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The Whys and Wherefores of Measuring Drum
Activity Decommissioning, decontamination, and normal processing of nuclear waste generates quantities of radioactive material in the form of many different matrices stored in many different sized containers. These containers must be measured to assay their radioactive content before moving, shipping offsite, burying, or placing in a storage area. Within their wide range of spectroscopy systems and products, ORTEC offers two products of great utility in this area, which specifically provide simple and cost-effective solutions to these gamma-ray measurement problems. The first product is a software program ISOTOPIC which solves the problems associated with sample matrices and geometries to determine activities. ISOTOPIC is added to existing gamma-ray spectroscopy systems using the ORTEC GammaVision® analysis package1.The second product is the ORTEC ISO-CART mobile assay system. It provides a "turn-key," cart-mounted, easy-to-use solution. All necessary hardware, including detector, shielding, collimator, MCA, computer, Cart and software are included. The ISO-CART is supplied from the ORTEC factory ready to go to work. User training is available on the use of the ISOTOPIC software or the complete ISO-CART system. Intended Audience This application note is intended for those who already own or are contemplating deploying either of these products for the first time. It provides a simplified procedure and "helpful hints" on the practicalities of these measurements. It is applicable to users of both the ISO-CART system or with "home built" gamma-spectroscopy systems incorporating ISOTOPIC and GammaVision software. The hints refer to "typical situations" based on extensive experience in measuring actual samples of this type. Before describing specific steps required to make the measurement, some consideration is given to the choice of equipment which will result in good quality, reliable results from such a system. The note concludes with a summary section of "frequently asked questions" relating to the application of ISOTOPIC to measurement problems. Choice of Equipment The quality of the results can be strongly influenced by the choice of equipment, both in terms of hardware and of software. Even when samples are in difficult measurement situations, with high attenuation, and nonhomogeneity, a poor choice of equipment will emphasize the situation and make results imprecise and unreliable. Equipment should be chosen with care. HPGe DetectorA HPGe detector with an efficiency of at least 50% relative2 is needed for low-level waste items. This detector is needed in order to separate the many gamma-rays and to have the sensitivity for the required detection limits. Larger efficiency HPGe detectors have higher peak-to-Compton ratios than smaller ones. A detector with a high peak-to-Compton ratio will provide a higher peak to background ratio at lower energies, improving lower limits of detection (Ref. 1). If most of the gamma-rays of interest are below 1.0 MeV, a detector with a larger diameter (for the same relative efficiency) will be more sensitive (Ref. 2). Most often, p-type (GEM) detectors are used for the gamma-ray detection. These detectors are useful for the measurement of gamma-ray energies down to about 60 keV. Most gamma-rays below this energy are absorbed in the matrix or wall of the drum. Signal Processing Electronics
 Collimator Considerations
Usually, waste drums are stored in a central location to be measured. The radioactivity in these drums may increase the background radiation field in which an individual drum is to be measured. To minimize the effect of background radiation, use a collimator to shield the detector and limit the field-of-view to a single drum. The wall of the collimator should be at least 1.6 cm (5/8") thick lead or equivalent stopping power. If additional activity is present in the measurement area, the collimator should extend from 2.5 cm (1") behind the detector and 2.5 cm (1") or greater in front of the detector. See Figure 1. The collimator can be made of tungsten or steel instead of lead. Table 1 shows the popular collimator materials and their stopping power at selected energies. If the fluorescence x rays from the collimator material overlap other nuclide reference gamma-ray peaks and interfere with the analysis of the container nuclides, the collimator should be lined with cadmium, tin, or copper to adsorb these x rays. Because tungsten is so dense, it is the best material for fabricating collimators. However, when cost is taken into account, lead may be an attractive alternative. A practical collimator (for stopping power and weight) is 2 cm of lead with 2 mm of tin or cadmium and 1 mm of copper on the inside.
Calibration of Equipment and Validation of Method The HPGe detector system is calibrated for energy and efficiency (necessary for nuclide quantification) using a NIST-traceable (or equivalent) mixed nuclide point source. The ISOTOPIC software uses this point source calibration to model the efficiency for the item being measured. This calibration provides the relationship between measured count-rate and the actual activity (becquerels or microcuries). It should be noted that ANY system of this type will require at least an energy calibration with a point source. The advantage of the ISOTOPIC approach is its simplicity. The detector may be calibrated for energy and efficiency at the same time using the same point source. This can be an entirely automated procedure because of the capabilities of GammaVision. Internally, ISOTOPIC contains all of the data needed to extrapolate from the point source to the actual measurement situation. (The procedure is effective for any HPGe detector. It is not necessary to employ special "vendor precalibrated" detectors.) The calibration source should contain gamma-rays that span the energy region of interest. For most applications, suitable standards are easily obtained. The overall system (including hardware and analysis software) should be validated by preparing standards containing: (1) activity traceable to a national standard and (2) matrices (either simulated or real) similar to that being analyzed. These simulated samples should be analyzed as if real and results compared with the known activity values of the standards. The overall accuracy of measurements can be established by this means.Data Taking Below is a step by step procedure covering the process of data taking. 1. Record the backgroundRecord a background measurement. If the background does not change due to movement of radioactivity nearby, one background measurement should suffice for the entire measurement period. Take a background measurement with a "blank" container in front of the detector. (A blank container contains material that is not radioactive but has similar weight to a typical sample.) If no container or an empty container is used to measure the background, then the recorded background will tend to be artificially high. The sample material and container reduce the background by absorbing many of the background gamma-rays. The thickness of the collimator may need to be increased if there are high energy gamma-rays from a nearby source that will interfere with the analysis. Some nuclear facilities use a portable shield behind the detector when backgrounds change in that direction. 2. Record the sample and container weights
If it is possible or likely that the container includes "hot spots," scan it with a 3¨ x 3¨ or larger hand-held NaI detector to locate them. If this procedure indicates that a "hot spot" is indeed present such that ~75% of the activity is there, then proceed as in step 6 below. If no hot spot was located, proceed with step 4. 4. Position your detector
Start out by counting the item for 30 minutes (typical). Items that indicate high activity may be counted as quickly as 5 minutes. Items with high density and very low activity may require additional counting time. Shorter counting times are possible if a higher efficiency detector (70% to 100% relative) is used. The ORTEC GammaVision software has a useful "count to MDA" feature. "Count to MDA" minimizes the count time necessary to achieve the desired detection limit for a specified nuclide. If the material in the drum is nonhomogeneous, then use a turntable to rotate it. Rotating the drum helps simulate an average matrix attenuation. Most solid items that need measurements are nonhomogeneous. The sample should be rotated several times during the counting period. Inexpensive turntables for 208-liter drums are available from ORTEC. Containers that are smaller than a 208-liter drum can be positioned closer to the detector. If the entire drum is assumed to contain radioactivity, then the entire container should be within the detector field-of-view. 6. Special considerations for hot spots
Analysis Before we describe the simple step by step analysis procedure, we will explain how GammaVision and ISOTOPIC interact: GammaVision is the ORTEC peak-fitting software for general purpose gamma-ray analysis of spectra taken with a HPGe detector. GammaVision is packed full of analysis features and options which provide a great deal of flexibility. ISOTOPIC is a "post processor" to GammaVision. GammaVision "hands over" to ISOTOPIC a list of peak-by-peak activities computed for each isotope in turn, uncorrected for geometry or absorption, and determined using a calibration which the user makes with a simple TRACEABLE multi-nuclide point source. ISOTOPIC uses the peak results from GammaVision along the source-to-detector distance employed for calibration. Sample physical data (size, distance, weight, material) are entered into ISOTOPIC via easy to use data entry screens. ISOTOPIC then performs the necessary attenuation corrections to the peak-by-peak activities and geometry corrections to the reference activity.1. Setting Up the GammaVision Analysis Library
Calibrate GammaVision in the usual way, with a point source at a "reasonable distance" (several detector diameters are recommended, say 25 cm). Positioning the source closer will introduce a larger percentage error due to uncertainty in the distance to the active volume of the detector. Use the (patented) autocalibrate for the energy and FWHM calibration. Delete any peaks returned by the autocalibration that obviously lie well away from the FWHM curve. A good FWHM calibration is critical to good peak fitting. 3. GammaVision Analysis Option Choices
 4. Entering Configuration Description
Enter a "best guess" for matrix characteristics as a starting point. The sample weight is very important. If it is not possible to weigh the sample then assume a nominal density. See Figure 3. As an alternative starting point, select a plausible matrix material from the drop down list.
Adjust the correction factor parameters on the isotope plot screen
iteratively until all the gamma-ray peaks of the specified nuclide have the same (or as
close as possible to the same)
5. Display the results before reporting 6. REPORT Stage
7. Review of unidentified peaks
Prediction of System Performance Estimation of likely minimum detectable activity (MDA) for a new type of sample or a new detector is possible, even before you have either one.It is very easy to model new sample configurations and to
determine the MDA to be expected for a given range of nuclides. What is needed is the
point source energy calibration and a background at least similar to what would be
measured by that detector in that facility. Data from previous measurements or
measurements in other facilities can often be adequate to make these estimates. It is not
necessary to have an actual sample, but the matrix must be reasonably known as well as the
list of nuclides of interest. The background is analyzed as if it is a sample. The MDA
activity obtained from GammaVision is then corrected for geometry, matrix and container
attenuation by ISOTOPIC. If counting times will be employed which are different to those
employed for the background analyzed, (which is usually the case) the following equation
can be used to correct the MDA to the new counting time: MDArt = MDA activity at the new counting time,
ISOTOPIC Measurement Uncertainty For measured items containing dense material, measurement uncertainties of 30% can be expected. It is difficult to estimate uncertainty accurately. The major sources of error are usually in the determination of the matrix correction factor. Usually, the higher the matrix density the larger the uncertainty in the matrix correction. When using ISOTOPIC, this uncertainty can be reduced by counting longer or using a more efficient detector so that weaker gamma-rays from a nuclide can be used to correct for the matrix. There will be smaller random errors due to uncertainty in positioning the item in front of the detector and counting statistics. If the item is not rotated during the measurements and the detector is close to the container, the uncertainty in the position of the activity (called geometry error) can become significant. ("Close," in this context is less than one drum diameter.) This error can be reduced by positioning the detector further away and rotating the container. In the cylinder configuration, there is a user-set option to identify when an item is rotated. The data is analyzed in the same way if the option is selected or not checked. However, the geometry error and the overall uncertainty will be reduced if the item is rotated during analysis. Other Approaches to this Measurement Problem Monte Carlo calculations, which are used in competing systems to compute activities, must assume a well-characterized matrix. This is seldom the case in the measurement of nuclear waste, which totally negates the apparent sophistication of the approach. In contrast, program ISOTOPIC can be used for a wide variety of container configurations and matrices; analyses are presented in units of activity and grams (necessary for accountable nuclides) for all nuclides. Conclusion ISOTOPIC and the ISO-CART mobile assay system provide a straightforward, good solution in many application situations involving ill-defined matrices and geometries. This application note has provided "getting started" information and answers to many of the more common questions. If you have further questions do not hesitate to ask us. Please send your query to rich.hagenauer@ametek.com, cc info@ortec-online.com or call 800-251-9750 for technical support. Frequently Asked Questions Do I need process knowledge of the nuclear facility to
analyze samples? Should the container exceed the field-of-view of the
collimator so that I am looking at part of it, or should I be able to see the whole
container? May I combine or average the results obtained by viewing the
container from different sides as an alternative to rotating it?
What if I collect data without a collimator?
If one peak is an "outlier" on the plot what
should I do? On the plot screen sometimes one nuclide will have a
negative slope and one nuclide will have a positive slope. Which nuclide do I adjust to
fine tune the corrections?
When using a daughter nuclide to quantify the parent,
which half-life do I use in the gamma-ray library?
How do I quantify the daughter activity if I place
references to their gamma-rays in the parent library?
I suspect that there are going to be inhomogeneties in the
drum and that rotating it will reduce the systematic error, but how fast should the drum
rotate? ISOTOPIC is easy and economical to use. How does it
compare in drum analysis with other techniques such as segmented gamma scanning (SGS) and
Tomographic Gamma Scanning (TGS)?
References 1R. M. Keyser, T. R. Twomey, and S. Wagner "The Benefits of Using Super-Large Germanium Gamma-Ray Detectors for the Quantitative Determination of Environmental Radionuclides," Radioact. & Radiochem,. Vol. 1, No. 2, pp. 47-55 (Spring 1990).2R. M. Keyser, T. R. Twomey and P. Sangsingkeow "Advances in HPGe Detectors for Real World Applications," The Journal of Radioanalytical and Nuclear Chemistry, Vol. 224, No. 3, (2000) Notes 1The ISOTOPIC Program runs in conjunction with ORTEC GammaVision-32 for HPGe detector gamma-ray analysis. GammaVision controls the acquisition, saves the spectrum and does the peak analysis. ISOTOPIC then "post processes" the results further to correct for matrix, container, collimator and geometry effects using operator-supplied information about dimensions, weights, distances and materials, along with internal databases of geometry and matrix correction information. 2Relative efficiency is defined relative to a 3" by 3" NaI detector, according to ANSI/IEEE 325-1996.Table 1. Stopping Power for Common Materials Used for Collimators.
Table 2. Detector sensitivities for a 208-liter Drum with a combustible matrix.
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