Sandwich Spectrometer Research Articles

Standardisation of 241Am activity for a key comparison

The JRC applied six measurement techniques to standardise the activity of an 241Am solution in the frame of the 2003 key comparison CCRI(II)-K2.Am-241. The methods used were alpha-particle counting at a defined small solid angle, high-efficiency particle and photon counting with a windowless 4π CsI(Tl) sandwich spectrometer, 4π alpha counting with a pressurised proportional counter, alpha-gamma coincidence counting and sum counting with a small pressurised proportional counter and a NaI(Tl) well detector, and 4π counting with a liquid scintillation counter. All results were consistent and an unusually low measurement uncertainty of 0.054% was achieved. An overview is presented of the outcome of the key comparison exercise, which demonstrates international equivalence.

Feasibility study of a SiC sandwich neutronspectrometer

Semiconductor sandwich neutron spectrometers are suitable for in-pile measurements of fast reactor spectra thanks to their compact and relatively simple design. We have assembled and tested a sandwich neutron spectrometer based on 4H-silicon carbide (4H-SiC) Schottky diodes. The SiC diodes detect neutrons via neutron-induced charged particles (tritons and alpha particles) produced by 6Li(n,α)3H reaction. 6LiF neutron converter layers are deposited on the front surface of Schottky diodes by magnetron sputtering. The responses of SiC diodes to charged particles were investigated with an 241Am alpha source. A sandwich neutron spectrometer was assembled with two SiC Schottky diodes selected based on the charged-particle-response experimental results. The low-energy neutron response of the sandwich spectrometer was measured in the neutron field of the Chinese Fast Burst Reactor-II (CFBR-II). Spectra of alpha particles and tritons from 6Li(n,α)3H reaction were obtained with two well-resolved peaks. The energy resolution of the sum spectrum was 8.8%. The primary experimental results confirmed the 4H-SiC sandwich neutron spectrometer's feasibility.

Measurement of the 225Ac half-life

The 225Ac half-life was determined by measuring the activity of 225Ac sources as a function of time, using various detection techniques: α-particle counting with a planar silicon detector at a defined small solid angle and in a nearly-2π geometry, 4πα+β counting with a windowless CsI sandwich spectrometer and with a pressurised proportional counter, gamma-ray spectrometry with a HPGe detector and with a NaI(Tl) well detector. Depending on the technique, the decay was followed for 59–141d, which is about 6–14 times the 225Ac half-life. The six measurement results were in good mutual agreement and their mean value is T1/2(225Ac)=9.920 (3)d. This half-life value is more precise and better documented than the currently recommended value of 10.0d, based on two old measurements lacking uncertainty evaluations.

Measurement of the 230U half-life

The 230U half-life was determined by measuring the decay curve of 230U sources by various nuclear detection techniques: α-particle counting at a defined small solid angle; 4πα+β counting with a windowless CsI sandwich spectrometer, a liquid scintillation counter and a pressurised proportional counter; gamma-ray spectrometry with a HPGe detector and nearly-2π α-particle counting with an ion-implanted silicon detector. Depending on the technique, the decay was followed for 100–200d, which is 5–10 times the 230U half-life. The measurement results of the various techniques were in good mutual agreement. The mean value, T1/2(230U)=20.23 (2)d, is lower than the literature value which is based on one measurement in 1948 and resulted in a half-life value of 20.8d without statement of uncertainty. A correction for the ingrowth of the long-lived 210Pb and its daughter products may have been overlooked in the past.

Experimental determination of the 233U half-life

The half-life of 233U was determined by a specific activity method, in the frame of an international collaboration project of four metrology institutes. A purified, highly enriched 233U solution was prepared with certified concentration of uranium isotopes. The isotopic composition of the uranium material was determined by thermal ionization mass spectrometry. The ingrowth of daughter nuclides was calculated and confirmed by α-particle and γ-ray spectrometry measurements. Quantitative sources were prepared from aliquots of the 233U solution and their disintegration rate was determined by means of α-particle counting at a defined small solid angle, liquid-scintillation counting and 4π counting in a pressurized proportional counter and in a CsI(Tl) sandwich spectrometer. A half-life value of 1.5867 (14) × 105 a was obtained, in which the relative standard uncertainty amounts to 0.09%. The result is lower than the currently recommended value of 1.592 (2) × 105 a and a new best estimate of T1/2(233U) = 1.5903 (13) × 105 a was calculated from a partially weighted mean of a set of published data.

Search for the radioactivity of 180mTa using an underground HPGe sandwich spectrometer

The radioactivity of 180mTa has never been detected. The present attempt to detect it was carried out using a newly developed HPGe sandwich spectrometer installed 500 m water equivalent underground in the HADES laboratory. The sample consisted of 6 discs of tantalum of natural isotopic composition with a total mass of 1500 g and a total mass for 180Ta of 180 mg. The sample was measured for 68 days and the resulting lower bound for the half-life of 180mTa was 2.0×10 16 y, which is a factor of 2.8 higher than the previous highest value.

The Sandwich spectrometer for ultra low-level γ-ray spectrometry

The technical details and performance of the newly developed Sandwich spectrometer for ultra low-level γ-ray spectrometry are presented. The spectrometer, which consists of two HPGe detectors, an active muon shield and a lead/copper shield with a convenient and rapid opening mechanism, is located in an underground laboratory at a depth of 500 m water equivalent. The data is collected in list mode, which enables off-line data analysis to identify muon-induced events and possible Ge detector crosstalk due to Compton scattering. The background count-rate from 40 to 2700 keV normalised to the mass of the Ge crystals is 220 day −1 kg −1.

Measurement of the spectral fluence rate of reference neutron sources with a liquid scintillation detector

Reference neutron sources such as (241)AmBe(alpha,n) and (252)Cf are commonly used to calibrate neutron detectors for radiation protection purposes. The calibration factors of these detectors depend on the spectral distribution of the neutron fluence from the source. Differences between the spectral fluence of the neutron source and the ISO-recommended reference spectra might be caused by the properties of the individual source. The spectral neutron fluence rates of different reference neutron sources used at PTB were measured with a liquid scintillation detector (NE213), using maximum entropy unfolding and a new, experimentally determined detector response matrix. The detector response matrix was determined by means of the time-of-flight technique at a pulsed neutron source with a broad energy distribution realised at the PTB accelerator facility. The results of the measurements of the reference sources are compared with the ISO-recommended reference spectra. For the PTB (241)AmBe(alpha,n) reference source, the spectral neutron fluence was determined by means of a high-resolution (3)He semiconductor sandwich spectrometer in 1982. These measurements were the basis for the ISO recommendations. The current measurements confirm the high-energy part (E(n) > 2 MeV) of this spectrum and demonstrate the suitability of this new method for high-resolution spectrometry of broad neutron spectra.

Standardisation of 125I using seven techniques for radioactivity measurement

Seven methods of radioactivity measurement were used to standardise an 125I solution within the frame of an international key comparison organised by BIPM: photon–photon coincidence counting with two NaI detectors, photon sum-peak counting in a NaI well detector and in a 4 π CsI(Tl) sandwich spectrometer, total emission counting in a windowless 4 π CsI(Tl) sandwich spectrometer, electron-X, γ coincidence counting and electron-X, γ sum counting in a pressurised proportional counter inside a NaI well detector and liquid scintillation counting with the CIEMAT/NIST method. The solid sources were prepared by quantitative drop deposition with addition of AgNO 3. The measurement methods, the results and the applied corrections are described and discussed.

Standardisation of 238Pu using four methods of measurement

Four independent methods of measurement were used to standardise a 238Pu solution within the frame of an international key comparison organised by the Bureau International des Poids et Mesures, Sèvres: 4 π liquid-scintillation spectrometry, alpha-particle counting at a defined solid angle, alpha spectrometry using a windowless 4 π CsI(Tl)-sandwich spectrometer and a 4 π pressurised proportional counter. The solid sources were prepared by quantitative drop deposition and a special source-drying technique. The weighted mean value of the activity concentration of 238Pu was found to be (360.2±1.0) kBq g −1 at the reference date (1 April 2001). The measurement methods, the results and the applied corrections are described and discussed.

Standardisation of 89Sr using three different methods

Three independent methods were used to standardise a 89Sr solution within the frame of an international comparison organised by BIPM/CCRI(II): 4 π liquid scintillation (LS) spectrometry, based on the CIEMAT/NIST efficiency tracing method, and direct activity measurements with a windowless 4 π CsI(Tl)-sandwich spectrometer and a 4 π pressurised gas proportional counter. Quantitative solid point-like sources were prepared paying special attention during the source drying phase. The impurities in the original solution were measured and corrected for in the results of all three methods. The activity concentration of 89Sr was found to be 26.21±0.08 kBq g −1. A new half-life value for 89Sr of 50.61±0.05 days was determined from the LS measurements. The measurement methods, including the impurity measurements, are described and the results compared, considering the advantages and disadvantages of each method.

High resolution measurements of neutron energy spectra from Am[sbnd]Be and Am[sbnd]B neutron sources

A 3He sandwich spectrometer incorporating two semiconductor detectors and a proportional counter region has been designed and constructed to perform high resolution measurements of neutron energy spectra in the energy range 100 keV to 15 MeV. The efficiency of the spectrometer was determined experimentally, using a 252Cf spontaneous fission source, in the low-scatter facility of the National Physical Laboratory. The Monte Carlo technique was also used to determine the efficiency and these calculated values were used to extrapolate the measured efficiency to higher energies. The neutron energy spectra from two different sized AmBe neutron sources and an AmB neutron source were measured. Although these spectra have been measured previously, the present work represents an improvement in the energy resolution and extends the energy range. Average values for dose equivalent quantities for neutrons from these sources are also presented. The spectral data is available from the authors, as a printed table or on a computer diskette, on request.

Absolute activity measurements with the windowless 4π-CsI(Tl)-sandwich spectrometer

The windowless 4π-CsI(Tl)-sandwich spectrometer consists of two scintillation crystals sandwiching radioactive sources deposited on thin plastic foils. This configuration has a solid angle very close to 4π sr. The detectors are sensitive to charged particles with energies > 15 keV and measure photons of 15–200 keV with a probability > 98%. Disintegration rates of samples of radionuclides with complex decay modes can be determined directly from the measured count rates with uncertainties below 0.3%. Radionuclide solutions of 57Co, 109Cd, 125I, 152Eu and 192Ir were standardised, partly in the framework of international comparisons. A detailed description of the spectrometer and the measurement procedure is given.

Measurement of the 59.5-keV gamma-ray emission probability in the decay of 241Am with a 4π-CsI(T1) - sandwich spectrometer

The 59.5-keV γ-ray emission rate of 241Am sources has been measured with a windowless 4 π-CsI(T1)-sandwich spectrometer. Photon attenuation by the α-particle absorbers was minimized by an optimal shape of these absorbers. The disintegration rates of the same 241Am sources were obtained by α-particle counting under defined low solid angles. The ratio of the two measured quantities, the 59.5-keV γ-ray emission probability, was found to be 0.3636 ± 0.0017 which is slightly higher than the recommended value of 0.359 ± 0.004.

A γ-discriminating 3He-semiconductor sandwich spectrometer

Abstract This report describes the construction and test of a 3 He-semiconductor sandwich spectrometer for in-core and out-of-core neutron spectrum measurements in the energy region from 100 keV to 5 MeV. The spectrometer is able to discriminate against γ-background and to correct for energy losses in the target. Therefore it has been possible to extend considerably the useful energy range of the classical 3 He-spectrometer. The geometrical efficiency of the spectrometer is calculated using a Monte Carlo program; in addition it is determined experimentally with monoenergetic neutrons. In a number of measurements the reliability of the spectrometer is demonstrated by comparison with other independent techniques.

The use of the lithium-6 semiconductor sandwich spectrometer for the measurement of fast-neutron spectra

In order to meet the stringent target accuracies for neutron spectrum measurements in fast reactor assemblies, the existing experimental methods must be refined and the sources of experimental error identified and, where possible, eliminated. The construction and properties of the lithium-6 semiconductor detector are discussed. The technique may be used to measure neutron spectra over the energy range (10 keV–6 MeV). Major sources of experimental error have been isolated, enabling improvements to be incorporated into the existing electronics system. The life-span of the detectors is severely limited by fast neutron radiation damage of the silicon diodes. The 6Li cross-section has been remeasured to an accuracy of ±4% (one standard deviation). The method of background subtraction has been tested and found to be adequate. Fast neutron spectra have been measured in three ZEBRA assemblies in the energy range (10 keV–6 MeV) and their accuracies assessed. The experimental results are compared with results from competing experimental techniques and with the theoretical predictions made using current methods and data. It is concluded that the results provide a sufficient overlap with existing techniques to give an accurate measurement of the fast-neutron spectrum above 1 MeV.

Spectral shape and nuclear matrix elements in the β-decay of 206Tl

The spectral shape, endpoint energy and half-life of the β-decay of 206Tl were measured. The β-particles were analysed in a 4π Si(Li) sandwich spectrometer. The sources used were isotopically separated and therefore extremely thin. The measured spectrum was corrected for the escape of bremsstrahlung. The extracted shape factor shows only small deviations from the statistical form, the endpoint energy of the decay is 1527 ± 4 keV, its half-life 4.27 ± 0.05 min. The nuclear matrix elements of this 0 − → 0 + decay have been calculated from single-particle wave functions. The resulting theoretical spectral form describes the experimental data very well, whereas the total transition probability is underestimated theoretically by a factor of 7.

Investigation of a 6Li semiconductor sandwich spectrometer for its characteristics and performances

Some characteristics and performances of a 6Li semiconductor neutron spectrometer such as low signal to background ratios outside a beam port, counting efficiency versus threshold energy setting and matching difficulty were reported and studied. Energy calibration can be made with thermal neutrons by the characteristics peaks of 2.73 MeV tritons and 2.05 MeV alphas instead of monoenergetic neutrons from a Van de Graaff accelerator.

Theoretical and experimental investigations for an improved application of the 6Li-semiconductor sandwich spectrometer

Theoretical and experimental investigations were carried out to improve the applicability of the 6Li-semiconductor sandwich spectrometer for neutron spectroscopy in fast zero power reactors and for the measurement of spectra from stationary radioactive neutron sources. The theoretical treatment presented is capable of calculating the efficiency of any geometry of the spectrometer as a function of the neutron energy in an isotropic flux as well as for beam geometry. The analytical model includes the incorporation of a discriminator threshold and its effect on the overall spectrometer efficiency. Calculated and measured geometrical efficiencies and energy resolutions were compared and showed good agreement. The application of the spectrometer to the determination of source neutron spectra as well as to neutron spectroscopy in the fast zero power facility SNEAK is reported.