stability [9
]. This is due to several characteristics such as i.e., high light response and duty-cycle (experimental live-time), appropriate signal-to-noise discrimination near the energy threshold, high stability and reproducibility of measurements, possibility to realize crystals with large masses, scalability and segmentation of the setup to increase sensitivity [9
The germanium detector array (GERDA) experiment searches for neutrino-less double-beta (0νββ) decay of 76Ge using an array of isotopically enriched high purity germanium (HPGe) detectors. The latter, made from high-purity (99.9999%) Ge material, are enriched in the 76Ge isotope from the natural abundance of 7.8% to more than 85%. The Ge detectors are both the source and detector for the 0νββ decay. Moreover, the detector modules developed for the cryogenic rare event search using superconducting thermometers (CRESST) experiment, a search for WIMP dark matter particles via their elastic scattering off nuclei, are based on scintillating crystals as absorbers.
In the framework of low-energy, ranging from few keV to MeV, and low background experiments as those previously mentioned, the choice of the isotopes as the interaction target and the chemical and radio-purity of the different components of the detectors together with surrounding materials is a prerequisite to guarantee a successful outcome.
Chemical purity is a key factor during crystal growth because the higher the contamination, the more difficult the crystallization [12
]. Furthermore, low concentration of transition metals and other elements like Mg, K, Mn, Bi, Sn, Mg, Al and Cu is a general requirement of the initial raw materials in order to obtain high-quality scintillating crystals, as reported by Cardenas et al. [13
]. Contaminants may distort the crystal lattice affecting, thus, the crystal features: the lack of homogeneity characterized by the presence of bulk defects like bubbles and metallic inclusions or veils and cracks may drastically affect the optical quality, light output, energy resolution, charge propagation and bolometric properties of such crystals [1
Besides a high crystal perfection in order to allow for a high sensitivity detector, a low concentration of radioactive impurities, typically below 10−13 g·g−1, is needed; thus, the use of radiopure materials with the lowest internal background is mandatory. Intrinsic contamination of the different materials employed coming from the impurity of raw materials or contamination during production processes as well as the exposure to cosmic rays must be assessed right from the start in order to reduce as much as possible detector background maximizing at the same time the signal-to-background ratio. This means a detailed screening of all materials to be used in the experimental setup, from the reagents and tools used for the synthesis of crystal powder to the finished crystal together with its handling, storage and transport before mounting in the experimental setup, not to mention each component of the final apparatus. Last, but not least, a great variation in radiopurity between different batches of the same raw material must be taken into account in selecting materials. This involves a large number of measurements to be done for each reagent and the final component involved.
The contamination may come from long-lived, naturally occurring isotopes, such as 238
K and their daughters and from cosmogenic activation of the detector materials and surroundings. The main sources of background interfering with the signals of the extremely rare events included cosmic rays, especially muons from the Earth’s atmosphere, gamma rays and neutrons from (α, n) reactions or 238
U spontaneous fission due to primordial radionuclides such as 232
U coming from the experimental environment such as laboratory walls or produced by surface and bulk contamination of the detector construction and shielding materials [14
]. In particular, the background resulting from 238
Th decay products include the noble gas radon able to diffuse out of materials, thus interfering with the radioactive equilibrium of the decay chains [15
Besides cosmic rays and radon isotopes, whose effects are usually negligible in deep underground facilities using a radon-free cleanroom for all the steps involved in the setup of the experiment or suitable filters fitting for this purpose, the main contribution to the background is represented by the radioactive contamination of the different components and chemicals employed in each step due to primordial radionuclides like 238U, 232Th and 40K and their decay products. For nuclear physics searches, an extremely low radioactive contamination is required, so sensitivities ranging from parts per trillion (10−12 g·g−1 or ppt) to parts per quadrillion (10−15 g·g−1 or ppq) levels are necessary to detect any trace of radioactivity typically below 10−13 g·g−1.
Whatever the nature of such rare events, detectors able to discriminate the weak and rare signals over the dominating background caused by natural radioactivity and cosmic rays are required. Since the energy range of the expected signals for rare events experiments is similar to those of the decay of radioactive nuclides or other possible nuclear reactions, these latter phenomena may cause background interfering with the signals of concern. For this reason, severe suppression of potential backgrounds is required. An extensive screening and material selection process must be conduct since radiopurity requirements are stringent.
For these reasons, analytical methods characterized by high sensitivity, low detection limit, high sample throughput and short analysis time are the best candidates to detect ultra-trace amounts of such radionuclides. The most frequently analytical techniques applied to measure the natural radioactivity and, therefore, to select highly radiopure materials for the experimental apparatus are ultra-low background γ-ray spectrometry with high purity germanium detectors (γ-HPGe), inductively coupled plasma mass spectrometry (ICP-MS), accelerator mass spectrometry (AMS) and neutron activation analysis (NAA). These different and complementary radioassay techniques are usually applied within the rare events nuclear physics experiments depending on the decay products, impurities and the matrix of each component involved.
Each one has its own advantages and disadvantages for a given application. From a laboratory perspective focused on radiopurity screening of materials, the most significant advantage of ICP-MS is its multi-elemental capability, in contrast to AMS and NAA, where only one radionuclide can be measured at a time.
Furthermore, an ICP-MS analysis requires small amounts of sample (few grams or less) and short times compared to γ-spectrometry, even if different sample preparation steps are necessary to dissolve the sample and to separate the elements of interest from the sample matrix, thus involving possible sample contaminations (time-consuming sample preparation). On the contrary, the nondestructive γ-spectrometry is able to determine the amount of different radioactive nuclides checking also for secular equilibrium with a detection limit of about 1 µBq/kg (232Th: 0.2 × 10−12 g·g−1, 238U: 0.08 × 10−12 g·g−1) achieved only using long measurement times and large amounts of sample.
AMS is based on the direct counting of atoms. It is characterized by a high isotope sensitivity and selectivity for extreme ultra-trace analysis of both radionuclides and stable nuclides (e.g., 14
I, Th, U and Pu). The analysis requires smaller samples (mg and even sub-mg amounts) and shorter measuring times (less than 1 h) to be performed compared to other radioanalytical techniques, but the experimental equipment is more complicated and expensive due to higher purchase and maintenance costs required by a particle accelerator [16
NAA is another powerful analytical technique able to perform a qualitative and quantitative analysis of long-lived primordial radionuclides at trace level by irradiating samples in a nuclear reactor. During irradiation, the naturally occurring stable isotopes of which the sample is made are converted into radioactive isotopes by neutron capture and subsequently, they decay according to their characteristic half-lives, emitting γ-rays with specific energies. As already explained for AMS, since NAA requires a nuclear reactor, other analytical techniques are preferred, for instance, ICP-MS, for which also stand-alone equipment is available. Moreover, since low detection limits are required to detect primordial radionuclides in detector construction materials, pre-and/or post-irradiation chemical treatments are required, thus, making NAA less attractive [14
1.1. ICP-MS Analytical Technique
Since its first development in the early 1980s, ICP-MS quickly became the leading analytical technique for fast and sensitive, trace and ultra-trace multi-elemental analysis. In addition, ICP-MS can be used to obtain precise isotope ratios of long-lived natural and artificial radionuclides in different materials. Due to the high precision and accuracy, excellent sensitivity, low detection limits, high throughput, small sample volume required, wide linear dynamic detection range, ICP-MS is widely used for quite different applications in areas ranging from environmental monitoring, health and food safety, high-purity materials screening (e.g., metals, alloys, semiconductors and insulators for microelectronics), geo and cosmochemistry, archeometry, forensic and pharmaceutical analysis [17
Over the years, continuous improvements in computer hardware and software, sample introduction, plasma efficiency, ion transmission, detector performance and automation increased its utility, sensitivity and resolution. Although quadrupole-based ICP-MS equipped with collision/reaction cell is the most used worldwide, at the same time, the development of high-resolution mass spectrometers (H-ICP-MS) has been necessary to overcome limitations due to spectral interferences (isobaric overlap, polyatomic ion interferences and double-charged ions).
The isobaric interferences are due to the direct overlap of isotopes having the same nominal mass coming from different elements (e.g., 114Sn and 114Cd, 148Nd and 148Sm, 160Gd and 160Dy). Low-resolution instruments cannot distinguish between the isotopes, but this type of interference can be overcome by choosing a non-interfered isotope in case of multi-isotopic elements or using mathematical corrections.
Polyatomic ions, mainly argides, hydrides, carbides, nitrides and oxides, are molecular species naturally formed in the argon plasma due to precursors in the argon gas, in solvents and acids used during sample preparation or in the sample matrix. Typical polyatomic interferences are those interfering with monoisotopic elements like 40
. Other examples of polyatomic interferences and the affected isotopes are reported as follows: 40
O or 40
Sm and 154
ArH and 23
O on 39
K. The lanthanide element isotopes are especially prone to molecular oxide formation. The use of H-ICP-MS acting on mass resolution (e.g., for the mass spectrometric separation of 187
ions, a mass resolution m/Δm = 3000 is required), cold plasma for selected elements with low ionization potential such as K, Na, Ca or Sr as reported by Nisi et al. [19
], reaction/collision cells, just to name a few approaches have resulted in reduction and, in some cases, complete elimination of many polyatomic interferences.
Doubly charged ions (e.g., 184
Sn) are common for elements having a low second ionization potential such as the alkaline earth, rare earth elements and, for instance, Th and U. Moreover, the greater the concentration of the element the higher the formation of M2+
interferences. Doubly charged ions were found to depend significantly on the inductively coupled plasma operating conditions and generally increase with increasing carrier gas flow rates. They also increase by about an order of magnitude when a desolvated aerosol is introduced to the ICP [20
Moreover, nonspectral interferences or matrix effects resulting in suppression or enhancement in analyte signal may occur during sample introduction, in the plasma during atomization, ionization and excitation as well as in mass spectrometry, during extraction, focusing and transport of ions from the plasma to the detector. They can be reduced by using an internal standard or a purification step of the sample solution. A total dissolved solids (TDS) content in the sample of <0.2% (2 g/L) is usually recommended in ICP-MS to reduce sample-specific matrix effects and a potential nebulizer clogging.
There are many ways to reduce interferences, among them also a proper tuning of the plasma and torch conditions, especially for oxides and doubly charged species, or a reduction of the sample argon gas flow rate to reduce oxides, the use of an internal standard, a standard addition method, an appropriate sample introduction system or simply a dilution of the sample is frequently used.
By using an H-ICP-MS operating in the high-resolution mode (m/Δm = 10,000), most target elements may be resolved from polyatomic interferences, but the enhanced mass resolution results in a reduced ion transmission and, therefore, in a loss of sensitivity (typically a signal intensity reduction of about one order of magnitude occurs).
In addition, dedicated and effective purification methods based on liquid–liquid extraction, selective precipitation, solid-phase chromatographic extraction have been developed in order to quantify the concentration of impurities as well as U, Th and K to ppt levels or below in different types of samples. For instance, Nisi et al. [19
] developed a procedure for Th and U separation and preconcentration from Te and
by means of UTEVA resins in the framework of the CUORE neutrino-less double-beta decay experiment, La Ferriere et al. [21
] reported an improved cleanliness procedure combined with anion exchange separation for the trace determination of Th and U in copper and lead shielding materials in support of the Majorana demonstrator experiment reaching sub-ppt sensitivity, Miyamoto et al. [22
] developed an automatic sequential separation of U, Th, Pb and lanthanides using an anion exchange column and pressurized gas. The use of TRU resin has been reported by Kaizer et al. [23
] for the determination of Th and U in selenium, aluminum and copper materials in the context of SuperNEMO experiment and AMS development.
Each step concerning sample treatments such as sample dissolution (dry ashing, wet digestion or closed-vessel acid digestion using microwave oven systems) and matrix separation, if required, must be performed in a cleanroom taking all the precautions needed to avoid any potential source of contamination.
Even if the so strict radiopurity requirements pose a considerable challenge on the materials selection and analysis in the framework of rare events experiments, ICP-MS, with its high sensitivity and high throughput, has become a key analytical technique well-suited to determine a wide range of minor, trace and ultra-trace elements.
1.2. Materials and Instrumentation
The need for reliable and accurate radiopurity measurements at ultra-trace levels requires ultra-pure reagents, inert and clean equipment as well as cleanroom in order to avoid any further contamination during all stages of sample preparation and analysis.
Highly purified HNO3 was prepared by double-distillation of trace analysis grade acid ( ≥ 69%, VWR Chemicals, Mississauga, ON, Canada) using a sub-boiling distillation system (Milestone, Bergamo, Italy). Single element 1000 mgL−1 stock standard solutions and multielement standard solution of trace analysis grade were purchased from Sigma-Aldrich. All aqueous solutions were prepared with ultra-pure water (UP H2O, 18.2 MΩ*cm resistivity) obtained from a Milli-Q IQ Element water system (Millipore, Burlington, MA, USA). Argon 5.0 of 99.999% purity was supplied by Air Liquide (Milano, Italy).
All analytical steps were performed in an ISO6 cleanroom at Gran Sasso National Laboratory (LNGS), and all the apparatus intended to come into direct contact with the sample and glassware were washed with 5% HNO3 aqueous solution and then rinsed with ultra-pure water. Measurements were carried out using a single quadrupole inductively coupled plasma mass spectrometer (ICP-MS, 7500a, Agilent, Santa Clara, CA, USA) and a high-resolution double-focusing magnetic sector field inductively coupled plasma mass spectrometer (HR-ICP-MS, Finnigan Element 2, Thermo Fisher Scientific, Bremen, Germany).