Oregon State University | College of Engineering | School of Nuclear Science and Engineering

NOVEL TEMPORAL GAMMA SPECTROSCOPY INSTRUMENTATION AND ANALYTICAL METHODS IN DETERMINING PHOTOFISSION PRODUCT YIELDS

ABSTRACTDespite the decades of previous research, many aspects of the available, empirical fission data are lacking. The evaluated data widely used in

computational tools suffer from inaccuracies and large relative uncertainties, particularly with respect to short-lived and low-yield fission products.

The existing data is often based on nuclear models and limited experimental measurements of these fission product yields have been performed. This

work seeks to validate and improve, where applicable, the current fission product yield data in the ENDF library using experimental measurements,

particularly for the fission of U-238 and Th-232 .

A reliable way to determine composition of radioactive materials is through the measurement and analysis of delayed gamma rays emitted from the

decay of fission products. High-purity germanium (HPGe) detectors, with standard and altered preamplifiers, are being employed for measurements

of short-lived fission products, collecting data between accelerator pulses in irradiations of U-238 and Th-232. One of the challenges associated with

measuring low-yield, short-lived fission products is the inability of commercial detectors and data acquisition systems to operate quickly, efficiently,

and in high-flux environments - a necessity for such measurements. In previous measurements, much of the measurable fission products would have

decayed away during sample transfer from the linear accelerator (linac) to the measurement setup. This results in low count rates and large

uncertainty in the calculated yield of fission products, especially for the short-lived ones.

This work leverages previous research examining pulse processing algorithms for high-throughput, high-resolution spectroscopy and the comparison

of measured and simulated delayed gamma ray spectra following photofission. Using established nuclear codes (e.g. ORIGEN-S and MCNP) delayed

gamma ray spectra will be simulated and compared to our measurement results. The quality and precision of data in the existing library, as

determined from this comparison, will be evaluated and reported.

Ari Foley a, Dr. Haori Yang a, Dr. Mathew Kinlaw b

a School of Nuclear Science and Engineering, Oregon State University, Corvallis, OR 97331, USA, b Idaho National Laboratory, Idaho Falls, ID 83415, USA

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Figure 1: Fractional mass yields produced by DT neutron fission, photofission (,f) (22 MeV); and fast neutron fission (500 KeV) of 238U.1,2

INTRODUCTIONNeutrons and photons can excite a nucleus and cause fission in nuclear materials. Following a fission event, emitted signals can be

measured for detection and identification purposes. Exploitable signatures can be based on neutrons and photons, both prompt and

delayed. Delayed gamma rays are more abundant, long-lasting, and are often easier to detect following a fission event. A unique gamma

ray intensity distribution exists for each fissionable isotope allowing material identification.

One of the challenges associated with measuring low-yield, short-lived fission products is the inability of commercial detectors and data

acquisitions systems to operate quickly, efficiently, and in high-flux environments, a necessity for such measurements. Similar

requirements exist for practical security and safeguards applications which rely upon observing and characterizing fission signatures in

order to detect and identify special nuclear material. Increasing the throughput capabilities—the amount of information the acquisition

system can process without loss—and limiting the processing time required for those measurements is of paramount importance.

Balancing the processing time inherent to those detection systems with the need for sufficient measurement precision requires

advancement to the current state of the art.

MOTIVATION➢ Improved nuclear data for isotopes of interest in SNM detection and nuclear forensics

➢ Photonuclear isotope production methods for nuclear forensics projects

Aluminum Beam ScrubPhoton Radiator

e-

Beam Exit Aluminum Beam ScrubPhoton Radiator

e-

Beam Exit

Figure 2: (left) MCNP6 mesh tally of electron beam into photon radiator (left, red/blue) (Right) Tally of electron beam (red/blue) with resulting photon spectrum (purple/yellow)

Figure 3: Experimental setup with pneumatic shuttle for irradiations in linac hall with the 25 MeV accelerator at the Idaho Accelerator Center in February 2018

ACKNOWLEDGEMENTThis research was performed using funding received from the DNDO Academic Research Initiative Program and portions of thisresearch were supported by the INL internship program

1England, T. R. and Rider, B. R.; “Evaluation and Compilation of Fission Product Yields,” ENDF-349, LA-UR-94-3106, Los Alamos National Laboratory (1994)2Belyshev, S. S., Ishkhanov, A. A. and Stopani, K. A.; “Mass yield distributions and fission modes in photofission of 238U below 20 MeV,” Phys Rev C 91, 034603 (2015).

EXPERIMENTAL SETUPTwo separate experimental setups have been tested: one with both modified and standard HPGe detectors facing the sample in an

experimental hall, adjacent to the linac hall, with a collimated photon beam from the 90 degree port, and the other utilizing a

pneumatic shuttle and the HPGe located within the linac hall during operation. Maximizing count rates while preventing loss in

energy resolution is a delicate balance. The intensity of the produced signal has a prompt drop following the end of an individual

accelerator pulse and an increasing amount of information is lost the later that measurement begins.

ANALYSESIn order to observe products with the lowest relative yields, cycles of irradiation and counting periods will be required. Custom software (coupling ORIGEN-S and MCNP6) is used in conjunction with MCNP6 simulations to determine

the optimal combination of counting and irradiation cycles. Measurements are analyzed to extract independent yields of the short-lived fission products based on Levenberg-Marquardt least squares fitting algorithms specific to the

isotope of interest’s decay chain.

Figure 4: Example of measured gamma spectrum from 0 to 8 seconds of irradiated U-238 at 22 MeV

Figure 5: Gamma spectrum signature of isotope of interest, Zr-99 (Data from Figure 5)

Figure 6: Illustration of decomposition of the peaks (at 469 keVand 465 keV) contributing to bulk peak

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Figure 7: Scope trace to visualize timing of linac charge measured by faraday target (yellow), HPGe preamp signal (cyan), and the “magic pulse” (magenta) to pneumatic

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Figure 7: Yield from 594 keV peak to verify Zr-99 peak against known half-life (Using data from Figure 6)