The DGF Pixie-4 spectrometer - compact digital readout electronics for HPGe clover detectors

Large volume HPGe detectors are commonly used in applications that require good energy resolution and high detection efficiency, but are expensive and difficult to grow. Clover detectors consisting of four smaller crystals in a common cryostat are a possible alternative, but traditionally require complex readout electronics. In contrast, the DGF Pixie-4 is a compact, digital spectrometer providing on a single 3U CompactPCI/PXI card all the electronics required for clover detectors, including computation of addback spectra. This paper describes the DGF Pixie-4 system architecture, characterizes its energy resolution and throughput, and presents results of test measurements with a clover detector.     

Fast analytical methods for the correction of signal random time-shifts and application to segmented HPGe detectors

Detection systems rely more and more on on-line or off-line comparison of detected signals with basis signals in order to determine the characteristics of the impinging particles. Unfortunately, these comparisons are very sensitive to the random time-shifts that may alter the signal delivered by the detectors. We present two fast algebraic methods to determine the value of the time-shift and to enhance the reliability of the comparison to the basis signals.     

An improved beam chopper setup at the Budapest PGAA facility

Modulating neutron beams with a beam chopper enables the investigation of time-dependent processes following neutron capture, such as emission of decay-gamma-radiation from short-lived nuclides. For this purpose, a redesigned beam chopper and controller electronics have been recently installed at the PGAA facility of the Budapest Research Reactor. The new electronics provides accurate timing and better control of routing of the signals. The upgraded instrumentation is capable of recording the events in the beam-on and beam-off phases in parallel. Simplified formulae were derived to relate the areas of prompt- and decay peaks to the partial γ-ray production cross-sections. Finally, preliminary results obtained with Ag- and Na-containing targets are presented.     

Use of silicon drift detectors for the detection of medium-light elements in PIXE

In order to fully exploit in PIXE the superior performance of silicon drift detectors especially for the detection of low- and medium-energy X-rays, avoiding in particular the negative effects of backscattered particles, we developed a custom spectrometer based on a 10 mm² chip with a thermoelectric Peltier cooler and home-designed front-end electronics, coupled to a weakly focusing polycapillary lens.This paper briefly describes the detector + lens assembly and reports the results of first tests carried out at an external beam line of the LABEC laboratory in Florence. Excellent energy resolution is achieved under real operating conditions in a PIXE run (measured FWHM at 1 keV is 81 eV with a count-rate of 480 cps) and also the lineshapes are very good (FW1/10M over FWHM ratio is 2.1). As a whole, our preliminary tests gave encouraging results and also helped to point out some aspects which it is worthwhile to investigate further (e.g. how X-ray peak intensity ratios may be affected by inaccurate lens alignment), in order to profit fully from such a good performance of the spectrometer.     

A digital spectrometer approach to obtaining multiple time-resolved gamma-ray spectra for pulsed spectroscopy

Neutron-induced gamma-ray emission and its detection using a pulsed neutron generator system is an established analytical technique for quantitative multi-element analysis. Traditional gamma-ray spectrometers used for this type of analysis are normally operated either in coincidence mode - for counting prompt gamma-rays following inelastic neutron scattering (INS) events when the neutron generator is ON, or in anti-coincidence mode - for counting prompt gamma-rays from thermal neutron capture (TNC) processes when the neutron generator is OFF. We have developed a digital gamma-ray spectrometer for concurrently measuring both the INS and TNC gamma-rays using a 14 MeV pulsed neutron generator. The spectrometer separates the gamma-ray counts into two independent spectra together with two separate sets of counting statistics based on the external gate level. Because the TNC gamma-ray yields are time dependent, additional accuracy in analyzing the data can be obtained by acquiring multiple time-resolved gamma-ray spectra at finer time intervals than simply ON or OFF. For that purpose we are developing a multi-gating system that will allow gamma-ray spectra to be acquired concurrently in real time with up to 16 time slots. The conceptual system design is presented, especially focusing on considerations for tracking counting statistics in multiple time slots and on the placement of pulse heights into multiple spectra in real time.     

PIXE characterization of CsI(Tl) scintillators used for particle detection in nuclear reactions

Particle-Induced X-ray Emission has been used to measure Thallium concentration in several CsI(Tl) scintillators from different manufacturers, in order to check their nominal declared values and correlate their behaviour with actual Tl concentration. Indeed, both Tl doping level and its uniformity affect light emission of these detectors, which are largely employed in nuclear physics experiments.In some of the examined crystals Tl concentration values from PIXE measurements came out to be quite different from those declared. This allowed us to explain apparent anomalies in the trend of their α/γ-induced light yield ratio versus Tl content. In some cases, the presence of unexpected contaminants was also pointed out.     

Quality assessment system for curved crystal X-ray optics

Doubly curved crystal X-ray optics provide intense focused monochromatic beams from laboratory X-ray tube sources. These optics are employed in crystallography and X-ray fluorescence systems and may find application to imaging. It is increasingly important to understand how optic defects impact performance for these systems. A simulation model was developed to assess the effects of misalignment and optic defects on system parameters such as intensity, beam size and resolution. Simulation results were compared to optics measurements. Rapid reproducible measurements of optics quality are important both for performing systematic studies of optics defects and for assessing individual optics. A simple operator-independent alignment technique was developed that was also beneficial in ensuring optimal beam intensity in analysis systems. The measurements and simulations were in good agreement and provided insight into essential optics parameters.     

Comparison of the analytical sensitivities for non-1/v elements in different neutron beams

Irregular nuclides such as ¹¹³Cd, and some rare-earth isotopes show different analytical sensitivities in PGAA performed at different facilities, because the cross-sections of these nuclides have strong low-energy resonances which partly overlap the energy range of typical neutron beams used for activation. A series of systematic measurements has been performed in the spectrally different cold and thermal neutron beams of the Budapest Research Reactor, Hungary and at the research reactor of NIST, Gaithersburg, USA to quantitatively study the non-1/v behaviour of irregular nuclides. Samples were prepared that contained one of the irregular nuclides and also a regular one (¹⁰B, ³⁵Cl and ⁵⁶Fe) and their activation ratios were compared as measured in five different beams. Theoretical values of the activation ratios were calculated from estimates of the actual neutron spectra and cross-section data, and show a generally good correspondence to the experimental results, although some details are still not reproduced.     

Phantom studies of Cd, Hg and Cl by prompt gamma neutron activation analysis using a Pu-Be neutron source

Prompt gamma neutron activation analysis is a means of non-invasive monitoring for occupational exposure to toxic heavy metals such as Cd and Hg. Preliminary kidney detection limits from previous phantom studies at McMaster were 13.6 ± 0.2 ppm for Cd (125 mL phantom) and 315 ± 24 ppm for Hg (125 mL phantom) using the ²³⁸Pu-Be neutron source and 0.88 ± 0.01 ppm for Cd (125 mL phantom) and 16.91 ± 0.05 ppm for Hg (30 mL phantom) using the thermal neutron beam port at the McMaster Nuclear Reactor. The detection limits vary greatly between the two methods due to differences in experimental set-up, neutron energy spectra and a difference in dose by more than a factor of 100. The Hg detection limit from preliminary data is much higher than expected for both neutron source types. In order to explain the apparent detection limit discrepancy, measurements of Hg and Cd phantoms were performed using the ²³⁸Pu-Be neutron source. The results were compared to phantom measurements of Cl, a well-known neutron activation element.     

Temperature and pressure conditions for the appropriate performance of charge and mass resolutions in balloon-borne CR-39 track detector for the heavy cosmic rays

We experimentally determined the suitable conditions of temperature and air pressure for measuring charge and mass compositions of heavy cosmic rays using CR-39 track detectors during long-duration, high-altitude balloon flights. The experiments were carried out utilizing Fe ion beams from Heavy Ion Medical Accelerator (HIMAC). Changes in the track registration sensitivity with temperature were studied at temperatures between 213 and 293 K. The charge and mass shifts around Fe ions are possibly 0.11 cu (charge unit) and 0.47 amu (atomic mass unit) at the temperature deviation between 227 and 243 K at balloon altitude. Decrease in track registration sensitivity as air pressure decreases has been observed between 13 Pa and 27 kPa. The charge and mass shifts around Fe ions are inferred to be 0.02 cu and 0.09 amu, respectively, when pressure varies between 20 and 27 kPa. During balloon flight, air pressure drops below this; therefore, care must be taken to keep the temperature and air pressure of CR-39 detectors within acceptable limits.     

A DSP-based multichannel analyzer for simultaneous acquisition of coincidence and anticoincidence spectra

A digital signal processor (DSP) based multichannel analyzer (MCA) has been developed for simultaneous acquisition of coincidence and anticoincidence γ-ray spectra. The shaped pulse from the spectroscopy amplifier is digitized by a flash analog-to-digital converter and then processed by a DSP. The coincidence mode operation is implemented by an external gate signal from a coincidence module. Fundamental performance was tested using NaI(Tl) detectors and compared with that of a standard NIM module. The new MCA is currently used for in vivo neutron activation analysis. Further development is in preparation toward full digital processing, which is free from the remaining analog component, that is the spectroscopy amplifier.     

Concentrations of U and Th in foodstuffs and radiation dose to a rural Moroccan population

Uranium (²³⁸U) and thorium (²³²Th) contents were measured in different foods widely consumed by the rural Moroccan population by using two different types of solid state nuclear track detectors (SSNTDs). The method utilized is based on determining the probabilities for α-particles emitted by the uranium and thorium series inside the studied materials to reach and be registered on the CR-39 and LR-115 type II SSNTDs and counting the resulting track densities. Total daily intakes of ²³⁸U and ²³²Th for a typical food basket were estimated to be 1.27 Bq d⁻¹ and 0.95 Bq d⁻¹, 1.55 Bq d⁻¹ and 1.18 Bq d⁻¹ and 2.01 Bq d⁻¹ and 1.52 Bq d⁻¹ for the 7-12 y, 12-17 y and adults age groups, respectively. Annual committed equivalent doses were evaluated in the human body compartments of members of the Moroccan rural population due to ²³⁸U and ²³²Th from the ingestion of the studied food materials by using the ICRP ingestion dose coefficients. Annual committed effective dose due to ²³⁸U and ²³²Th intakes were found to be 3.16 × 10⁻⁵ Sv and 10.10 × 10⁻⁵ Sv, 3.81 × 10⁻⁵ Sv and 10.80 ×  10⁻⁵ Sv and 3.32 × 10⁻⁵ Sv and 12.80 × 10⁻⁵ Sv for the 7-12 y, 12-17 y and adults age groups, respectively.     

Be AMS measurements at low energies (E < 1 MeV)

The longlived isotope ¹⁰Be is of great importance in earth sciences for dating applications, reconstruction of the solar activity or in climate research. Routine AMS measurements with BeO samples are performed on accelerators with a terminal voltage above 2 MV. Applying the degrader foil technique for boron suppression, first tests with BeO samples on the 0.6 MV ETH/PSI machine were limited by background to a ¹⁰Be/⁹Be ratio of 10⁻¹³. The background was identified as ⁹Be which reaches the detector by scattering processes. By applying an additional magnetic mass filter to the high energy mass spectrometer the background was effectively removed. A ¹⁰Be/⁹Be background ratio of <5 × 10⁻¹⁵ was achieved. The overall efficiency (detected ¹⁰Be compared to BeO⁻ injected into the accelerator) was 7-8%.     

Micro-SR-XRF and micro-PIXE studies for archaeological gold identification - The case of Carpathian (Transylvanian) gold and of Dacian bracelets

Trace-elements are more significant for provenancing archaeological metallic artifacts than the main components. For gold, the most promising elements are platinum group elements (PGE), Sn, Te, Sb, Hg and Pb. Several small fragments of natural Transylvanian gold - placer and primary - were studied by using micro-PIXE technique at the Legnaro National Laboratory AN2000 microbeam facility, Italy and at the AGLAE accelerator, C2RMF, Paris, France and by using micro synchrotron radiation X-ray fluorescence (micro-SR-XRF) at BESSY synchrotron, Berlin, Germany. The goal of the study was to identify the trace-elements, especially Sn, Sb and Te. A spectacular application to five Dacian gold bracelets authentication is presented (Sn and Sb traces).     

Determining Pu isotopic composition and Pu content of PuBe sources by neutron coincidence technique

The paper describes a pure neutron method for determining both Pu content and Pu isotopic composition of PuBe neutron sources by neutron coincidence technique, without using gamma-spectrometry. The new procedure based on the R/T-T relationship is a developed version of the R/T-method based on R/T-M_Pu calibration curve described in [C.T. Nguyen, J. Bagi, L. Lakosi, A novel method of quantitative assay of PuBe neutron sources by neutron coincidence technique, Nucl. Instr. and Meth. B 246 (2006) 409], utilizing Pu isotopic correlations; here R, T, M_Pu are double count rate, single count rate and total Pu content, respectively. Accuracy of the method was found to be about 2-3% and 15% for ²³⁹Pu component and Pu content, respectively. Measurement time as a function of detector efficiency is treated in detail. It is shown that in a system of frame, a transuranium neutron source can be characterized by a pair of co-ordinates [R/T,  T].     

Improved energy resolution of a cyclotron beam for RBS measurements

For RBS (Rutherford Back Scattering) analysis, the quality of the beam is of premium importance because the depth profile resolution of the method is strongly dependent on the energy resolution of the probing beam. A magnetic analyzer, consisting of two 90 left-right bending magnets forming an achromatic doublet has been adapted to the Liege 20 MeV (proton) AVF (Azimuthal Varying Field) cyclotron. The energy resolution of that system has been measured by recording the resonance width of a ³²S(p,p′γ)³²S (3.38 MeV. p⁺ lab. energy). We have obtained a value of ΔE = ± 2 keV, reducing by a factor of 20 the natural dispersion of our cyclotron.We describe our magnetic analyzer system and present the results of our RBS measurements at energies up to 14 MeV α.     

Cell irradiation setup and dosimetry for radiobiological studies at ELBE

The radiation source ELBE delivers different types of secondary radiation, which is used for cell irradiation studies in radiobiological research. Thereby an important issue is the determination of the biological effectiveness of photon radiation as a function of photon energy by using low-energetic, monochromatic channeling radiation (10-100 keV) and high-energetic bremsstrahlung (up to 40 MV). Radiobiological studies at the research facility ELBE demand special technical and dosimetric prerequisites. Therefore, a cell irradiation system (CIS) has been designed, constructed and installed at the beam line. The CIS allows automatic irradiation of a larger cell sample number and the compensation of spatial inhomogeneity of the dose distribution within the beam spot. The recently introduced GafChromic^® EBT radiochromic film model has been used to verify the cell irradiation dose deposition achieving a dose uncertainty of <5%. Both, the installed cell irradiation system and the developed dosimetric procedure based on the use of the EBT film have been experimentally tested at ELBE. The biological effectiveness of 34 MV bremsstrahlung with respect to 200 kV X-rays from a conventional X-ray tube has been determined. An RBE value of 0.75 has been measured in good agreement with literature.     

Determination of the effective atomic numbers and electron densities for YBaCuO superconductor in the range 59.5-136 keV

The effective atomic numbers and electron densities of YBa₂Cu₃O_7−δ superconductor at 59.5, 65.2, 77.1, 94.6, 122 and 136 keV were calculated by using the measured mass attenuation coefficients. Measurements were made by performing transmission experiments in a well-collimated narrow beam geometry set-up by employing Si(Li) detector with a resolution of 0.16 keV at 5.9 keV. These values are found to be in good agreement with theoretical values calculated based on XCOM data. The observed crystal structure of YBa₂Cu₃O_7−δ superconductor is close to the theoretical structure. Z_eff and N_el experimental values showed good agreement with the theoretical values for calcined and sintered YBa₂Cu₃O_7−δ.     

Investigation of the Zn(p, 2p)Cu nuclear reaction: New measurements up to 40 MeV and compilation up to 100 MeV

The excitation function was measured for the ⁶⁸Zn(p, 2p)⁶⁷Cu nuclear reaction from its threshold energy up to 40 MeV. Nine pieces of highly enriched ⁶⁸Zn (>98%) metal foils were irradiated to obtain reliable cross-sections using the usual stacked-foil technique. All foils were subjected to high efficiency radiochemical separation before the activity measurements. A critical compilation of the available experimental cross-section results was also performed. Thick target yields of ⁶⁷Cu and the longer-lived copper radio-contaminants (⁶¹Cu and ⁶⁴Cu) were calculated using the reliable literature results up to 100 MeV. Additionally, EOB (End Of Bombardment) contamination levels as a function of bombarding energy and irradiation time were deduced.     

Differential and scanning-mode external PIXE for the analysis of the painting “Ritratto Trivulzio” by Antonello da Messina

Extensive PIXE investigations have been performed at the LABEC accelerator laboratory in Florence on the painting “Ritratto Trivulzio” by Antonello da Messina, one of the great Italian masters of XV Century and a pioneer in modern oil painting.It is well known that a complete and unambiguous characterisation of materials in paintings is often difficult, owing to their complex structure. For the “Ritratto Trivulzio”, the combination of advanced variants of PIXE, such as differential and scanning-mode analysis, provided a relevant contribution to the characterisation of paint layers, in terms of composition and structure, in a totally non-invasive and non-destructive way.Single-spot mode PIXE with external proton beams of about half millimetre diameter was first used for a general characterisation of different areas of the painting. Differential PIXE performed in many spots led to reconstruct the sequence of paints employed by the artist and to estimate in some cases the local paint layer thickness.A second run carried out with the external scanning microbeam facility, using a 3 MeV proton beam of about 80 μm size, was then crucial to clarify some issues raised by the first analysis. In particular, elemental maps from selected areas helped to understand the way some colour shades had been obtained on the red mantle of the portrayed gentleman, using different pigments irregularly distributed on the surface.