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RML Facilities

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IAEA-MEL's underground counting laboratory: CAVE (Counting lAboratory for enVironmental radionuclidEs)

The use of underground laboratories for low-level gamma-ray spectrometry of environmental radionuclides is, together with the availability of large volume HPGe detectors (up to 200% relative efficiency compared to 75 mm diameter and 75 mm long NaI(Tl) detectors), the most important breakthrough in low-level counting. Why are underground laboratories needed for environmental radionuclide analysis? There are several reasons: (i) Highly accurate and precise data on natural and anthropogenic radionuclides often used as tracers in the marine and terrestrial environments are required for environmental and climate change studies; (ii) Forty years after the main introduction of anthropogenic radionuclides into the environment as a result of large-scale atmospheric nuclear weapons tests (global fallout) their concentrations in the environment have decreased considerably. (iii) With the development of highly accurate and precise mass spectrometry techniques such as Inductively Coupled Plasma Mass Spectrometry (ICPMS), Thermal Ionisation Mass Spectrometry (TIMS) and Accelerator Mass Spectrometry (AMS) the required sample size for analysis has decreased considerably (at least by a factor of 10). This poses limits on radiometric analyses as well, as only smaller sample volumes are available for analysis, e.g. from oceanographic expeditions.

The decrease in sample volume has also important economic aspects, e.g. for marine sampling for radionuclide measurements, where previously large volume seawater samples were needed (up to 500 L) taken at different water depths down to several thousands of meters, which required several days of sampling at one station. With the present state of the art technique using a Rosette system equipped with Niskin bottles of 20-30 L, the complete water column can be sampled with a small number of casts and within a few hours.

Radiometric gamma-spectrometric techniques have improved recently with the availability of large volume HPGe detectors, often placed underground to decrease the cosmic-ray component of their background, but also by the application of anti-cosmic and/or anti-Compton shielding of detectors and the use of coincidence gamma-ray spectrometry modes of operation, e.g. beta-gamma, or gamma-gamma coincidences. These new radiometric and mass spectrometry techniques have opened doors for investigations which previously were not possible because too large samples were required, or were not possible because of the lack of sensitivity and precision.

IAEA-MEL has been heavily engaged in marine radioactivity studies for radioprotection purposes, as well as in the applications of natural and anthropogenic radionuclides as tracers for studying oceanic processes. There has been a constant need for analyses of radionuclides in many marine samples at very low levels, therefore an underground Counting lAboratory for enVironmental radionuclidEs (CAVE) was constructed to meet growing requirements. The main objectives were: (i) To decrease background levels and improve the sensitivity and precision of gamma- and beta-spectrometric measurements; (ii) To decrease the size of marine samples needed for analysis by at least a factor of 10; (iii) To decrease measuring time and analyse more samples per week; (iv) To stabilize the characteristics of gamma- and beta-spectrometers against temperature and humidity changes, but also against changes in the radon content of air and variations in cosmic-ray flux.

The CAVE laboratory is situated at a depth of 35 m w.e. (water equivalent) in an underground car park in Monaco, about 300 m from IAEA-MEL premises. There is easy access by car for the weekly supply of liquid nitrogen, as well as for frequent changes of samples for counting. As the data acquisition system in the CAVE is part of the local area network operated at IAEA-MEL, the spectrometers can be controlled and the data evaluated at MEL. The operation of the CAVE is very convenient as it does not disrupt regular laboratory work and requires no extra effort.

The CAVE is equipped with ventilation and air conditioning maintaining overpressure and stable humidity and temperature levels. The radon content of air inside the laboratory varies around a few Bq m-3 mainly depending on radon concentrations in the open air outside of the laboratory. However, there is still a significant contribution to the detectors background from radon emanations in the detector chambers. Therefore the chambers must be flushed by liquid nitrogen from the Dewars containers.

Figure 1 Cross-section of the lead shield with HPGe detectors.

As the CAVE is situated only 35 m w.e. below ground level, the muon component of cosmic rays has been reduced only negligibly. However, the nucleonic component of the cosmic rays has been reduced considerably, by about 4 orders of magnitude. Therefore, anti-cosmic shielding has to be installed to decrease the muon contribution to the detectors background. Four HPGe detectors are placed in a common lead shield (Figure1, Figure 2). This structure has been adopted for the purpose of effectively protecting the detectors against cosmic ray muons using anti-cosmic shielding. The lead shield (total thickness 125 mm) is comprised of two layers. The outer layer (75 mm) is made of ordinary lead, which has been used for shielding of detectors. However, the second internal layer (50 mm) is made of very low activity lead (210Pb activity is below 0.1 Bq kg-1) to improve the measurements of the detectors by reducing background interference. The internal layers separating the detectors are 100 mm thick and are also made of very low activity lead.

Figure 2 General view of the anti-cosmic system and construction details of passive shielding and Ge detectors inside the anti-cosmic shield.

The types of HPGe detectors housed in the shield were: (i) 150% relative efficiency well-detector; (ii) 170% relative efficiency coaxial detector; (iii) 200% relative efficiency well-detector; (iv) Ge-Ge telescope (sandwich) detector working in anti-Compton mode.

The anti-cosmic shielding made of plastic scintillators (70 mm thick) is viewed by 50 mm diameter photomultipliers surrounding the lead shield. Each of the HPGe detectors in the lead shield is connected in anticoincidence with the plastic scintillator shielding, thus events caused by penetrating cosmic ray muons are not evaluated in the measured HPGe spectra. The time spectrum from HPGe detectors in coincidence with the plastic scintillator was studied to correctly assess the delay and time constants in the electronics system.

The second spectrometric system installed in the CAVE is the coincidence and anti-Compton spectrometer(Figure 3, Figure 4). It consists of an n-type HPGe detector (100% relative efficiency), top NaI(Tl) detector (75 mm diameter, 75 mm long) and an NaI(Tl) annulus (outside diameter 300 mm, 400 mm long), both mounted in a cover made of electrolytic copper.

Figure 3 General view of anti-Compton system.

Figure 4 Anti-Compton shield.

ORTEC NIM modular electronics and software have been used for signal processing, data acquisition and spectra evaluation. Two systems have been installed. The first one houses the electronics for the detectors inside the anti-cosmic shielding, and the second one the coincidence-anti-Compton spectrometer. A block schema of the electronics used for the second application is shown in Figure 5. Time amplitude converters have been used for gating signals from specific detectors, including a flat Si beta-detector situated between the HPGe detector and the top NaI(Tl) detector.

Figure 1 Cross-section of the lead shield with HPGe detectors.

The gamma-ray spectrometers equipped with modular electronics can operate in several modes: (i) single gamma-ray spectrometer; (ii) gamma-ray spectrometer with anti-cosmic shielding; (iii) anti-Compton gamma-ray spectrometer; (iv) coincidence (β-γ, γ-γ, β-γ-γ, etc.) gamma-ray spectrometer with anti-coincidence shielding.

The background of two large volume HPGe detectors was studied in two modes: (i) direct mode - a single gamma-ray spectrometer with lead shielding only (passive shielding); (ii) anti-cosmic mode - the HPGe detector in anti-coincidence with plastic scintillator shielding (active anti-cosmic shielding).

Figure 6 Background gamma-ray spectrum of 170% relative efficiency HPGe detector operating without (top) and with (bottom) anti-cosmic shielding.

Figure 7 Background gamma-ray spectrum of 200 % relative efficiency HPGe detector operating without (top) and with (bottom) anti-cosmic shielding.

Figure 6 compares the background spectra of the 170% detector in both modes. The anti-cosmic mode (the bottom spectrum) has decreased the detector background by about an order of magnitude, which is very well documented by the decrease in the counting rate of the annihilation peak (511 keV). Similar background spectra are presented in Figure 7 for the 200% detector. Both background spectra document the significance of the cosmic ray continuum in the spectrum which is produced by the penetrating cosmic ray muons and their secondaries. However, in both cases the detectors and/or their surroundings had been contaminated by natural radionuclides such as 40K, as well as 238U and 232Th decay products (e.g. 214Bi at 609 keV and 208Tl at 2614 keV), this emphasises the importance of using radioactively pure materials for the detectors and shields.

Figure 8 Gamma-ray spectrum of the reference material IAEA-414 (Irish Sea and North Sea fish) without (top) and with (bottom) anti-Compton shielding.

The performance of the anti-Compton spectrometer is illustrated in Figure 8 where a single (top) as well as an anti-Compton (bottom) spectrum are compared for the reference material IAEA-414 (Irish Sea and North Sea fish). The anti-Compton shielding has depressed the Compton continuum in the spectrum by about an order of magnitude. The anti-Compton spectrometer is especially effective for measuring marine samples where 40K usually dominates in the spectrum, and due to its wide Compton continuum complicates the analysis of radionuclides with lower energies.

Table 1 Integral background counting rate of HPGe detectors

The background characteristics of four HPGe detectors studied in the CAVE are compared in Table 1. Two of the detectors were coaxial (100 and 170% relative efficiency) and two were well-type detectors (150 and 200% relative efficiency). The lowest background per kg Ge was obtained with the 150% detector, mainly due to the low contamination of the detector's cryostat, in contrast to the 170% detector which shows the highest background per kg Ge, probably due to contamination of the Al cryostat. This documents the fact that commercial companies manufacturing HPGe detectors for low-level gamma-ray spectrometry need to considerably decrease the radioactive contamination of constructions parts.

Figure 9 Background gamma-ray spectra of the Ge-Ge telescope (sandwich) spectrometer operating as a single spectrometer (top) and as an anti-Compton spectrometer with external anti-cosmic shielding (middle). Gamma-ray spectrum of a Mediterranean sediment sample measured in the last mode is also presented (bottom).

The Ge-Ge telescope (sandwich) detector has been designed for the analysis of low energy gamma-emitters (such as 210Pb) in marine samples. It consists of a planar HPGe detector attached to a coaxial detector. Both detectors use their individual preamplifiers. If coincidence pulses between the planar detector and the coaxial detector are not evaluated, the planar detector operates as an anti-Compton spectrometer, discriminating gamma-rays with higher energies registered in both detectors. Figure 9 compares a direct background measurement of the planar detector (top) with that of the Ge-Ge anti-Compton spectrometer placed in the lead shield with anti-cosmic shielding (middle). The bottom spectrum shows a gamma-spectrum of a real sample (Mediterranean sediment) measured in the same mode. The advantages of the Ge-Ge telescope detector for analysis of low-energy gamma-emitters are clearly visible.

Figure 10 Integral background counting rates of HPGe detectors (from 40 to 2700 keV) divided by the mass of the Ge-crystal as a function of the operating depth of different underground laboratories. The solid line shows the muon fluence rate in arbitrary units normalised to the background counting rate above ground. All detectors have only passive shielding, except the IAEA-MEL detector (after Laubenstein et al., this volume).

Figure 10 compares the integral backgrounds of HPGe detectors (from 40 to 2700 keV) divided by the mass of the Ge crystal, operating in different underground laboratories. The solid line is the muon fluence rate in arbitrary units normalized to the background counting rate above ground (after Laubenstein et al., this volume). All detectors have passive shielding only, except for the IAEA-MEL detector which has the anti-cosmic shielding made of plastic scintillators (as described above). Although the operating depth of the IAEA-MEL underground laboratory is only 35 m w.e., with the installation of the anti-cosmic shielding, the background is similar to a background level measured in a laboratory at 250 m w.e. depth. Therefore, anti-cosmic shielding in an underground laboratory of shallow depth is extremely important for reducing the detectors background and should be widely used.

The Alpha-Spectrometry Systems

The alpha spectrometry systems are used to measure natural (polonium, uranium, thorium) and anthropogenic (plutonium, americium, neptunium) alpha emitters in marine samples (seawater, sediment and biota) and atmospheric samples (wet and dry fallout, atmospheric aerosols). The elements are first separated and purified chemically before being either electroplated or precipitated in order to be in the proper geometry needed for alpha particles counting.

Figure 11

Seven EG&G Ortec systems are in operation currently in RML (Figure 11): three have 16 alpha spectrometry chambers each (model n° 576), connected to an amplifier, a multichannel analyser and a multiplexer, the other four are integrated Octete PC systems with 8 chambers each. The programmes used for spectrum analysis and data evaluation are Maestro and Alphavision, both provided by the EG&G Ortec company.

A total of eighty silicon surface barrier detectors are available for measurements, this giving the possibility to measure more than a thousand samples per year and also to apply long counting times in order to improve the quality of data. The estimated levels may be as low as the range of µBq which is usually found in seawater samples for isotopes like plutonium and americium. The samples are counted under vacuum and, because the levels are low, good background conditions are requested as well as reliable calibration parameters. The efficiency of an analysis is estimated with a tracer added to the original sample, this tracer being an isotope of the same chemical element but absent from the original sample. The quality of the separation has an incidence on the resolution of the peaks which correspond to the alpha emission of an isotope at a specific energy.

Figure 12 and 13

Figure 12 shows a spectrum of uranium measured in a seawater sample. The tracer used is the U232. Figure 13 shows a spectrum of plutonium obtained from an Intercomparison Sediment prepared at the IAEA-MEL. In this case, the tracer is the Pu242.

The XRF Analysis Spectrometer

Figure 14

These analyses are carried out using a SPECTRO-XLABPRO instrument designed by the company SPECTRO Analytical Instruments. With this instrument X-ray fluorescence elemental analyses of solid, powder and liquid samples are carried out (Figure 14).

Analysis is conducted based on the characteristic x-ray radiation emitted from the atoms excited in a sample. The excitation is performed with x-ray radiation emitted from x-ray tube and polarized by a target. The x-ray radiations of the sample are detected with a Si(Li)-semiconductor detector linked to detection electronics so that the energy spectrum of the incoming x-ray radiation can be measured with the best possible energy resolution and highest possible impulse rate. Although the limits of detection are variable for different elements, the measured statistical errors are typically within one sigma confidence interval. The equipment is also very stable and reliable, as shown by the reproducibility of the data in sediments, which has been tested regularly over the years.

The instrument has proven to be particularly useful in case of analysis of intercomparison samples for homogeneity tests, and also in the case of fresh water discharges in coastal zones. In that specific case, the sensitivity of the measuring system allowed to separate different water sources at a very first stage of the analysis and without any preliminary chemical processing of the samples. Is non-destructive aspect also makes the method suitable for samples in small quantity which would subsequently be subjected to other analyses, as for example suspended matter or dry precipitation.

Laser Diffraction Granulometer

The Mastersizer Micro from Malvern Instruments is a particle size analysis instrument using laser diffraction and diffusion.

In practice, the particles are diluted and circulated in a cell where they deviate a laser beam. The quantity of light deviated and the angle of deviation allow to measure precisely the size of particles, as the laser granulometry is very sensitive to the volume and mass of particles. Measurable size of diameters of particles range from 0.05 to 900 µm.

This instrument is mainly used for sediments, as the particle size spectrum is a basic information needed for interpreting the results of contaminant analyses.