Exploration of an Electron LINAC-Driven Photoneutron Source Based on Scorpius

Project # 23-044 | Year 1 of 2

Amber Guckes^a, J. Andrew Green^a, Christine Evans^a, James Mellott^a,
Kevin Yim^b, Alexander Barzilov^b, David Schwellenbach^c, Allan Ortiz^d, Jon Stoner^e, Kevin Folkman^e, Chad O’Neill^e, Brian Berls^e

^aNorth Las Vegas, ^bUniversity of Nevada Las Vegas, ^cKeystone International, ^dLos Alamos Operations (LAO), ^eIdaho State University
_{This work was done by Mission Support and Test Services, LLC, under Contract No. DE-NA0003624 with the U.S. Department of Energy, the NNSA Office of Defense Programs, and supported by the Site-Directed Research and Development Program. DOE/NV/03624–1886.}

Abstract

Neutron production can be realized with an electron linear accelerator (LINAC) and an appropriately selected photodisintegration target configuration. Photoneutron sources can be valuable to the stockpile stewardship and global security missions. Specifically, an electron LINAC-driven photoneutron source can provide short-pulsed neutrons to enable diagnostics such as neutron radiography and neutron resonance spectroscopy for subcritical experiments. While an electron LINAC-driven photoneutron source currently does not exist at the NNSS, the highly anticipated Scorpius linear induction accelerator (LIA) could be leveraged in this way. This project will assess the viability of using Scorpius to produce neutrons that can be used for neutron diagnostics crucial to Stockpile Stewardship.

Background

A multi-probe (e.g., x-rays and neutrons) diagnostic capability for subcritical experiments would provide a unique opportunity to field two distinctly different sets of measurement techniques within the same testbed resulting in twice as much data. Typically, two different diagnostic capabilities and thus, two different testbeds, would be required to achieve the same amount of data. The Scorpius machine coming to the NNSS in the next 5–10 years could eventually be used as a multi-probe diagnostic capability.

The Scorpius LIA is a multi-pulse linear induction accelerator with an electron energy of ~20 MeV. Electrons produced by Scorpius will impinge on a Bremsstrahlung target enabling x-ray radiography of subcritical experiments; this is the primary mission for Scorpius. Unintentional production of photoneutrons at the Dual-Axis Radiographic Hydrodynamic Test (DARHT) facility, with a similar configuration to Scorpius, has been studied extensively by the Los Alamos National Laboratory (LANL). This informed the Scorpius Bremsstrahlung x-ray target design so to minimize the production of photoneutrons. LANL did not pursue the intentional production of photoneutrons using this configuration.

Photoneutron production in and of itself is not novel. However, the idea of producing photoneutrons intentionally with Scorpius is. Leveraging Scorpius in this way could provide the NNSS and the National Weapons Laboratories with a multi-probe diagnostic capability which is currently not envisioned for the Scorpius testbed at the NNSS Principal Underground Laboratory for Subcritical Experimentation (PULSE).

Technical Approach

A viable neutron source could be devised by adding an independent photoneutron target in line with or at a 90-degree angle to the existing Scorpius Bremsstrahlung x-ray target. An entirely new Bremsstrahlung x-ray and photoneutron target could be designed as well. Various materials such as heavy water, high-density polyethylene, beryllium, tungsten, lead, and tantalum have been used as targets employed with electron accelerators to produce photoneutrons previously. Low-Z materials can be used to optimize the production of slow neutrons which are required to perform neutron resonance spectroscopy. High-Z materials can be used to optimize the production of fast neutrons which are useful for radiography.

Our team is exploring this potential Bremsstrahlung x-ray and photoneutron target trade space considering the 20 MeV electrons from Scorpius as the source term. Our approach is to inform a photoneutron target design through Monte Carlo N-Particle (MCNP) simulations and evaluate the performance of the photoneutron target design through experimental measurements.

We are collaborating with the University of Nevada, Las Vegas to prepare an MCNP model of the Scorpius electron beam source term and potential photoneutron target configurations. These simulations inform our photoneutron target design and provide a prediction of expected neutron yield and energy spectrum.

At least three measurement campaigns to first confirm our neutron diagnostics and target design methodology and then evaluate the neutron yield and energy spectrum of our Scorpius photoneutron target design are planned over the course of this 2-year project. Two of these measurement campaigns will take place at the Idaho State University (ISU) Idaho Accelerator Center (IAC). The third will take place at the DARHT facility at LANL.

The IAC’s L-band LINAC has an electron endpoint energy that is tunable from approximately 1 MeV to 25 MeV. It also has a pulse width of approximately 60 ps (operated in bunched mode) and from 2 ns to 2 us. It has a repetition rate from single-pulse to 200 Hz. The IAC’s S-band multiport LINAC has similar performance. It is not capable of achieving sub-nanosecond pulse width but is capable of higher beam current. These specifications encompass the electron endpoint energy and timing of Scorpius. Scaling the beam currents offered by these two IAC LINACs to that of Scorpius, while achieving its exact electron endpoint energy and timing specifications, provides an experimental basis for evaluating photoneutron production before Scorpius is installed at the NNSS. Furthermore, the IAC staff has a breadth of experience in photoneutron production which we will leverage in the design of our photoneutron targets and experimental setup. Photoneutron measurements at lower electron energies (410 MeV) and with a beryllium photoneutron target were performed in Year 1 at the IAC. Photoneutron measurements with our Scorpius photoneutron target and electron energies of 20 MeV are planned in Year 2 of this project ahead of fielding at DARHT.

An opportunity to field our Scorpius photoneutron target on DARHT was presented to the team by LANL’s Mark Crawford. DARHT is a dual-axis linear induction accelerator with beam energy, timing, and current comparable to Scorpius. Fielding our Scorpius photoneutron target on DARHT would be the ultimate test ahead of Scorpius being commissioned at the NNSS. We plan to take advantage of this opportunity in Year 2 of this project.

Results and Technical Accomplishments

It is predicted that the Scorpius accelerator will yield about 4.4E14 electrons per 50 ns pulse assuming a beam current of 1400 amps. The resultant Bremsstrahlung x-ray flux and, therefore, the potential photoneutron flux will vary based on the target material and its material properties (e.g., macroscopic photoneutron cross-section, geometry, configuration). MCNP simulations performed in Year 1 of this project indicate that a neutron yield of up to ~6E-5 integrated neutrons/cm²/source electron, and ~1E-6 at a peak photoneutron energy of 1 MeV may be plausible with Scorpius and our initial designs of a photoneutron target for it.

The opportunity to perform photoneutron measurements with 4-10 MeV electron energy and a beryllium photoneutron target at the ISU IAC was presented to the team during a tour of the ISU IAC on May 10, 2023. We were able to secure beam time at the ISU IAC June 28–30, 2023 and completed measurements with our suite of neutron diagnostics including four neutron time-of-flight (nToF) detectors, a Cs₂LiYCl6:Ce (CLYC) dual neutron/photon spectroscopy detector, and neutron activation foils consisting of indium, gold, manganese-copper, and tungsten.

The data analysis is complete from the first IAC measurement campaign. The most successful aspect is from the neutron activation measurement with the S-band beamline. Results from this data indicate neutron yields of ~1E^-7 eV-scale neutrons/cm²/source electron with the beryllium photoneutron target at an electron energy of 7.8 MeV. For the same beam, the intensity of 1.2 MeV photoneutrons (from the indium foil measurement) was measured to be ~5E-^‑5 neutrons/cm²/source electron. Higher neutron yields are expected with the Scorpius photoneutron target and 20 MeV electron energy. The nToF and CLYC detectors were limited by the availability of the short-pulse L-band beamline, which was offline for much of the period. All technical reports were formally written up and were uploaded to the SDRD monthly reporting archive for August and September.

Several important lessons learned were also attained from this first set of measurements which will be applied during future experimental campaigns.

Conclusions and Path Forward

All the work completed in the first year of this project was aimed towards priming ourselves to fabricate, field, and take data with a Scorpius photoneutron target in the second year of this project. We performed MCNP simulations to inform a 4 MeV photoneutron target design which we then fielded along with our neutron diagnostics at the ISU IAC. These measurements proved our diagnostics and our methodology for informing the photoneutron target design. With this data in hand, we performed another set of MCNP simulations, this time informing a 20 MeV Scorpius photoneutron target design.

Looking towards the second year, we will procure the Scorpius photoneutron target and field it at both the ISU IAC and DARHT along with our neutron diagnostics. The data collected from these measurements will answer the question: can we use Scorpius to produce useful amounts and energies of neutrons?

Publications

• Title: Exploration of an electron LINAC-driven photoneutron source based on Scorpius
  Journal / Conference: JOWOG 32 HDT Plenary Hydrodynamic Diagnostic Technology – Radiographic Systems and Imaging Analysis
  Year: 2023
  Author(s): Amber Guckes, J. Andrew Green, David Schwellenbach, Alexander Barzilov, Kaleab Ayalew, Kevin Yim
  
• Title: Exploration of an electron LINAC-driven photoneutron source based on Scorpius
  Journal / Conference: Linear Induction Accelerator Working Group
  Year: 2023
  Author(s): Amber Guckes, J. Andrew Green, David Schwellenbach, Alexander Barzilov, Kaleab Ayalew, Kevin Yim

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