BOMAB phantom variability: do small dimensional changes matter?

The Human Monitoring Laboratory (HML) has had a number of BOMAB phantoms built over the years. Upon characterization, it has been found that the dimensions of the phantoms are always slightly different. This study has looked at the effect of these small variances in dimensions of the phantoms and compared the results to what is required in the industry standard using Monte Carlo simulations for three counting geometries: the HML's scanning detector whole body counter, the StandFast whole body counter, and the W-chair whole body counter. It has been found that the effect of these small variations on the performance of these phantoms is very minor (<5%). It is reassuring to find that small variations in manufacturing, even if individual sections are non-compliant, have such a minor effect on performance as to be considered a negligible effect for any counting system's geometry.

Effect of an aerosol deposition pattern in the lung on the counting efficiency of a large area germanium detector array.

The Human Monitoring Laboratory has extended the use of sliced lungs containing planar sources to simulate heterogeneous radionuclide deposition patterns. This work examined two deposition patterns and their effect on the counting efficiency of low-energy photons. The results have shown that heterogenous distributions can be difficult to detect in some cases and can still lead to large uncertainties (up to a factor of 2.5) in the activity estimate, especially at low photon energies. At higher energies ( approximately 60 keV), the effect of the heterogeneous distribution is greatly reduced and errors in the activity estimate reduced to approximately 25%. The presence of a heterogenous distribution can be detected by comparing the ratio of the individual detector counts with the expected values obtained from measuring multiple lungs sets that contained a homogeneous distribution. The distributions tested in this paper were detectable (at 2sigma) as heterogeneous by two of the four detectors in the counting array.

Can the HML's sliced BOMAB phantom be used in any whole body counter with a reduced number of sources?

The sliced Bottle Manikin Absorber (BOMAB) phantom was originally proposed as an alternative to a commercially available phantom, but it suffers from the disadvantage of containing over 160 sources that need to be manufactured; however, it was found that the number of slices could be reduced substantially and that two slices in the sliced phantom gave the same performance characteristics over a wide energy range as a conventional BOMAB phantom for a particular counting system. This work explores the adaptability of this phantom to another counting geometry. The response of the Human Monitoring Laboratory's whole-body counter measuring this phantom with a decreasing number of planar sources has been modelled using MCNP5 over a wide energy range (122-2754 keV). It was found that the best agreement was obtained when the phantom contained 10 sources, 1 in the mid point of each section. As this is a different result from a previous finding, any other counting geometry will have to be assessed to determine the optimum loading if the sliced phantom is to be used. Also, it is clear that this type of phantom cannot be used for an intercomparison that will encounter different counting geometries, unless it contains a full loading of sources.

Fundamental uncertainties in lung counting.

The HML has investigated the effect the uncertainty introduced into an activity estimate from a lung count due to 1) replicate counts and 2) lung set variability. Replicate counts in the HML seem to only be affected by random statistics as the uncertainty can be predicted by Monte Carlo simulations. These findings from the lung set variability experiments suggest that a lung set has an unquantified uncertainty on its activity that adds a component to the uncertainty on the counting efficiency, and ultimately the activity estimate, as they can differ by as much as 30% at 17.5 keV or about 13% at 185.7 keV, when one is expecting only a 3% difference.

The Human Monitoring Laboratory's whole body counter: monitoring the liquid nitrogen level as a quality control tool.

The Human Monitoring Laboratory (HML) has developed a method to measure the liquid nitrogen boil-off rate from the whole body counter's single dewar as a function of time. The device consists of a commercially available instrument that was modified to fit the HML's whole body counter's dewar; unfortunately, the modification was not perfect requiring an alternative approach to using the maximum fill value. The boil-off rate is now measured by taking two measurements and calculating the loss rate. Resulting boil-off rates are plotted on a control chart so that long-term trends can be easily assessed.

Evaluation of two commercially available portal monitors for emergency response.

The Human Monitoring Laboratory has compared two types (the P3 and the MiniSentry) of portal monitors that can be field deployed in response to an emergency. They can be used to screen persons for internal or external radioactive contamination by fission activation products (neither unit is capable of detecting alpha or beta radiation, and the amount of material required to alarm the monitors is unacceptably high for low energy x rays or gamma rays) following an incident involving the release of radioactive material (accidental or intentional). It was found that the P3 benefits from simplicity but requires slightly more activity to alarm than the MiniSentry, although for emergency response, the amount of activity that can be detected is far below a level where significant health effects will occur. The MiniSentry was found to have more capability than the P3, but these benefits also bring their own disadvantages. It was also found that the MiniSentry would be difficult to deploy in an outdoor setting whereas the P3 is well designed for a field setting. Despite the differences found, the HML has concluded that both have a distinct place in emergency monitoring. In the future, the HML plans to have both instruments ready for field deployment.

Exercise Maritime Response (EXMR): lessons learned for population monitoring and communications.

Exercise Maritime Response was the third in a series of four emergency response exercises sponsored by the Chemical, Biological, Radiological and Nuclear Research and Technology Initiative. It was designed to test the Canadian Federal, Provincial and Municipal response to a terrorist attack using radioactive materials. The complexity of this exercise had been increased over previous exercises to now include simulated contaminated members of the public. This paper summarizes the experiences, and the lessons learned, of the Health Canada (HC) team. The largest issues identified by the HC team were: crowd control, insufficiency of staff to deal with surge capacity, and communications. The exercise did prove that the population monitoring equipment worked well and that small amounts of radioactivity were easily identified and quantified to within 20% of their true value.

Large area germanium detector arrays for lung counting: what is the optimum number of detectors?

Using the Lawrence Livermore National Laboratory (LLNL) torso phantom to calibrate a lung counting system can lead to the conclusion that three large area (i.e. >70 mm diameter) Ge detectors will outperform a four-detector array and provide a lower MDA as a four-detector array of large area Ge detectors covers a significant portion of inactive tissue (i.e. non-lung tissue). The lungs of the LLNL phantom, which are approximately 10 cm too short compared with real lungs, also suggests that a two-detector array could be used under limited circumstances. When tested with modified lungs that are more human-like, it was found that the four-detector array showed the best counting efficiency and the lowest MDA. Fortunately, these findings indicate that, although the LLNL phantom's lungs are too short, there is no adverse impact on the calibration of a lung counter.

Are Portable Personnel Portal Monitors too sensitive?

The Human Monitoring Laboratory has field tested its Portable Personnel Portal (P3) Monitors using sources up to 1700 MBq (47 mCi) to determine the alarm distance as a function of activity. The results show that the P3 monitors are highly sensitive, so much so that siting will be a problem for multiple units if multiple alarms are to be avoided. Building materials will shield the monitors allowing units to be placed closer together than in the open where there is no shielding, but windows and doors reduce shielding and complicate the siting of multiple units. In either situation, careful prior thought should be given to siting the monitors and the logistics of the crowd control techniques.

The sliced bomab phantom: a new variant for intercomparison.

Previously, this laboratory conceptualized a new phantom for calibration or performance testing of whole body counters using Monte Carlo simulations. This paper describes the physical reality that was created from the Monte Carlo design project and compares its counting efficiency to that of a conventional BOMAB phantom using two whole body counters. In one counter (NaI based) the agreement between the two phantoms was +/-8% and in the second counter (Ge based) the agreement was +/-5% at all the energies measured (126 keV, 661 keV, 1172 keV, 1330 keV). The advantage of the sliced phantom is that the sources are solid, sealed, and cannot leak activity thereby simplifying packing for shipment if the phantom is classified as a Dangerous Good. The new phantom is, therefore, ideal for uses that involve shipment, such as an intercomparison exercise. The phantom is also re-usable as the sources can be changed.

Sensitivity of portable personnel portal monitors: potential problems when dealing with contaminated persons.

Health physicists are usually concerned with small amounts of radioactivity and strive to develop techniques to measure them; however, following a terrorist attack involving radioactive materials the converse might be the case, and exposed persons may be heavily contaminated. The Human Monitoring Laboratory (HML) has field tested its Portable Personnel Portal (P3) monitors using sources up to 1,700 MBq (47 mCi) to determine the alarm distance as a function of activity. The results show that the P3 monitors are highly sensitive, so much so that siting will be a problem for multiple units if multiple alarms are to be avoided. Building materials will shield the monitors allowing units to be placed closer together than in the open where there is no shielding, but windows and doors reduce shielding and complicate the siting of multiple units. In either situation, careful prior thought should be given to siting the monitors and the logistics of crowd control techniques.

The Human Monitoring Laboratory's new lung counter: calibration and comparison with the previous system and the Cameco Corporation's lung counter.

The Human Monitoring Laboratory has replaced its lung counting system with four large area (85 mm x 30 mm) HPGe detectors, electronics, and software. The system has been calibrated with the same lung set and phantom that was used to calibrate the Human Monitoring Laboratory's previous lung counting system and the Cameco Corporation's mobile lung counter. The performance characteristics (efficiency and sensitivity) of all three systems are compared, with the Human Monitoring Laboratory's new system being more sensitive than the other systems by factor of 1.3. The large area detectors highlight the design deficiency of the Lawrence Livermore National Laboratory's torso phantom, namely short lungs, as the lower two detectors are over inactive tissue (approximately 40%). As a result, both a two-detector and a three-detector array are actually more sensitive than a four-detector array in certain circumstances. This is, however, an unrealistic finding as human lungs are much longer (approximately 10 cm) than the Lawrence Livermore National Laboratory's phantom's lungs. The dosimetric implications of the new system's minimum detectable activities are put into perspective using (57)Co, (235)U, (238)U, (239)Pu, (241)Am, and natural uranium as example radionuclides.

Performance characteristics of a commercially available whole body counting phantom measured experimentally and using Monte Carlo simulations.

The performance characteristics of a commercially available whole body counting phantom have been examined experimentally and using Monte Carlo simulations. The counting efficiency as a function of photon energy (126 keV to 2,754 keV) obtained using this phantom has been compared with those obtained using a real or virtual BOMAB phantom in a scanning detector and a FastScan whole body counter. The results suggest that the commercially available whole body counting phantom is equivalent to a BOMAB phantom under certain circumstances but that under other measurement conditions it may be necessary to develop correction factors.

The HML's new field deployable, high-resolution whole body counter.

The Human Monitoring Laboratory has found an alternate use for a hyperpure germanium field deployable instrument that was originally designed to be used in a search and identify mode for contraband radioactive material. The instrument, the Ortec Detective, becomes a fully functional spectroscopy system when connected to a laptop computer. In this configuration it can be used as a high-resolution portable whole body counter. This work has determined that the instrument has adequate sensitivity for emergency response with respect to fission and activation products, but not actinides. The use of Monte Carlo simulations has allowed the HML to calibrate the instrument, partially optimize the counting geometry, and develop a calibration curve that is a function of photon energy and a person's size. Similarly for thyroid counting, a function has been found that fits counting efficiency to a person's height. The MDA's are a few kilo Becquerels for fission and activation products for a 5-min count in an unshielded environment using a male subject.

Evaluation of the new graded Z liner in the Human Monitoring Laboratory's lung counter.

The Human Monitoring Laboratory has recently completed the graded Z shielding in its lung counting chamber. The addition of a layer of tin and a layer of copper over the existing layer of lead that lined the thick steel walls has improved the background, as confirmed by long counts before and after the shielding was added. The graded Z liner was successful in reducing the general background in the lower energy regions by an average of 16% over the eight regions studied in the range 15-200 keV, over previous values.

Counting 241Am in the BfS human skull phantom on contact-evaluation in the human monitoring laboratory.

Skull counting can be used to assess the activity of radionuclides internally deposited in the bone. The Human Monitoring Laboratory (HML) at Health Canada conducted the measurement of 241Am in the BfS (Bundesamt für Strahlenschuts) skull phantom on contact with the skull for various positions. By placing the detector in contact, the HML can improve the counting efficiency by over 20% compared to placing the detector 1 cm above the surface of the skull. Among all the positions tested, the forehead position is the preferred counting geometry due to the design of HML's counting facility and the comfort it would provide to the individual being counted, although this counting position did not offer the highest counting efficiency for the gamma rays (either the 59.5 keV or the 26.3 keV) emitted by 241Am.

Managing children during a radiological or nuclear emergency--Canadian perspectives.

Children have a higher chance of being contaminated by radioactive materials during a radiological or nuclear (RN) emergency. They are more sensitive to radiation health effects and suffer more significant psychosocial impacts than adults during emergency response. This paper presents a summary of recommendations on effective management of children during an RN emergency. These recommendations intend to be additional considerations for existing RN response protocols and procedures implemented at local, state/provincial, or national level.

Summing coincidence errors using 152Eu lungs to calibrate a lung-counting system: are they significant?

The use of a lung phantom containing 152Eu/241 Am activity can provide a sufficient number of energy lines to generate an efficiency calibration for the in vivo measurements of radioactive materials in the lungs. However, due to the number of energy lines associated with 152Eu, coincidence summing occurs and can present a problem when using such a phantom for calibrating lung-counting systems. A Summing Peak Effect Study was conducted at three laboratories to determine the effect of using an efficiency calibration based on a 152Eu/241 Am lung phantom. The measurement data at all three laboratories showed the presence of sum peaks. While one of the laboratories found only small biases (< 5%) when using the 152Eu/241 Am calibration, the other facilities noted up to 30% positive bias in the 140 keV to 190 keV energy range that prevents the use of the 152Eu/241 Am lung phantom for routine calibrations. Although manufactured by different vendors, the three facilities use similar types of germanium detectors (38 cm2 by 25 mm thick or 38 cm2 by 30 mm thick) for counting. These results underscore the need to evaluate the coincidence summing effect, which appear system dependent, when using a nuclide such as 152Eu for the calibration of low-energy lung counting systems and highlight the problem of using a general calibration curve in place of specific nuclide calibration factors.

The use of autoradiography for investigating the distribution of radioactivity in lung counter calibration sources.

This paper shows that autoradiography is a useful technique for investigating radioactivity distributions in lung phantoms and planar sources. It was applied to a sliced lung phantom that had activity homogeneously distributed throughout the tissue substitute material and to laminated planar sources in an attempt to answer three questions: 1) Was the activity distribution the same in each slice of the sliced homogeneous lung set? 2) Was the activity distribution the same for each of the laminated planar sources? and 3) Were the activity distributions the same between slices and planar sources? The activity distribution, including identification of some locations of elevated activity in the sliced homogeneous lungs, was easily obtained using autoradiography. This study demonstrates that neither the sliced homogeneous lung sets nor the laminated planar sources had a homogeneous distribution of radioactivity, as had been previously thought.

Lung counting: summing techniques to reduce the MDA.

The new dose limits recently adopted in Canada (and elsewhere in the world) have made it more difficult to detect some radionuclides by in vivo counting at the average dose limit of 20 mSv. This is particularly true for natural uranium. Two techniques have been developed by the Human Monitoring Laboratory to reduce the Minimum Detectable Activity (MDA) for the lung counting of this nuclide. The first technique, developed in collaboration with Cameco, is to either sum sequential counts of an individual or to sum spectra of a group of workers similarly occupationally exposed. This technique offers a reduction in the MDA of up to a factor of three. The second technique, developed in collaboration with CNEN, involves the summing of photopeaks within an individual spectrum and offers a reduction in the MDA of up to a factor of two.