Evaluating the intensity of the 803-keV γ ray of 210Po using a 4παβ(LS)-γ(HPGe) measurement system

The absolute intensity for the 803-keV γ ray of ²¹⁰Po was evaluated by α-γ coincidence technique. A liquid sample with a known amount of ²¹⁰Po embedded in scintillation fluid was measured in a coincidence-based system that comprises a Liquid Scintillator (LS) detector and a High-Purity Germanium (HPGe) detector. A photo-reflector assembly that contains the ²¹⁰Po sample provides 100% efficiency for detecting the α particles. The combination between the HPGe and the LS detectors allows to reject non-coincident α-γ events while maintaining high resolution γ spectroscopy. Consequently, the faint 803-keV photopeak from ²¹⁰Po could be observed in a background-free environment, and its intensity could be evaluated with good accuracy. Sample measurements were carried out over nine months to gather statistics and verify the reliability of the experimental procedure. The absolute intensity of the 803-keV line was found to be (1.22 ± 0.03) × 10^-5, in excellent agreement with the adopted value in a recent data compilation and consistent with previous experimental works.

Characterization of a 4παβ(LS)-γ(HPGe) prototype system for low-background measurements

A ground-level prototype system for low-background measurements was developed and tested. The system consists of a high-purity germanium (HPGe) detector used for detecting γ rays and coupled to a liquid scintillator (LS) used for detecting α and β particles. Both detectors are surrounded by shielding materials and anti-cosmic detectors ("veto") used to suppress background events. The energy and timestamp of detected α, β and γ emissions are recorded event-by-event and analyzed offline. By requiring timing coincidence between the HPGe and LS detectors, background events originating from outside the volume of the measured sample can be effectively rejected. The system performance was evaluated using liquid samples containing known activities of an α emitter (²⁴¹Am) or a β emitter (⁶⁰Co) whose decays are accompanied by γ rays. The LS detector was found to provide a solid angle of almost 4π for α and β particles. Compared to the traditional γ-singles mode, operating the system in coincidence mode (i.e., α-γ or β-γ) reduced the background counts by a factor of ∼100. Consequently, the minimal detectable activity for ²⁴¹Am and ⁶⁰Co was improved by a factor of 9, being 4 mBq and 1 mBq for an 11-d measurement, respectively. Furthermore, by applying a spectrometric cut in the LS spectrum that corresponds to α emission from ²⁴¹Am, a background reduction factor of ∼2400 (compared to γ-singles mode) was achieved. Beyond low-background measurements, this prototype exhibits additional compelling features, such as the ability to focus on certain decay channels and study their properties. This concept for a measurement system may be of interest to laboratories that monitor environmental radioactivity, studies involving environmental measurements and/or trace-level radioactivity.

A rapid method for determining low concentrations of 210Pb in drinking water using MnO2 fibers

A rapid method for determining low activity concentrations of ²¹⁰Pb in drinking water was developed and tested. The method consists of a few stages for sample preparation that involve passing 12 L of water through a column with acrylic fibers implanted with MnO₂ (used to adsorb ²¹⁰Pb). The MnO₂ fibers are oven-dried, compressed and measured by a broad-energy germanium detector used to quantify ²¹⁰Pb via its characteristic 46.5 keV γ-ray. The time taken for sample preparation is approximately 4 h and recovery factors for lead in tap water of 87 ± 3% were achieved. After a measurement duration of 4 h, the minimum detectable activity concentration reaches 0.02 Bq/L for ²¹⁰Pb, being well below the respective limit for drinking water in Israel (0.2 Bq/L) as well as the value recommended by the World Health Organization (0.1 Bq/L). Furthermore, a measurement duration of 48 h provides a minimum detectable activity concentration of ∼0.006 Bq/L, which is similar in magnitude to other, well-established methods that rely on lengthy and rather complex procedures. Thus, the combination of MnO₂ fibers and gamma-ray spectrometry may be attractive for routine use by analytical laboratories that monitor radioactivity in drinking water.

Evaluating the intensity of the 'prompt' 140.5 keV γ-ray of 99Mo using a 4παβ(LS)-γ(HPGe) measurement system

The absolute intensity for the 'prompt' 140.5 keV gamma-ray of ⁹⁹Mo was evaluated using the β-γ coincidence technique. A liquid sample of ⁹⁹Mo was prepared from a⁹⁹Mo/^99mTc generator and measured in a 4παβ(LS)-γ(HPGe) system that comprises a Liquid Scintillator (LS) detector and a High-Purity Germanium (HPGe) detector. The sample was introduced into scintillation fluid embedded in a photo-reflector assembly that provides almost 100% efficiency for detecting β particles (in the energy range of intreset). The combination of the HPGe and the LS detectors provided a highly effective rejection mechanism for non-coincident events. Thereby, the distinction between the detected 140.5 keV events originating from decays of ^99mTc (IT) and those from transitions bypassing the metastable state could be obtained and the 'prompt' intensity was evaluated directly. The system was calibrated for detecting β particles and γ-rays using radioactive sources of known activities and having identical geometry as the sample containing ⁹⁹Mo. The absolute intensity of the 'prompt' 140.5 keV was found to be (5.21 ± 0.02_stat±0.16_sys)%, in good agreement with results from more recently reported works.

A METHOD TO IDENTIFY AND LOCALIZE A SINGLE HOT PARTICLE IN THE LUNGS USING AN ARRAY OF HIGH-PURITY GERMANIUM DETECTORS FOR IMPROVED ESTIMATE OF THE DEPOSITED ACTIVITY

A new method has been developed to identify and localize a single hot particle in the lungs using an array of four high-purity germanium detectors. The method is based upon calculating a set of three count rate ratios (generated by each individual detector in the array) that are evaluated in sequence to designate whether the measured deposition can be associated with a hot particle rather than the default assumption of a uniform activity distribution. Identification and localization of the hot particle are determined from a single in vivo measurement in which detectors are positioned above and below the thorax. The method was tested using an anthropomorphic thorax phantom in which point sources of 241Am, 137Cs and 60Co were individually inserted in the lungs at 15 different locations and were measured using a scanning bed whole-body counter. Depending upon source location and photon energy, a bias of -35% up to +76% could be introduced by falsely assuming a uniform activity distribution in the lungs. This bias would directly translate to an erroneous dose estimate to the lungs. It was demonstrated that by using the appropriate detector efficiencies for the single hot particle, the bias associated with the activity determination is reduced to <10% and ~2% in average.

Oculomotor anticipation reveals a multitude of learning processes underlying the serial reaction time task

Sequence learning is the cognitive faculty enabling everyday skill acquisition. In the lab, it is typically measured in speed of response to sequential stimuli, whereby faster responses are taken to indicate improved anticipation. However, response speed is an indirect measure of anticipation, that can provide only limited information on underlying processes. As a result, little is known about what is learned during sequence learning, and how that unfolds over time. In this work, eye movements that occurred before targets appeared on screen in an ocular serial reaction time (O-SRT) task provided an online indication of where participants anticipated upcoming targets. When analyzed in the context of the stimuli preceding them, oculomotor anticipations revealed several simultaneous learning processes. These processes influenced each other, as learning the task grammar facilitated acquisition of the target sequence. However, they were dissociable, as the grammar was similarly learned whether a repeating sequence inhabited the task or not. Individual differences were found in how the different learning processes progressed, allowing for similar performance to be produced for different latent reasons. This study provides new insights into the processes subserving sequence learning, and a new method for high-resolution study of it.

Direct and indirect measures of context in patients with mild-to-severe traumatic brain injury (TBI): The additive contribution of eye tracking

Introduction: The facilitation of memory for target stimuli due to the similarity of context in the learning and testing phases is known as the "Context-Effect" (CE). Previous studies reported that TBI affects memory for contextual information when tested directly. However, the indirect effect of contextual information on memory of target (i.e., CE) is preserved. Several studies have demonstrated that CE is composed of multiple, distinct cognitive processes. The present study includes four context conditions to enable identification of the exact process affected by TBI. In addition, eye movements were monitored to test three hypotheses: first, that the TBI group's dwell time on target (DTOT) at encoding would be less than that of controls. Second, that DTOT at encoding would be more highly associated with recognition at test for the control group than for the TBI group. Third, that overall DTOT at encoding on new, as compared to old items ("repetition effect"), would be less pronounced for the TBI group as compared to controls. Methods: Twenty-four patients with mild-to-severe TBI and 23 matched controls participated in this study. We presented participants with photographs of male faces shown wearing distinctive, trial-unique hats (yielding specific Target-Context pairing). Eye movements were recorded throughout the test task. Results: Memory for faces following TBI is impaired compared to that of controls. The magnitude and pattern of CE are the same for both groups. The TBI group has a lower DTOT compared to that of controls. However, the relative length of DTOT in the various conditions is similar in both groups. Conclusions: Behavioral results indicate that although the TBI group has impaired memory for faces, the CE pattern is similar to that of controls. Similarly, in terms of eye movements, although the TBI group focuses less on target, relations between the various conditions are similar in both groups.

RETENTION OF 7Be IN THE LUNGS FOLLOWING INTAKE BY INHALATION

Several workers were internally exposed to 7Be particles following their dispersion in air from a damaged electrodeposited source. A series of in vivo measurements performed with one worker up to 108 days post exposure determined that retention of 7Be in the thoracic region of the respiratory tract was best described by a two-component exponential function with half-lives of ~0.4 and ~109 days. The initial deposition in the thoracic region was estimated to be 6.8 kBq. The concentration of 7Be in single void urine samples collected from this worker up to 3 days post intake ranged from 1 to 10 Bq/l. In the absence of specific knowledge about the physical and chemical characteristics of the inhaled particles, the committed effective dose was estimated to be 0.3 μSv.

Response of a Shadow-Shield Whole-Body Counter to a Variety of Physical Phantoms

The performance characteristics of a shadow-shield whole-body counter system with an array of four high-resolution germanium detectors using whole-body and organ-specific (lungs, liver, head, knee and thyroid) physical phantoms are described. Detection efficiency and minimum detectable activities for selected radionuclides and several measurement configurations are presented. Results demonstrate that the system meets the requirements for direct radio bioassay and that detection efficiency and minimum detectable activities are similar in magnitude to other whole-body (or organ) counting systems installed in fully shielded structures.

Note: Proton irradiation at kilowatt-power and neutron production from a free-surface liquid-lithium target

The free-surface Liquid-Lithium Target, recently developed at Soreq Applied Research Accelerator Facility (SARAF), was successfully used with a 1.9 MeV, 1.2 mA (2.3 kW) continuous-wave proton beam. Neutrons (~2 × 10(10) n/s having a peak energy of ~27 keV) from the (7)Li(p,n)(7)Be reaction were detected with a fission-chamber detector and by gold activation targets positioned in the forward direction. The setup is being used for nuclear astrophysics experiments to study neutron-induced reactions at stellar energies and to demonstrate the feasibility of accelerator-based boron neutron capture therapy.

Absolute photo-destruction and photo-fragmentation cross section measurements using an electrostatic ion beam trap

We describe a technique to measure absolute photo-induced cross sections for cluster anions stored in an electrostatic ion beam trap (EIBT) with a central deflector. The setup allows determination of total photo-destruction cross sections as well as partial cross sections for fragmentation and electron detachment. The unique properties of this special EIBT setup are investigated and illustrated using small Al(n)(-) clusters.

A bent electrostatic ion beam trap for simultaneous measurements of fragmentation and ionization of cluster ions

We describe a bent electrostatic ion beam trap in which cluster ions of several keV kinetic energy can be stored on a V-shaped trajectory by means of an electrostatic deflector placed between two electrostatic mirrors. While maintaining all the advantages of its linear counterpart [Zajfman et al., Phys. Rev. A 55, R1577 (1997); Dahan et al., Rev. Sci. Instrum. 69, 76 (1998)], such as long storage times, straight segments, and a field-free region for merged or crossed beam experiments, the bent trap allows for simultaneous measurement of charged and neutral fragments and determination of the average kinetic energy released in the fragmentation. These unique properties of the bent trap are illustrated by first results concerning the competition between delayed fragmentation and ionization of Al(n) (-) clusters after irradiation by a short laser pulse.