Detection, identification, and characterization of shielded Special Nuclear Material (SNM), and especially of Highly Enriched Uranium (HEU), often require active interrogation. Active interrogation with neutrons or high energy photons induces fission and thus causes the emission of characteristic gamma and neutron radiation. The detection of the emitted radiation with simple detector systems often only indicates the presence of SNM. Analysis of the correlations of the neutron emissions can provide characteristics of a material, such as multiplication and mass. Such analyses are used in well counters for characterizing SNM. Our system consists of a portable pulsed neutron generator and a portable list -mode neutron detection system. The neutron correlations between the intense interrogating pulses from the neutron generator are analyzed to determine characteristics of the SNM. After the last interrogating pulse, the die away is analyzed to identify the type of SNM. We report data from a variety of SNM objects and present preliminary analysis results.

Improving the quality of safeguards measurements at Gas Centrifuge Enrichment Plants while reducing the inspection effort is an important objective given the number of existing and new plants that need to be safeguarded. A useful tool in many safeguards approaches is the on-line monitoring of enrichment in process pipes. One requirement of such a monitor is a simple, reliable and precise passive measurement of the 186-keV line from 235U. The other information required is the amount of gas in the pipe, which can be obtained by a transmission or pressure measurement. We describe our research to develop such a passive measurement system. Unfortunately, a complication arises in the interpretation of the gamma measurements, from the contribution of uranium deposits on the wall of the pipe to the 186-keV peak. A multi-detector approach to address this complication is presented where two measurements, one with signal primarily from gas and one with signal primarily from deposits, are performed simultaneously with different detectors and geometries. This allows a correction to be made to the 186-keV peak for the contribution from the deposit. We present the design of the multi-detector system and the results of the experimental calibration of the proof-of-principle prototype built at LANL.

In this paper we report our progress toward the development of an advanced enrichment monitoring technology for safeguarding gas centrifuge enrichment plants. We compare the UF6 gas pipe attenuation and sensitivity to X-ray tube HV variations for two transmission energies: 22 keV and 25.5 keV. The first experimental enrichment results taken with a static UF6 gaseous source and X-ray tube based transmission source over a wide gas pressure range are presented.

We present a method of determining the wall thickness of a pipe in a Gas Centrifuge Enrichment Plant (GCEP) when an empty pipe measurement is not feasible. Our method uses an X-ray tube for transmission measurements and a lanthanum bromide (LaBr3) scintillation detector on the opposite side of the pipe. Two filters, molybdenum (K-edge 20.0keV) and palladium (K-edge 24.35keV) are used to transform the bremsstrahlung spectra produced by the X-ray tube into more useful, sharply peaked, spectra. The maximum energies of the peaks are determined by the K-edges of the filters. The attenuation properties of the uranium hexafluoride (UF6) gas allow us to determine wall thickness by looking at the ratio of selected regions of interest (ROIs) of the Mo and Pd transmitted spectra. While the attenuation factor at these two transmission energies in the UF6 gas is nearly equal, attenuation in the aluminum pipe wall at these two energies differs by a factor of about 60. This difference allows measurement of attenuation in the pipe independent of attenuation in the UF6 gas. Feasibility studies were performed using analytical calculations, and filter thicknesses were optimized. In order to experimentally validate our attenuation measurement method, a UF6 source with variable enrichment and pipe thickness was built. We describe the experimental procedure used to verify our previous calculations and present recent results.

Transmission measurements of radiation through process pipes provide a non-intrusive method of determining the amount of product present in the pipes. The product could be a liquid, a slurry, or a gas, which is the most challenging because of the low density. Traditionally, these techniques have used a radioactive source that has to be replaced periodically. We have developed a transmission technique based on an X-ray tube instead of a decaying source. A notch filter is used to provide a narrow transmission line, and a thin silicon transmission detector is used to monitor the X-ray tube output. The transmitted X-rays are measured with a high-throughput gamma spectrometer that consists of a NaI(Tl) detector and an MCA with precise dead time correction. This spectrometer provides stable transmission measurements with an accuracy of a fraction of a percent. The shielding and collimator are made of machinable tungsten for thermal mechanical stability, as well low-cost, low-weight tungsten powder in polymer castings. We describe two methods of measuring the pipe wall thickness without evacuating the pipe. Our particular application was for enrichment monitors for UF(6) in process pipes. Enrichment monitors that are independent of the plant data require two measurements: a transmission measurement to determine the total amount of uranium in the pipe and a measurement of the 186-keV gamma-ray line to determine the amount of (235)U present. The ratio of these values gives the enrichment. Previous designs used a decaying radioactive source such as (57)Co (122 keV, T(½)=272 days) or (109)Cd (22 keV, T(½)=1.2 years). A major effort was required to access and periodically replace these sources in operating plants. In this report, we describe the use of an X-ray tube, which eliminated the source problem, and other innovations. Then we present data from an enrichment monitor that incorporates these innovations.

Improving the quality of safeguards measurements at Gas Centrifuge Enrichment Plants (GCEPs), whilst reducing the inspection effort, is an important objective given the number of existing and new plants that need to be safeguarded. A useful tool in many safeguards approaches is the on-line monitoring of enrichment in process pipes. One aspect of this measurement is a simple, reliable and precise passive measurement of the 186-keV line from {sup 235}U. (The other information required is the amount of gas in the pipe. This can be obtained by transmission measurements or pressure measurements). In this paper we describe our research efforts towards such a passive measurement system. The system includes redundant measurements of the 186-keV line from the gas and separately from the wall deposits. The design also includes measures to reduce the effect of the potentially important background. Such an approach would practically eliminate false alarms and can maintain the operation of the system even with a hardware malfunction in one of the channels. The work involves Monte Carlo modeling and the construction of a proof-of-principle prototype. We will carry out experimental tests with UF{sub 6} gas in pipes with and without deposits in order to demonstrate the deposit correction.

A flux monitor diode is being explored as an option for measurement of the output of an X-ray tube that is used for active transmission measurements on a pipe containing UF6 gas. The measured flux can be used to correct for any instabilities in the X-ray tube or the high voltage power supply. For this measurement, we are using a silicon junction p–n photodiode, model AXUV100GX, developed by International Radiation Detectors, Inc. (IRD, Inc.). This diode has a silicon thickness of 104μ and a thin (3–7nm) silicon dioxide junction passivating, protective entrance window. These diodes have been extensively tested for radiation hardness in the UV range. However, we intend to operate mainly in the 10–40keV X-ray region. We are performing radiation hardness testing over this energy range, with the energy spectrum that would pass through the diode during normal operation. A long-term measurement was performed at a high flux, which simulated over 80 years of operation. No significant degradation was seen over this time. Fluctuations were found to be within the 0.1% operationally acceptable error range. After irradiation, an I–V characterization showed a temporary irradiation effect which decayed over time. This effect is small because we operate the diode without external bias.

In support of the development of an x-ray tube based source for transmission measurements of UF6 gas, we have developed a one-dimensional, spreadsheet-based model of the source. Starting with the spectrum produced by an x-ray tube we apply the linear attenuation coefficients for various notch filters, the aluminum pipe, and UF6 gas. This model allows calculation of the transmitted spectrum based on the type of filter, the thickness of the filter, the x-ray tube high voltage, the Al pipe thickness, and the UF6 gas pressure. The sensitivity of the magnitude of the transmission peak produced by the notch filter to any of these variables can be explored quickly and easily to narrow the choices for experimental measurements. To validate the spreadsheet based model, comparisons have been made to various experimental data.

We report our developments of the next generation of uranium enrichment monitoring technology for gas centrifuge enrichment plants (GCEPs). The main challenge presented by current technology is the need for periodic replacement of the short half-life (1.27 year) 109 Cd transmission source. We report on a transmission source at the 22.1 keV K-edge of ruthenium based on an X-ray tube with a “notch” filter. As part of the design we have modeled the NaI detector passive shielding with the MCNP code. Some preliminary results from experiments and modeling will be presented.

The recent General Dental Council (GDC) statements on ‘The practice of dentistry by non GDC registrants’ and the British Association of Oral and Maxillofacial Surgeons (BAOMS) position statement on the ‘The practice of Oral and Maxillofacial Surgery (OMFS) by non-GDC medical registrants’ are both timely and welcome (Newsletter from the President of the Br Assoc Oral Maxillofac Surg, April, 2008). These position statements will enable medical graduates to take up OMFS posts prior to attending dental school and go some way to addressing concerns we raised over the future recruitment of Maxillofacial Surgeons.1Gibbons A.J. Moss C.E. From where will future Maxillofacial Surgeons be recruited?.Br J Oral Maxillofac Surg. 2008; 46: 423Abstract Full Text Full Text PDF PubMed Scopus (2) Google ScholarThe new Medical Foundation Programme requires doctors to make decisions on their surgical careers much earlier than before and the opportunities for doctors to gain exposure to OMFS are very limited. Where possible we would encourage all OMFS Units that currently give Senior House Officer (SHO) posts to medical graduates to consider providing slots on their training programmes for Foundation Year 2 doctors. This would maintain the pool of potential medical graduate applicants for careers in OMFS. However, medical deans may be reluctant to fund this and dental deans are keen to transfer all SHO posts into the new Dental Foundation Programme.The excellent attendance by both medical and dental graduates pursuing a career in oral and maxillofacial surgery at this year's OMFS Junior Trainees Conference was extremely encouraging. Nevertheless, despite the GDC and BAOMS statements, the effect on OMFS recruitment of replacing SHOs with time limited Dental Foundation Programme trainees and the re-introduction of the speciality of Oral Surgery remains to be established. The recent General Dental Council (GDC) statements on ‘The practice of dentistry by non GDC registrants’ and the British Association of Oral and Maxillofacial Surgeons (BAOMS) position statement on the ‘The practice of Oral and Maxillofacial Surgery (OMFS) by non-GDC medical registrants’ are both timely and welcome (Newsletter from the President of the Br Assoc Oral Maxillofac Surg, April, 2008). These position statements will enable medical graduates to take up OMFS posts prior to attending dental school and go some way to addressing concerns we raised over the future recruitment of Maxillofacial Surgeons.1Gibbons A.J. Moss C.E. From where will future Maxillofacial Surgeons be recruited?.Br J Oral Maxillofac Surg. 2008; 46: 423Abstract Full Text Full Text PDF PubMed Scopus (2) Google Scholar The new Medical Foundation Programme requires doctors to make decisions on their surgical careers much earlier than before and the opportunities for doctors to gain exposure to OMFS are very limited. Where possible we would encourage all OMFS Units that currently give Senior House Officer (SHO) posts to medical graduates to consider providing slots on their training programmes for Foundation Year 2 doctors. This would maintain the pool of potential medical graduate applicants for careers in OMFS. However, medical deans may be reluctant to fund this and dental deans are keen to transfer all SHO posts into the new Dental Foundation Programme. The excellent attendance by both medical and dental graduates pursuing a career in oral and maxillofacial surgery at this year's OMFS Junior Trainees Conference was extremely encouraging. Nevertheless, despite the GDC and BAOMS statements, the effect on OMFS recruitment of replacing SHOs with time limited Dental Foundation Programme trainees and the re-introduction of the speciality of Oral Surgery remains to be established.

We report our progress toward development of new generation on-line enrichment monitoring technology for UF6 gas centrifuge plants based on a transmission source and a NaI spectrometer. We use an X-ray tube with transmission filters instead of a decaying isotopic transmission source to eliminate the costly replacement of this source. The UF6 gas density measurement is based on the energy dependency of the mass attenuation for two characteristic X-ray lines generated by the transmission filters. An analytical expression for the UF6 density is derived and criteria for the selection of transmission energies are discussed. Because of the differential method of measurement, the UF6 gas density does not depend on the intensity of the X-ray source. We describe a design of a sealed UF6 gas test stand for development testing and calibration of various on-line enrichment monitoring instruments. The sealed source is intended to replace a UF6 gaseous loop currently used for calibration.

Single neutron and gamma charge pulses were captured using a fast ADC. These data were used to populate simulated 5-µs active interrogation flash events. Analysis of these simulations revealed that long pulse tail accumulation during the flash leads to an unpredictable charge background for several microseconds after the flash. The existence of this changing background will make retrieving prompt neutron information immediately after the flash problematic. The tail accumulation is accentuated for every pulse in the flash, making the time just after the last pulse in the flash the worst possible time to measure prompt neutron data. Obtaining prompt neutron data during the flash—particularly near the beginning of the flash, where background tail accumulation is minimal—is feasible if shielding is used to control the count rates, but requires fast gamma-neutron pulse shape discrimination. A means of accentuating the pulse shape difference between neutron and gamma data is given, along with a means of compensating for the long pulse components in the dominant gamma pulses.

We have used a 150 MBq [sup 56]Co source to perform gamma-ray total absorption cross section measurements with very high precision. The use of [sup 56]Co allowed us to simultaneously measure the total cross section at 15 energies ranging from 0.847 MeV to 3.451 MeV. Seven sample materials were measured: Be, C, Cu, Ta, W, Pb, and U. In general, the measurement precision is better than 0.5% and in all cases better than 1.3%.

A Compton-backscatter capability has recently become available at the Duke University Free Electron Laser Laboratory. This capability allows one to produce high fluxes of tunable, nearly monoenergetic gamma rays. Using these gamma-ray beams, we have made high-precision (~0.5%) measurements of the gamma-ray total cross section at 3.45, 4, 5, 6, 8, 10, and 12 MeV. The nuclei measured were Be, C, Cu, Ta, W, Pb, and U