The U.S Environmental Protection Agency (USEPA) Airborne Spectral Photometric Environmental Collection Technology (ASPECT) program is designed in part to provide first responders with radiological mapping of potentially hazardous locations. This program utilizes an aircraft fitted with a gamma-ray spectrometer capable of remote detection of radioisotopes. The challenges present in detecting a radioisotope signal remotely are strongly tied to the signal-to-noise ratio of the collected gamma-ray spectra and the specific signal processing and pattern recognition methods used in the data analysis. Depending on the distance from the detector to the radioisotope source, Compton scattering can significantly reduce the analyte signal, and weakened signals pose a significant challenge when attempting to design an effective classifier for detecting radioisotopes of interest. In this research, a basic methodology has been developed for the detection of cesium-137 (¹³⁷Cs) and cobalt-60(⁶⁰Co) utilizing only laboratory collected spectra and backgrounds from the field collected only once. The presented classifier methodology has been proven to provide a fundamental structure for which more advanced algorithms can be developed. Furthermore, this methodology has demonstrated the ability to strongly associate a level of confidence in a detection which allows for intelligent decision making. From this basic methodology, more sensitive and selective algorithms can be designed. The Compton effect has previously been problematic in the development of gamma-ray pattern recognition systems. In this research, a background suppression methodology utilizing linear regression has been implemented to enhance the basic pattern recognition methodology. This background correction strategy has led to significant improvements in the remote detection of radioisotopes and enables the classification of more complex radioisotopes such as europium-152 (¹⁵²Eu). This research demonstrates not only the capabilities of the pattern recognition methodology, but also the flexibility of the procedure. In the remote detection of radioisotopes, false detections and low sensitivity are the key challenges when developing a classifier. While the background corrected methodology was shown to greatly enhance the classifier performance, further advances can be made into the methodology through the use of committee classifiers. Since every classifier utilizes a different dataspace, a standardization procedure has been developed from which the classifier result can be averaged and generate a committee classifier result. This classifier methodology has been demonstrated to further improve the radioisotope classifier performance without sacrificing either sensitivity or selectivity. While the development of targeted radioisotope classifiers is invaluable to the first responder, developing a general gamma-ray anomaly classifier can handle those radioisotopes that have no dedicated classifier. From this objective, a unique anomaly classifier based on the Compton region of the gamma-ray spectrum has been developed and demonstrated to operate in the field. Utilizing all of the strategies and techniques developed for single radioisotope classifiers, the anomaly classifier has been proven to detect background and natural radioactive sources as well as controlled and man-made radioisotope targets. From the developed remote detection classifiers via pattern recognition techniques, robust radiation detection classifiers have been developed. This pattern recognition methodology eschews the need for extensive field data collection for training the algorithms, while also removing the need for on-site calibrations. When used in conjunction with one another, the dedicated radioisotope and anomaly classifiers provide a thorough and rugged remote detection capability to the first responder. The presented methodology also demonstrates that any radioisotope classifier can be generated, implying that this method can be used for the detection of any radioisotope in the field.

Nuclear techniques such as perturbed angular correlation (PAC) sample the hyperfine interactions of a large number of probe atoms in specific crystallographic sites. Real crystals contain static defects producing a distribution of electric field gradients (EFGs) that add to the ideal EFG of the crystal at any given probe site. Also, dynamic defects like moving vacancies and interstitial atoms can be present in the crystal and contribute to the distribution of EFGs. The distribution of EFGs leads to line broadening and a change in the observed asymmetry parameter η since the total EFG no longer has the symmetry of the perfect crystal. When both defects are present in a material, obtaining quantitative information from the analysis of PAC spectra is usually very difficult since great care has to be taken to ensure that the source of line broadening is identified correctly. In order to relate the relationship between the static line broadening and changes in the asymmetry parameter η, a uniform random distribution of point charges was used to simulate the static defect EFG. PAC spectra collected on cubic niobium metal, cubic stabilized zirconia and Nb-doped tetragonal zirconia were fitted with this model. Although the quality of the fits is good, more work is needed to clarify the relationship between the new model parameters and the line broadening and asymmetry parameter derived from conventional model fits. The PAC spectra of Nb-doped tetragonal zirconia were fitted with a conventional static model to establish a reliable relationship between line broadening and the asymmetry parameter when only static defects are present in a sample. To account for effects of dynamic defects, a four state stochastic model for vacancy motion was adapted in order to include the line broadening and changes in the asymmetry produced by static defects. As a result, the activation energies corresponding to the rates at which a oxygen vacancy is trapped by, detraps from, and hops among equivalent sites about a PAC probe atom were calculated. The values that were found are physically reasonable, indicating that the dynamics of an oxygen vacancy around a PAC probe atom are satisfactorily described. / Graduation date: 2003

A system intended to detect, classify, and track radioactive sources in moving vehicle traffic is under development at Lawrence Livermore National Laboratory (LLNL). This system will fuse information from a network of sensor suites to provide real time tracking of the location of vehicles emitting gamma and/or neutron radiation. This work examined aspects of the source terms of interest and applicable gamma detection technologies for passive detection of emitted gamma radiation. The severe restriction placed on the length of count due to motion of the source is presented. Legitimate gamma sources expected in traffic are discussed. The requirement to accurately classify and discriminate against these "nuisance" sources and cost restraints dictate the choice of NaI(Tl) detectors for this application. The development of a capability to automatically analyze short duration, low signal-to-noise NaI(Tl) spectra collected from vehicles passing a large, stationary detector is reported. The analysis must reliably discriminate between sources commonly transported in motor vehicles and alert on the presence of sources of interest. A library of NaI(Tl) spectra for 33 gamma emitting sources was generated with MCNP4B Monte Carlo modeling. These simulated spectra were used as parent distributions, from which multiple realizations of short duration spectra were sampled. Principal component analysis (PCA) of this data set yielded eigenvectors that enable the conversion of unknown spectra into principal component space (PCS). An algorithm using least squares fitting of the positions of library sources in PCS as basis functions, capable of identifying library nuclides in unidentified spectra, is reported. Analysis results for experimental spectra are compared against those achieved using simulated spectra. A valuable characteristic of this method is its ability to identify sources despite unknown shielding geometries. The successful identification of radionuclides and false identification rates found were excellent for the signal levels involved. For many of the sources, identification performance against experimental spectra was somewhat poorer than found using simulated spectra. The results demonstrate that the PCA-based algorithm is capable of high success rates in identifying sources in short duration, low signal-to-noise NaI(Tl) spectra. / Graduation date: 2001

In case of a release of radioactive nuclides into the environment it is necessary to  have reliable methods to estimate the potential effect on people and the ecosystem. In this context the total activity deposited, the elemental composition and the depth distribution are of importance. An efficient in situ method to estimate the average contamination over larger areas using high purity germanium (HPGe) detectors has been developed. The method combines simulation of photon transport with measured detector properties. The total activity of 137Cs determined from gamma-ray spectra recorded in situ are compared to results from soil sampling. Another in situ method has been developed to determine the depth distribution of a radionuclide contamination, using an array of small detectors inserted into the ground, as an alternative to the standard procedure of soil sampling. The possibility to use cadmium telluride (CdTe) detectors as well as lanthanum bromide (LaBr3) detectors has been investigated. As a demonstration of the developed method the small-scale variation of the activity distributions in an area covering 350 m2 have been measured and is compared to results obtained by other methods. / At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Submitted.

A delayed-coincidence spectrometer system was used to measure nuclear lifetimes in the picosecond region by the centroid-shift method. Timing signals were derived from two plastic scintillators; each plastic scintillator was located within a specially constructed NaI(Tl) detector. Each NaI(Tl) detector was used to detect the gamma ray which Compton scattered from an associated plastic scintillator. Energy identification was achieved by summing the signals from the plastic and the NaI(Tl) detectors to give full energy gamma-ray photopeaks. To test the effectiveness of the system, the half-lives of the first excited state of 134Ba and the first and second excited states of 144Nd were measured. The half life of the first excited state of 134Ba was measured as 4.4 - 1.3 psec; this result is in agreement with a previously reported value of 5.0 t 1.5 psec. The half-life for the first excited state of 144Nd was determined to be 1.5 ± 1.0 psec while the half-life for the second excited. state of 144Nd was determined to be 5.1 ± 0.5 psec. The half-life value for the second excited state of 144Nd is in disagreement with previously reported values of 21 ± 2 psec and 100 ± 30 psec which were also determined by delayed-coincidence methods. Although the result of 1.5 ± 1.0 psec for the first excited state lifetime differed from a previous result of 3.4 * 0.1 psec obtained by Coulomb excitation, this difference was due to the inability of the present spectrometer system to resolve the 618- and 696-keV gamma rays of 144Nd. It is concluded that a lower lifetime limit measurable with the system described in this study is about one picosecond, provided that the gamma rays can be resolved in the sum energy spectrum of the plastic and NaI(T1) detectors.Ball State UniversityMuncie, IN 47306

A delayed-coincidence timing spectrometer system was used to experimentally measure the lifetime of the 344-keV level of 152 64Gd by means of the self comparison technique. A major objective of the study was to determine the effectiveness of a NaI(Tl)-plastic detector assembly for energy spectrum reconstruction when measuring the lifetime of a level in a complex nuclear decay scheme. The experimentally measured mean life was 38 ± 5 psec. This result does not agree with two previously reported values of 76 psec. It was concluded that gamma-ray energy misidentification was primarily responsible for the disagreement. Correction factors would have to be applied to the experimental data is such a system is to be effective in a nuclear decay scheme study which involves many gamma rays.Ball State UniversityMuncie, IN 47306

No description available.

After a Radiological Dispersal Device (RDD) event, there will not be time to transport people to a whole-body-counter (WBC), since it is a specialized instrument. This work will assess the feasibility of using handheld spectrometers for measuring the radioactivity that may have been inhaled by a victim as a consequence of an RDD event. Measurements were made with a handheld isotope identifier using a slab phantom and several radioactive point sources. A Lawrence Livermore National Laboratory (LLNL) Realistic Torso Phantom and a set of phantoms based on Medical Internal Radiation Dose (MIRD) reports were also used in this work. These phantoms include the human skeleton and have tissue-equivalent organs. Computational models were developed of all of the phantoms using the Monte Carlo Transport code MCNP. After validation of the computer model, MCNP runs were conducted using other sources that are likely to be used in a RDD. Calculations were then done to find the Minimum Detectable Activity (MDA) of all sources used. The Minimum Detectable Dose (MDD) was then calculated for the MIRD phantoms at various times after inhalation.

Thesis--M. Phil., University of Hong Kong, 1983.

Thesis (Ph.D)--Mechanical Engineering, Georgia Institute of Technology, 2010. / Committee Chair: Hertel, Nolan; Committee Member: Kulp, William David; Committee Member: Lee, Eva; Committee Member: Pagh, Richard; Committee Member: Petrovic, Bojan; Committee Member: Rahnema, Farzad; Committee Member: Smith, Eric; Committee Member: Wang, Chris. Part of the SMARTech Electronic Thesis and Dissertation Collection.