Unambiguous identification of the second 2+ state in 12C and the structure of the Hoyle state.

The second J(π)=2+ state of 12C, predicted over 50 years ago as an excitation of the Hoyle state, has been unambiguously identified using the 12C(γ,α0)(8)Be reaction. The alpha particles produced by the photodisintegration of 12C were detected using an optical time projection chamber. Data were collected at beam energies between 9.1 and 10.7 MeV using the intense nearly monoenergetic gamma-ray beams at the HIγS facility. The measured angular distributions determine the cross section and the E1-E2 relative phases as a function of energy leading to an unambiguous identification of the second 2+ state in 12C at 10.03(11) MeV, with a total width of 800(130) keV and a ground state gamma-decay width of 60(10) meV; B(E2:2(2)+→0(1)+)=0.73(13)e(2) fm(4) [or 0.45(8) W.u.]. The Hoyle state and its rotational 2+ state that are more extended than the ground state of 12C presents a challenge and constraints for models attempting to reveal the nature of three alpha-particle states in 12C. Specifically, it challenges the ab initio lattice effective field theory calculations that predict similar rms radii for the ground state and the Hoyle state.

PROPOSAL FOR A EUROPEAN METROLOGY NETWORK ON BIOLOGICAL IONISING RADIATION EFFECTS.

Progress in the field of ionising radiation (IR) metrology achieved in the BioQuaRT project raised the question to what extent radiobiological investigations would benefit from metrological support of the applied methodologies. A panel of experts from the medical field, fundamental research and radiation protection attended a workshop at Physikalisch-Technische Bundesanstalt to consult on metrology needs related to biological radiation effects. The panel identified a number of metrological needs including the further development of experimental and computational techniques for micro- and nanodosimetry, together with the determination of related fundamental material properties and the establishment of rigorous uncertainty budgets. In addition to this, a call to develop a metrology support for assisting quality assurance of radiobiology experiments was expressed. Conclusions from the workshop were presented at several international conferences for further discussion with the scientific community and stakeholder groups that led to an initiative within the metrology community to establish a European Metrology Network on biological effects of IR.

A compact high-energy neutron spectrometer.

A compact liquid organic neutron spectrometer based on a single NE213 liquid scintillator (5 cm diameter x 5 cm) is described. The spectrometer is designed to measure neutron fluence spectra over the energy range 2-200 MeV and is suitable for use in neutron fields having any type of time structure. Neutron fluence spectra are obtained from measurements of two-parameter distributions (counts versus pulse-height and pulse shape) using the Bayesian unfolding code MAXED. Calibration and test measurements made using a pulsed neutron beam with a continuous energy spectrum are described and the application of the spectrometer to radiation dose measurements is discussed.

Quantitative discrimination between oil and water in drilled bore cores via Fast-Neutron Resonance Transmission Radiography.

A novel method utilizing the Fast Neutron Resonance Transmission Radiography is proposed for non-destructive, quantitative determination of the weight percentages of oil and water in cores taken from subterranean or underwater geological formations. The ability of the method to distinguish water from oil stems from the unambiguously-specific energy dependence of the neutron cross-sections for the principal elemental constituents. Monte-Carlo simulations and initial results of experimental investigations indicate that the technique may provide a rapid, accurate and non-destructive method for quantitative evaluation of core fluids in thick intact cores, including those of tight shales for which the use of conventional core analytical approaches appears to be questionable.

Radiation exposure at ground level by secondary cosmic radiation.

The contribution of the charged component of secondary cosmic radiation to the ambient dose equivalent H*(10) at ground level is investigated using the muon detector MUDOS and a TEPC detector surrounded by the coincidence detector CACS to identify charged particles. The ambient dose equivalent rate H*(10)T as measured with the TEPC/CACS is used to calibrate the MUDOS count rate in terms of H*(10). First results from long-term measurements at the PTB reference site for ambient radiation dosimetry are reported. The air pressure corrected dose rate shows, as expected, a strong correlation with the neutron count rate as measured with the Kiel neutron monitor. The measured seasonal variations exhibit a negative correlation with the temperature changes in the upper layers of the atmosphere where the ground level muons are produced.

Experimental tools for track structure investigations: new approaches for dosimetry and microdosimetry.

In recent years, several new methods have been developed for track structure investigations. They use macroscopic gas volumes to transfer the results to microscopic dimensions and are based on either the cluster counting principle or on optical methods for full track analysis. The first type of method allows cluster size distributions in nanometre dimensions to be measured. The second type reveals the full topology of the ionisation pattern of tracks or track segments within a volume of a few micrometres with a spatial resolution down to 5 nm. The basic principles of these instruments and their main features are summarised and a selection of results obtained to date are shown.

Imaging of microscopic features of charged particle tracks in a low-pressure gas.

An imaging system for measuring the spatial distribution of charged particle tracks in a low-pressure gas is presented. The method is based on an optically read out time projection chamber. Results of experiments with fast heavy ions are shown.

Measurement of neutron fluence spectra up to 150 MeV using a stacked scintillator neutron spectrometer.

A stacked scintillator neutron spectrometer (S3N) consisting of three slabs of liquid organic scintillator is described. A pulsed beam providing a broad spectrum of neutron energies is used to determine the detection efficiency of the spectrometer as a function of incident neutron energy and to measure the pulse height response matrix of the system. Neutron spectra can then be determined for beams with any kind of time structure by unfolding pulse height spectra measured by the S3N. Examples of fluence spectrum measurements in the energy range 20-150 MeV are presented.

Quasi-monoenergetic neutron reference fields in the energy range from thermal to 200 MeV.

Well-characterised neutron fields are a prerequisite for the investigation of neutron detectors. Partly in collaboration with external partners, the PTB neutron metrology group makes available for other users neutron reference fields covering the full energy range from thermal to 200 MeV. The specification of the neutron fluence in these beams is traceable to primary standard cross sections.

A novel fast-neutron detector concept for energy-selective imaging and imaging spectroscopy.

We present and discuss the operational principle of a new fast-neutron detector concept suitable for either energy-selective imaging or for imaging spectroscopy. The detector is comprised of a series of energy-selective stacks of converter foils immersed in a noble-gas based mixture, coupled to a position-sensitive charge readout. Each foil in the various stacks is made of two layers of different thicknesses, fastened together: a hydrogen-rich (plastic) layer for neutron-to-proton conversion, and a hydrogen-free coating to selectively stop/absorb the recoil protons below a certain energy cut-off. The neutron-induced recoil protons, that escape the converter foils, release ionization electrons in the gas gaps between consecutive foils. The electrons are then drifted towards and localized by a position-sensitive charge amplification and readout stage. Comparison of the images detected by stacks with different energy cut-offs allows energy-selective imaging. Neutron energy spectrometry is realized by analyzing the responses of a sufficient large number of stacks of different energy response and unfolding techniques. In this paper, we present the results of computer simulation studies and discuss the expected performance of the new detector concept. Potential applications in various fields are also briefly discussed, in particularly, the application of energy-selective fast-neutron imaging for nuclear safeguards application, with the aim of determining the plutonium content in Mixed Oxide (MOX) fuels.

High-frame rate imaging of two-phase flow in a thin rectangular channel using fast neutrons.

We have demonstrated the feasibility of performing high-frame-rate, fast neutron radiography of air-water two-phase flows in a thin channel with rectangular cross section. The experiments have been carried out at the accelerator facility of the Physikalisch-Technische Bundesanstalt. A polychromatic, high-intensity fast neutron beam with average energy of 6 MeV was produced by 11.5 MeV deuterons hitting a thick Be target. Image sequences down to 10 ms exposure times were obtained using a fast-neutron imaging detector developed in the context of fast-neutron resonance imaging. Different two-phase flow regimes such as bubbly slug and churn flows have been examined. Two phase flow parameters like the volumetric gas fraction, bubble size and mean bubble velocities have been measured. The first results are promising, improvements for future experiments are also discussed.