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A gamma-ray-based, active (A) and passive (P) computed tomography (CT) technology has been developed that locates, identifies and quantifies gamma-ray emitting isotopes, transuranic (TRU) and others, in nuclear waste drums. ACT uses a collimated external source and a HPGe detector to measure selected mono-energetic gamma-rays that are attenuated by waste-drum contents; a separate PCT measurement uses the HPGe detector to record the spectra of gamma-rays emitted from within a drum. The ACT attenuation images and the PCT emission spectra are coupled to quantitatively assay drum contents for ∼0.1–200 g of TRU isotopes. Calibration requires a single measurement of a known radioactive standard; construction of waste-drum surrogates is not required. Fixed and mobile systems demonstrated compliance with a DOE quality assurance program via several independent blind tests.

Excited states in ${}^{208}$Pb have been studied by measuring $\ensuremath{\gamma}$ rays in time coincidence with reaction charged particles. ${}^{208}$Pb states were produced with both the ${}^{207}$Pb${(d,p)}^{208}$Pb and ${}^{209}$Bi$(t,\ensuremath{\alpha})$${}^{208}$Pb reactions. The energy resolution of the particle spectra of 100 keV allowed a rough determination of level excitation, which then was determined with high resolution from the coincident $\ensuremath{\gamma}$ decay measured in Ge detectors. Many new $\ensuremath{\gamma}$ transitions have been found and previously unresolved multiplets of states resolved. The data give spectroscopic factors for neutron transfer and proton pickup. Spins and parities of levels could be deduced from their $\ensuremath{\gamma}$ decays. A least squares fit of all $\ensuremath{\gamma}$ energies gave very precise (0.1 keV) level energies. All states predicted by the shell model below 4.6 MeV are now found and their spins determined unambiguously.

The next generation, multi-detector active and passive computed tomography (A&PCT) scanner will be optimized for speed and accuracy. At the Lawrence Livermore National Lab (LLNL) we have demonstrated the trade-offs between different A&PCT design parameters that affect the speed and quality of the assay results. These fundamental parameters govern the optimum system design. Although the multi-detector scanner design has priority put on speed to increase waste drum throughput, higher speed should not compromise assay accuracy. One way to increase the speed of the A&PCT technology is to use multiple detectors. This yields a linear speedup by a factor approximately equal to the number of detectors used without a compromise in system accuracy. There are many different design scenarios that can be developed using multiple detectors. Here we describe four different scenarios and discuss the trade-offs between them. Also, some considerations are given in this design description for the implementation of a multiple detector technology in a field- deployable mobile trailer system.

Gamma-ray-based computed tomography (CT) requires that two different measurements be made on a closed waste container. [MAR92 and ROB94] When the results from these two measurements are combined, it becomes possible to identify and quantify all detectable gamma-ray emitting radioisotopes within a container. All measurements are made in a tomographic manner, i.e., the container is moved sequentially through well- known and accurately reproducible translation, rotation, and elevation positions in order to obtain gamma-ray data that is reconstructed by computer into images that represent waste contents. [ROB94] The two measurements modes are called active (A) and passive (P) CT. In the ACT mode, a collimated gamma-ray source external to the waste container emits multiple, mono-energetic gamma rays that pass through the container and are detected on the opposite side. The attenuated gamma-rays transmitted are measured as a function of both energy and position of the container. Thus, container contents are `mapped` via the measured amount of attenuation suffered at each gamma-ray energy. In effect, a three dimensional (3D) image of gamma- ray attenuation versus waste content is obtained. In the PCT measurement mode, the external radioactive source is shuttered turned- off, and the waste container, is moved through similar positions usedmore » for the ACT measurements. However, this time the radiation detectors record any gamma-rays emitted by radioactive sources on the inside of the waste container. Thus, internal radioactive content is mapped or 3D-imaged in the same tomographic manner as the attenuating matrix materials were in the ACT measurement mode.« less

The authors use Monte Carlo calculations to study a new design for a high-sensitivity gamma-ray detection system. The system uses an array of high-purity germanium detectors operating with an event-mode data acquisition system. The calculations show that the proposed design could produce a factor of 10 increase in the sensitivity of these measurements compared to currently employed systems.

Discrete transitions between very high spin states in 170W were studied with the 154Gd(20Ne, 4n) reaction. The second band crossing occurs at a rotational frequency of ≈ 0.48 MeV, which is considerably higher than in other nuclei. The data are interpreted within a pairing selfconsistent HFBC model which suggests that the proton h 9 2 2 (ω = 1 2 ) alignment is predominantly responsible for the second band crossing.

Gamma decay of the four previously known 10 + states between 4.8 and 5.9 MeV of excitation in 208 Pb, and isomerism of the lowest 10 + [T 1 2 =500(50) ns ] and 8 + [T 1 2 =3.2(5) ns ] states have been established with α - γ coincidence measurements in 209 Bi(t, αγ ) 208 Pb. Wavefunctions for the 10 + states were deduced in the model space involving 1p-1h excitations only. These show that the π h 9 2 h 11 2 −1 strength is nearly evenly distributed among the 10 + states, in disagreement with results from (e, e′) data.