B. L. Goldblum

Relative {sup 235}U(n,{gamma}) and (n,f) cross sections from {sup 235}U(d,p{gamma}) and (d,pf)

Author(s): Allmond, J.M. | Abstract: The internal surrogate ratio method allows for the determination of an unknown cross section, such as (n,y), relative to a better-known cross section, such as (n,f), by measuring the relative exitchannel probabilities of a surrogate reaction that proceeds through the same compound nucleus. The validity of the internal surrogate ratio method is tested by comparing the relative gamma and fission exit-channel probabilities of a 236U* compound nucleus, formed in the 235U(d,p) reaction, to the known 235U(n,y) and (n,f) cross sections. A model-independent method for measuring the gamma-channel yield is presented and used. PACS numbers: 24.87.+y, 24.75.+i, 24.50.+g, 25.85.Ge

Neutron Spectroscopy for pulsed beams with frame overlap using a double time-of-flight technique

Abstract A new double time-of-flight (dTOF) neutron spectroscopy technique has been developed for pulsed broad spectrum sources with a duty cycle that results in frame overlap, where fast neutrons from a given pulse overtake slower neutrons from previous pulses. Using a tunable beam at the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory, neutrons were produced via thick-target breakup of 16 MeV deuterons on a beryllium target in the cyclotron vault. The breakup spectral shape was deduced from a dTOF measurement using an array of EJ-309 organic liquid scintillators. Simulation of the neutron detection efficiency of the scintillator array was performed using both GEANT4 and MCNP6. The efficiency-corrected spectral shape was normalized using a foil activation technique to obtain the energy-dependent flux of the neutron beam at zero degrees with respect to the incoming deuteron beam. The dTOF neutron spectrum was compared to spectra obtained using HEPROW and GRAVEL pulse height spectrum unfolding techniques. While the unfolding and dTOF results exhibit some discrepancies in shape, the integrated flux values agree within two standard deviations. This method obviates neutron time-of-flight spectroscopy challenges posed by pulsed beams with frame overlap and opens new opportunities for pulsed white neutron source facilities.

Surrogate measurement of the 238Pu(n,f) cross section

The neutron-induced fission cross section of 238 Pu was determined using the surrogate ratio method. The (n,f) cross section over an equivalent neutron energy range 5‐20 MeV was deduced from inelastic α-induced fission reactions on 239 Pu, with 235 U(α, α � f )a nd 236 U(α, α � f) used as references. These reference reactions reflect 234 U(n,f )a nd 235 U(n,f) yields, respectively. The deduced 238 Pu(n, f) cross section agrees well with standard data libraries up to ∼10 MeV, although larger values are seen at higher energies. The difference at higher energies is less than 20%.

Proton light yield in organic scintillators using a double time-of-flight technique

Recent progress in the development of novel organic scintillators necessitates modern characterization capabilities. As the primary means of energy deposition by neutrons in these materials is n-p elastic scattering, knowledge of the proton light yield is paramount. This work establishes a new model-independent method to continuously measure proton light yield in organic scintillators over a broad energy range. Using a deuteron breakup neutron source at the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory and an array of organic scintillators, the proton light yield of EJ-301 and EJ-309, commercially available organic liquid scintillators from Eljen Technology, were measured via a double time-of-flight technique. The light yield was determined using a kinematically over-constrained system in the proton energy range of 1-20 MeV. The effect of pulse integration length on the magnitude and shape of the proton light yield relation was also explored. This work enables accurate simulation of the performance of advanced neutron detectors and supports the development of next-generation neutron imaging systems.

A Novel Framework for Safeguarding Naval Nuclear Material

ABSTRACT The present international standard allows non-nuclear weapon states (NNWS) to forego safeguards when nuclear material is used in a “non-proscribed military activity,” though no criteria have been established to determine when NNWS can remove naval nuclear material from safeguards. Though at present, only nuclear-armed states possess nuclear submarines, the global nuclear naval landscape may soon change with the advancement of Brazils fledgling program and the possible precedent it would set for other NNWS. A framework is needed to shore up nuclear security and prevent nuclear material diversion from the nuclear naval sector. Proposed and existing nonproliferation frameworks, including a Fissile Material Cut-off Treaty and commitments through the nuclear security summits, are insufficient to close this loophole. A Naval Use Safeguards Agreement (NUSA), modeled after the Additional Protocol of the International Atomic Energy Agency, would provide a framework to remove the opacity surrounding nuclear material in the naval sector. Designed for NNWS and encouraged as confidence-building measures for nuclear weapon states, NUSA would explicitly outline those stages in the naval nuclear fuel cycle where safeguards are to be applied and in what context. This viewpoint also further provides direction for targeted research and development in technical naval nuclear safeguards solutions.

A framework for assessing alternate proliferation pathways in the age of non-state actors

While the nonproliferation community has long acknowledged the possibility of nuclear terrorism, its prevention has become a central focus area in recent years. For decades it has been assumed that, with access to special nuclear material (SNM), the steps to indigenously develop an improvised nuclear device are within the reach of non-state actors. At the same time, indigenous production of SNM has generally been dismissed as infeasible. Recognizing how recent trends of technology democratization, the open exchange of information, and globalization have eroded certain barriers to proliferation by non-state actors, this research explores the pathways a non-state actor could take to indigenously develop SNM and develops a method to determine the comparative attractiveness of each pathway, given the current capabilities of non-state actors. Additionally, it considers avenues for further investigation to identify countermeasures for SNM production, including countermeasures specific to non-state actors.

All hands on deck: advancing safeguards for naval nuclear materials

ABSTRACT With the continued use of unsafeguarded naval nuclear-propulsion programs in all nuclear-weapon states, the commissioning of an Indian nuclear submarine, and the potential investment in such programs by non-nuclear-weapon states including Brazil and South Korea, movement toward a regulatory regime for nuclear material in the naval sector has become imperative. Such a framework faces a recurring debate on adequately protecting sensitive military technology while delivering assurances that naval nuclear material is not diverted to nuclear-weapon programs. In this viewpoint, we examine various prospective mechanisms to regulate naval nuclear stocks and assess them in terms of their effectiveness and scope. Drawing on lessons from the drafting, negotiation, and implementation of the Model Additional Protocol, we recommend a safeguards regime for naval nuclear material via a protocol that supplements the existing global nuclear-governance system. This protocol provides a standardized yet flexible approach to naval nuclear-material safeguards across all states (whether nuclear-weapon states, non-nuclear-weapon states, or outside the Treaty on the Non-Proliferation of Nuclear Weapons) to handle variations among naval nuclear fuel cycles and technologies.

Photon Strength and the Low-Energy Enhancement

Several measurements in medium mass nuclei have reported a low-energy enhancement in the photon strength function. Although, much effort has been invested in unraveling the mysteries of this effect, its physical origin is still not conclusively understood. Here, a completely model-independent experimental approach to investigate the existence of this enhancement is presented. The experiment was designed to study statistical feeding from the quasi-continuum (below the neutron separation energy) to individual low-lying discrete levels in 95Mo produced in the (d, p) reaction. A key aspect to successfully study gamma decay from the region of high-level density is the detection and extraction of correlated particle-gamma-gamma events which was accomplished using an array of Clover HPGe detectors and large area annular silicon detectors. The entrance channel excitation energy into the residual nucleus produced in the reaction was inferred from the detected proton energies in the silicon detectors. Gating on gam...

Surrogate ratio methodology for the indirect determination of neutron capture cross sections

The relative gamma-decay probabilities of the {sup 162}Dy to {sup 161}Dy and {sup 162}Dy to {sup 164}Dy residual nuclei, produced using light-ion-induced direct reactions, were measured as a function of excitation energy using the CACTUS array at the Oslo Cyclotron Laboratory. The external surrogate ratio method (SRM) was used to convert these relative gamma-decay probabilities into the {sup 161}Dy(n,gamma) cross section in an equivalent neutron energy range of 130-560 keV. The directly measured {sup 161}Dy(n,gamma) cross section, obtained from the Evaluated Nuclear Data Files (ENDF/B-VII.0), was compared to the experimentally determined surrogate {sup 161}Dy(n,gamma) cross section obtained using compound-nucleus pairs with both similar ({sup 162}Dy to {sup 164}Dy) and dissimilar ({sup 162}Dy to {sup 161}Dy) nuclear structures. A gamma-ray energy threshold was identified, based upon pairing gap parameters, that provides a first-order correction to the statistical gamma-ray tagging approach and improves the agreement between the surrogate cross-section data and the evaluated result.

RelativeU235(n,γ)and(n,f)cross sections fromU235(d,pγ)and(d,pf)

Author(s): Allmond, J.M. | Abstract: The internal surrogate ratio method allows for the determination of an unknown cross section, such as (n,y), relative to a better-known cross section, such as (n,f), by measuring the relative exitchannel probabilities of a surrogate reaction that proceeds through the same compound nucleus. The validity of the internal surrogate ratio method is tested by comparing the relative gamma and fission exit-channel probabilities of a 236U* compound nucleus, formed in the 235U(d,p) reaction, to the known 235U(n,y) and (n,f) cross sections. A model-independent method for measuring the gamma-channel yield is presented and used. PACS numbers: 24.87.+y, 24.75.+i, 24.50.+g, 25.85.Ge