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Performance evaluation of an energy tuning assembly for neutron spectral shaping

TL;DR: In this paper, an energy tuning assembly (ETA) was designed to be fielded at the National Ignition Facility (NIF) to modify the characteristic D-T fusion spectrum to include a prompt fission neutron spectral component.
Abstract: Author(s): Bevins, JE; Sweger, Z; Munshi, N; Goldblum, BL; Brown, JA; Bleuel, DL; Bernstein, LA; Slaybaugh, RN | Abstract: An energy tuning assembly (ETA) was designed to be fielded at the National Ignition Facility (NIF) to modify the characteristic D-T fusion spectrum to include a prompt fission neutron spectral component. The ETA was characterized at the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory to measure the shaped spectrum from an incident deuteron breakup neutron source, test the proposed neutron spectroscopy techniques used to inform the flux measurements at NIF, and validate the ability to predict ETA performance using a Monte Carlo Neutral Particle (MCNP) simulation. Activation foils (i.e., Ni, In, Au, Al) were exposed to a collimated 33-MeV deuteron-breakup beam originating from a tantalum breakup target. The source spectrum absent the ETA was characterized using a set of activation foils and the STAYSL unfolding code. Finally, the ETA-modified spectrum was obtained using activation foil unfolding with a [Formula presented]=1.32. The ETA-modified unfolded spectrum agreed with the MCNP-simulated prediction in the energy range of 0.1–14 MeV, but exhibited disagreements in the 10 eV–100 keV region. This work demonstrates shaping of the NIF neutron spectrum via the ETA to be a viable path forward for tailored neutron beams at NIF.

Summary (4 min read)

1. Introduction

  • Very early on, it was deemed desirable to modify neutron spectra for basic and applied scientific research and development.
  • Materials science and nuclear physics have used neutron filters on cold neutron beams to filter neutrons with wavelengths less than the critical wavelength, resulting in a high purity, low energy beam [4,5].
  • Section 4 describes the methods used to analyze the foil activation data, and Section 5 details the performance achieved.

2. Energy tuning assembly design

  • For the TNF application, the goal is to develop a spectrum that combines a D-T fusion, or thermonuclear (TN), plus a prompt fission neutron spectrum (PFNS) component [13].
  • It is worth noting that this is a representative but notional TNFrelevant neutron spectrum, and there exists a variety of different neutron spectra that would be of interest for the purposes of generating synthetic debris.
  • To avoid a man-power intensive ‘‘trial and error" approach to generating point designs, a software suite to automate the design of ETAs was developed.
  • The ETA, as designed, will only generate a TN+PFNS when exposed to a source neutron spectrum similar to the NIF D-T fusion neutron spectrum.

3.1. Beam design

  • Deuterons, with a neutron separation energy of 2.22 MeV, are weakly bound and will produce neutrons via breakup in the Coulombic field of a heavier nucleus (elastic breakup), proton stripping reactions (inelastic breakup), and pre-equilibrium and evaporation emission from the excited compound nucleus formed by deuteron absorption [28–31].
  • Eq. (1) describes the downward shift in the peak and average neutron energy with increasing 𝑍 of the breakup target.
  • In contrast, the pre-equilibrium and evaporative emission channels will be roughly isotropic and have energy distributions peaked at much lower energies based on the characteristic temperature of the nucleus [31,33,34].
  • SlaybaughLab/NIF_TNF_ETA/tree/master/AsConstructed/CAD_Models/ peaked near 14 MeV—NIF-relevant energies thereby probing the same interaction mechanisms—and with as limited a high energy component as possible (∼2% of the total spectrum is above 20 MeV).

3.2. Foil irradiation

  • The deuteron beam was run at a current of ∼8.2 μA during the foil irradiation for the measurement of the source beam and ∼10.8 μA during the ETA foil irradiation.
  • The Cave 0 beam line was optically aligned using a phosphor located in the Cave 0-1 beam box shown in Fig. 3 [27].
  • A Faraday cup was located at the breakup target location shown in Section 3 and equipped with a 4-mm-thick tantalum breakup target placed in the Cave 0 beam line [30,36].
  • The correction factors were necessary because of the non-ideal counting geometry (i.e., 50-mm-diameter foils placed 1 cm from the detector) that was used to compensate for the drastically reduced flux (as compared to the NIF experiment design) and other experimental constraints (e.g., count time and number of detectors).
  • The second foil set was irradiated to measure the source spectrum.

3.3. Foil counting

  • Foil counting was conducted using the 88-Inch Cyclotron counting lab’s Ortec coaxial HPGe GMX-50220-S detector.
  • An ORTEC ASPEC-927 multichannel analyzer with two 14-bit analog-to-digital converter (ADCs) was used to collect data and interface with MAESTRO software [37].
  • The efficiency function was calculated using a least squares method.
  • The function, given by Eq. (2), was derived from fitting the experimental calibration data; the resulting chi-square per degrees of freedom between Eq. (2) and the experimental data was 2.6.

4.1. Activation analysis

  • To perform a neutron spectrum unfold, the activity of each foil immediately following irradiation must be determined.
  • At the counting location 1 cm from the detector, there can be large uncertainties in the calculated coincident summing and geometric correction factors.

4.2. Irradiation foil measurement sets

  • To correct for geometry and coincident summing, a normalization foil pack was used to generate data for the method described in Section 4.1.
  • The foil pack was attached directly to the face of the beam box at BLC in the Cave 0-1 beam line.
  • The foil characteristics are described in Table 1; for simplicity, each foil will be referred to by the shorthand name shown in the tables, where ‘‘1" is the normalization foil set, ‘‘2" is the source spectrum measurement, and ‘‘3" is the ETA spectrum measurement.

4.3. Monte Carlo simulations

  • Monte Carlo simulations of the ETA experimental setup were performed using MCNP v6.1.3.
  • A cross-sectional view of the 88-Inch Cyclotron Cave 0 experimental facility and beam line is shown in Fig.
  • Simulated activation of the foils was calculated using IRDFF v1.05 data libraries [41].
  • Published neutron spectra for 33 MeV deuteron breakup on tantalum (or similar energies) do not address the low energy component (<3 MeV) necessary for this measurement [30,32], so the measured deuteronbreakup source spectrum from this work was used as the initial source term for the ETA performance simulation.
  • The deviation from uniformity over this range has minimal impact on modeled results given the rapid fall off of neutron importance outside of the ∼0.4◦ line of sight.

4.4. Spectrum unfolding

  • Unfolding of the activation results was accomplished using the STAYSL v1.2 suite [26] using the IRDFF v1.05 data libraries, which contain data for select reactions up to 60 MeV [41].
  • STAYSL has several modules available to account for correction factors due to self-shielding, decay, and burn-up.
  • The Beam Correction Factor (BCF) module was used to correct for irradiation time history using the 88-Inch Cyclotron’s beam current monitor (BCM).
  • No burn-up correction factors were applied given the low reaction rates achieved.
  • 𝐴◦ is the foil activities, 𝑃 is the neutron flux convolved with the cross-section, 𝑁𝑃 is the co-variance matrix from the flux and nuclear data convolution, and 𝑁𝐴◦ is the activity co-variance matrix.

5.1. Beam measurement

  • The post-irradiation activities of the source spectrum measurement foils calculated from Eq. (9) are listed in Table 2.
  • For nuclides with multiple gamma rays with high branching ratios, the activity reported here is the average of the activities calculated using each gamma ray.
  • The model of Cave 0 and the ETA are available at https://github.com/.
  • The resulting normalized STAYSL unfolded deuteron breakup source spectrum is shown in comparison to the MCNP simulated ETA-modified spectrum in Fig.
  • This region is below the threshold reactions and above the highly sensitive thermal region for the (n, 𝛾) reactions.

5.2. ETA measurement

  • The ETA-modified beam foil activities post-irradiation were calculated using Eq. (9) and are listed in Table 3.
  • Again, the activities listed here ignore gamma self-shielding.
  • While this indicates a great overall correlation and shape match, the PCC does not account for magnitude shifts.
  • As before, the largest discrepancy between the modeled and simulated result is in the 10 eV to 10 keV region below the threshold reactions and above the high-cross section region (< 10 eV) for (n, 𝛾) reactions.
  • If the spectrum above 10 keV is used for the KS two-sample test, 𝐷=0.10, and the 𝑝 is 0.78 – a stronger indication that the distributions are the same and the simulation is consistent with the measured spectrum.

6. Summary

  • An ETA was designed to produce a TN+PFNS at NIF through spectral modification techniques.
  • This work sought to validate the ability to model the ETA using MCNP, a challenging proposition given the weighted impact of component cross-sections on the resulting spectrum modification that could highlight nuclear data deficiencies.
  • The direct exposure foils were used to obtain an unfolded neutron spectrum that was used as the starting source spectrum in subsequent MCNP models to predict the ETA performance.
  • The largest discrepancies between the model and the STAYSL unfolded spectrum are in the 10 eV to 10 keV region, where the activation foil pack had limited sensitivity and coverage.

Acknowledgments

  • The authors thank the 88-Inch Cyclotron operations and facilities staff for their help in performing these experiments.
  • Thanks also to Ethan Boado, Matthew Harasty, Keegan Harrig, Will Kable, Thibault Laplace, Jørgen Mitbø, and Andrew Voyles for assistance with experimental execution.
  • This material is based upon work supported in part by the Department of Energy National Nuclear Security Administration through the Nuclear Science and Security Consortium under Award Number DENA0003180, and performed under the auspices of the U.S. Department of Energy by Lawrence National Security, LLC, Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
  • This work is also supported by the Lawrence Berkeley National Laboratory under Contract No. DE-AC02-05CH11231 for the US Nuclear Data Program and the National Science Foundation Graduate Research Fellowship under Grant No. NSF 11-582.
  • The views expressed in this article are those of the authors and do not reflect the official policy or position of the United States Air Force, Department of Defense, or the U.S. Government.

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Recent Work
Title
Performance evaluation of an energy tuning assembly for neutron spectral shaping
Permalink
https://escholarship.org/uc/item/4dk339s7
Authors
Bevins, JE
Sweger, Z
Munshi, N
et al.
Publication Date
2019-04-11
DOI
10.1016/j.nima.2019.01.049
Peer reviewed
eScholarship.org Powered by the California Digital Library
University of California

Nuclear Inst. and Methods in Physics Research, A 923 (2019) 79–87
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A
journal homepage: www.elsevier.com/locate/nima
Performance evaluation of an energy tuning assembly for neutron spectral
shaping
James E. Bevins
a,
, Z. Sweger
b
, N. Munshi
b
, B.L. Goldblum
c
, J.A. Brown
c
, D.L. Bleuel
d
,
L.A. Bernstein
c
, R.N. Slaybaugh
c
a
Department of Engineering Physics, Air Force Institute of Technology, Wright Patterson Air Force Base, OH 45433, USA
b
Department Physics, University of California, Berkeley, CA 94720, USA
c
Department of Nuclear Engineering, University of California, Berkeley, CA 94720, USA
d
Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
A R T I C L E I N F O
Keywords:
Neutron energy tuning
Beam shaping
Neutron activation analysis
Spectrum unfolding
Fast neutrons
Surrogate debris
A B S T R A C T
An energy tuning assembly (ETA) was designed to be fielded at the National Ignition Facility (NIF) to modify
the characteristic D-T fusion spectrum to include a prompt fission neutron spectral component. The ETA was
characterized at the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory to measure the shaped spectrum
from an incident deuteron breakup neutron source, test the proposed neutron spectroscopy techniques used to
inform the flux measurements at NIF, and validate the ability to predict ETA performance using a Monte Carlo
Neutral Particle (MCNP) simulation. Activation foils (i.e., Ni, In, Au, Al) were exposed to a collimated 33-MeV
deuteron-breakup beam originating from a tantalum breakup target. The source spectrum absent the ETA was
characterized using a set of activation foils and the STAYSL unfolding code. Finally, the ETA-modified spectrum
was obtained using activation foil unfolding with a
𝜒
2
𝜈
= 0.92. The ETA-modified unfolded spectrum agreed
with the MCNP-simulated prediction in the energy range of 0.1–14 MeV, but exhibited disagreements in the
10 eV–100 keV region. This work demonstrates shaping of the NIF neutron spectrum via the ETA to be a viable
path forward for tailored neutron beams at NIF.
1. Introduction
Very early on, it was deemed desirable to modify neutron spectra for
basic and applied scientific research and development. By 1935, only
three years after the discovery of the neutron, hydrogen thermalization
and cadmium filters were employed to modify or remove portions of
neutron spectra [1,2]. Over 80 years later, neutron spectral modification
techniques have not changed significantly.
Neutron filters, sometimes called neutron screens, have been used
in a wide variety of nuclear science and engineering applications. In
activation analysis, highly absorbing filters employing Cd, B, Gd, Hf,
etc. are often used to suppress the thermal neutron flux allowing for epi-
thermal reactions to dominate [3]. Materials science and nuclear physics
have used neutron filters on cold neutron beams to filter neutrons
with wavelengths less than the critical wavelength, resulting in a high
purity, low energy beam [4,5]. To certify epi-thermal and fast reactor
designs, solid and liquid neutron filters have been used for irradiation
degradation and power transient studies, respectively [6]. In the field of
radiation detection, filters have been used to enhance the gamma signal
by suppressing high neutron fields and to selectively detect thermal and
Corresponding author.
E-mail address: james.bevins@afit.edu (J.E. Bevins).
fast neutrons [7,8]. Neutron filters can also be used to produce quasi-
monochromatic neutron beams with V, Mn, S,
56
Fe, or other nuclei
that have deep interference minima [9]. Parametrically optimized,
sometimes layered, neutron filter designs for several applications such
as neutron radiography, boron neutron capture therapy, and neutron
transmutation doping of silicon have also been developed by combining
multiple elements from the list of applications above [1012].
Recent research introduced the concept of an energy tuning assembly
(ETA) using modern optimization techniques to advance the state-of-
the-art for designing custom neutron spectra [13,14]. The first ETA
was designed for the technical nuclear forensics (TNF) community
to produce fission and activation products that provide the charac-
teristic ‘‘fingerprint" used to aid in the attribution of the originating
source of a detonated nuclear weapon (aka synthetic post-detonation
debris) [1517].
This work investigates the ability to model and measure the per-
formance of the TNF ETA designed to be fielded at the National
Ignition Facility (NIF) as a passive diagnostic with the desired neu-
tron spectrum achieved across the small experimental cavity, labeled
Element 6 in Fig. 1. The nature of the experiment and ETA design
https://doi.org/10.1016/j.nima.2019.01.049
Received 26 November 2018; Accepted 15 January 2019
Available online 21 January 2019
0168-9002/Published by Elsevier B.V.

J.E. Bevins, Z. Sweger, N. Munshi et al. Nuclear Inst. and Methods in Physics Research, A 923 (2019) 79–87
Fig. 1. ETA designed to generate a thermonuclear plus prompt fission spectrum at the
National Ignition Facility [13].
limits the neutron spectroscopy options available to measure the neu-
tron spectrum achieved in the cavity. While typical techniques such
as time-of-flight [18], proton recoil spectrometers [19], pulse height
spectrum unfolding with organic scintillators [20], and capture gated
spectrometers [21] will not work, foil activation analysis and unfolding
is a viable technique to measure the neutron energy spectrum in the
ETA experimental cavity. Foil activation analysis for neutron spectrum
unfolding is a well established method that has been used to measure
spectra for reactors [22], the Spallation Neutron Source [23], spallation
at the Isotope Production facility [24],
252
Cf [25], etc. For this work, the
STAYSL PNNL code developed by Greenwood et al. was used to unfold
the activation results [26].
Unfolding neutron spectra using foil activation analysis is often
dependent on the initial a priori spectrum. This a priori spectrum is
generally developed using a model of the system under consideration.
However, modeling the ETA performance was considered a challenging
proposition given the weighted impact of component cross-sections on
the resulting spectrum modification that could highlight nuclear data
deficiencies. Thus, the goal of this work was to validate the ability to
model the ETA performance to address the concerns about the nuclear
data impacts on the model. This was accomplished through a series of
experiments at the 88-Inch Cyclotron at Lawrence Berkeley National
Laboratory (LBNL).
The paper is organized as follows. Section 2 explains how the ETA
was developed and the intended application as context for the current
measurement. Section 3 describes how the 88-Inch Cyclotron at LBNL
was used to measure the ETA performance in preparation for full
implementation at the NIF. Section 4 describes the methods used to
analyze the foil activation data, and Section 5 details the performance
achieved. Concluding remarks are presented in Section 6.
2. Energy tuning assembly design
For the TNF application, the goal is to develop a spectrum that
combines a D-T fusion, or thermonuclear (TN), plus a prompt fission
neutron spectrum (PFNS) component [13]. For the purposes of brevity,
this objective spectrum will be referred to as the TN+PFNS and is shown
in Fig. 2. It is worth noting that this is a representative but notional TNF-
relevant neutron spectrum, and there exists a variety of different neutron
spectra that would be of interest for the purposes of generating synthetic
debris. To avoid a man-power intensive ‘‘trial and error" approach to
generating point designs, a software suite to automate the design of
ETAs was developed. This resulted in the creation of a new optimization
method, Gnowee [14], and a ETA design software, Coues [13].
These tools were used to create an ETA assembly to be fielded at the
NIF as a proof of concept to generate synthetic debris. A cross-sectional
view of the ETA design generated by Coeus for the TN+PFNS objective
spectrum with NIF-imposed constraints of weight (75 kg), efficiency
(1×10
9
fissions), and outer design envelope (to avoid 1𝜔 scattered light)
is shown in Fig. 1. The outer diameter is 280 mm, the overall length is
240.2 mm, and the central sample cavity is 8.93 mm tall with a diameter
of 53.1 mm.
1
The ETA was built by American Elements.
The ETA, as designed, will only generate a TN+PFNS when exposed
to a source neutron spectrum similar to the NIF D-T fusion neutron
spectrum. However, validation of the ability to model the results and
unfold the neutron spectrum can be carried out using a surrogate
facility. For this purpose, exploratory experiments were conducted at
the 88-Inch Cyclotron to determine the ability to model and measure
the performance of the ETA as a developmental step toward creating
synthetic post-detonation fission products at the NIF.
3. Experimental setup
3.1. Beam design
The LBNL 88-Inch Cyclotron is capable of accelerating deuterons
up to a maximum energy of 65 MeV with maximum currents on the
order of 10 μA [27]. Deuterons, with a neutron separation energy of
2.22 MeV, are weakly bound and will produce neutrons via breakup
in the Coulombic field of a heavier nucleus (elastic breakup), proton
stripping reactions (inelastic breakup), and pre-equilibrium and evapo-
ration emission from the excited compound nucleus formed by deuteron
absorption [2831]. Each production mechanism produces neutrons
with different angular and energy distributions. The elastic and inelastic
breakup reactions result in neutron distributions that are highly forward
peaked with an average energy of
𝐸
𝑛
=
1
2
𝐸
𝑑
𝑍𝑒
2
𝑅
𝑏𝑢
𝑄
, (1)
where 𝑅
𝑏𝑢
is the breakup radius, 𝐸
𝑑
is the energy of the deuteron, and
𝑄 is the Q value of the reaction, 𝑍 is the atomic number of the target
nucleus, and 𝑒 is the elementary charge of a electron (proton) [29,30].
Eq. (1) describes the downward shift in the peak and average
neutron energy with increasing 𝑍 of the breakup target. The neutron
distribution is narrows with increasing 𝑍 due to the higher 𝑑𝐸𝑑𝑍
of the heavier target nucleus and the increasing relative fraction of
the elastic channels [29,32,33]. In contrast, the pre-equilibrium and
evaporative emission channels will be roughly isotropic and have energy
distributions peaked at much lower energies based on the characteristic
temperature of the nucleus [31,33,34]. As the neutron emission angle
increases, the relative contribution of the pre-equilibrium and evapora-
tive emission channels increase, and the resulting energy distribution is
far broader and less intense [3133,35].
This diversity in neutron spectra as a function of incoming deuteron
energy, outgoing angle, and the target enables the 88-Inch Cyclotron to
be used to perform a variety of neutron related experiments. For the ETA
experiments, a beam was designed to have a neutron spectrum that is
1
Full mechanical drawing are available from https://github.com/
SlaybaughLab/NIF_TNF_ETA/tree/master/AsConstructed/CAD_Models/
AssemblyDrawings.
80

J.E. Bevins, Z. Sweger, N. Munshi et al. Nuclear Inst. and Methods in Physics Research, A 923 (2019) 79–87
Fig. 2. TN+PFNS derived for use as the objective spectrum for ETA optimization and design [13].
peaked near 14 MeV—NIF-relevant energies thereby probing the same
interaction mechanisms—and with as limited a high energy component
as possible (2% of the total spectrum is above 20 MeV). This was
accomplished using a
2
H
+
beam accelerated to 33 MeV and directed
at a tantalum breakup target.
3.2. Foil irradiation
The deuteron beam was run at a current of 8.2 μA during the
foil irradiation for the measurement of the source beam and 10.8 μA
during the ETA foil irradiation. The Cave 0 beam line was optically
aligned using a phosphor located in the Cave 0-1 beam box shown in
Fig. 3 [27]. A Faraday cup was located at the breakup target location
shown in Section 3 and equipped with a 4-mm-thick tantalum breakup
target placed in the Cave 0 beam line [30,36]. The tantalum target was
backed by a 14.5-mm-thick copper cooling assembly with a 38-mm-
radius cutout centered on the tantalum target. The resulting neutrons
entering the Cave 0-2 experimental area were collimated by 1.5 m of
concrete and 1.5 m of concrete and sand bags encasing the beam pipe,
producing a high contrast, open-air neutron beam. [27].
The origin is taken as beam line center (BLC) in the 𝑦 and 𝑧 directions,
and the Cave 0-2 side of the Cave 0-1/Cave 0-2 wall shown in Fig. 3 in
the 𝑥 direction. The activation foils for both measurements were placed
at BLC and 708.3 cm from the front face of the breakup target (61 cm
from origin). This resulted in a neutron flux of ∼3.6 × 10
5
n s
1
cm
2
at the experimental location, which is approximately seven orders of
magnitude below the NIF source fluence, ∼5.7 × 10
12
n s
1
cm
2
, for
which the ETA was designed. Some of this difference is compensated
for in run time, but the drastically reduced flux places a limit on the
experimental analysis techniques as described further in Section 4.
Three sets of activation foils were irradiated during this experi-
ment. The first set was irradiated to provide geometry and coincidence
summing correction factors for the high purity germanium (HPGe)
detector. The correction factors were necessary because of the non-ideal
counting geometry (i.e., 50-mm-diameter foils placed 1 cm from the
detector) that was used to compensate for the drastically reduced flux
(as compared to the NIF experiment design) and other experimental
constraints (e.g., count time and number of detectors). The second foil
set was irradiated to measure the source spectrum. The third foil set
was irradiated in the ETA sample cavity to measure the ETA-modified
spectrum.
2
All three sets of foils are described further in Section 4.2.
2
All experimental data are available at https://github.com/SlaybaughLab/
88_Data/blob/master/Experiments/Activation/33MeVTa_25Apr/.
Fig. 3. Schematic representation of the 88-Inch Cyclotron vault and beam line to Cave 0.
The Cave 0 experimental end station is comprised of two enclosures, Cave 0-1 and Cave 0-
2, separated by a lead-lined door outfitted with a beam port.
3.3. Foil counting
Foil counting was conducted using the 88-Inch Cyclotron counting
lab’s Ortec coaxial HPGe GMX-50220-S detector. The detector has a
46.8% relative efficiency and was oriented in an upward facing direction
in a lead lined case. An ORTEC ASPEC-927 multichannel analyzer with
two 14-bit analog-to-digital converter (ADCs) was used to collect data
and interface with MAESTRO software [37]. Efficiency calibration data
were taken at positions 1 cm and 18 cm from the detector surface using
the following sources and lines:
241
Am (59.54 keV),
133
Ba (80.998,
276.40, 302.85, and 356.01 keV),
137
Cs (661.657 keV),
60
Co (1173.23
81

J.E. Bevins, Z. Sweger, N. Munshi et al. Nuclear Inst. and Methods in Physics Research, A 923 (2019) 79–87
Fig. 4. HPGe efficiency calibration performed at 18 cm from the detector.
and 1332.49 keV), and
152
Eu (121.78, 244.70, 344.28, 778.90, 964.06,
1085.847, 1112.08, and 1408.01 keV). Additionally,
88
Y (392.87 and
1836.06 keV) and
88
Zr (898.04 keV) were used for energy-only calibra-
tion lines.
The efficiency function was calculated using a least squares method.
On physical grounds, the efficiency curve asymptotically approaches a
𝐸
−1
𝛾
relationship at high energy, so the inverse of the efficiencies was fit
using a linear least squares regression with the basis vectors x, 1,
1
x
,
1
x
2
.
The function, given by Eq. (2), was derived from fitting the experimental
calibration data; the resulting chi-square per degrees of freedom be-
tween Eq. (2) and the experimental data was 2.6. The resulting efficiency
curve and calibration data used are displayed in Fig. 4, where the error
bars represent statistical and systematic uncertainty in the activity of
each standard nuclide.
𝜖
18
(𝐸
𝛾
) =
𝐸
𝛾
2
(0.677)𝐸
𝛾
3
+ (168.92)𝐸
𝛾
2
(15106.4)𝐸
𝛾
+ 765419
(2)
4. Analysis
4.1. Activation analysis
To perform a neutron spectrum unfold, the activity of each foil im-
mediately following irradiation must be determined. The initial activity
of a sample at the end of irradiation is given as:
𝐴
0
=
𝐶(𝐸
𝛾
)𝜆𝑒
𝜆𝛥𝑡
𝑗
(1 𝑒
𝜆𝛥𝑡
𝑐
)𝜖
𝑑
(𝐸
𝛾
)𝑓
𝑙
𝐼
𝛾
(𝐸
𝛾
)
, (3)
where 𝐶 (𝐸
𝛾
) is the number of gamma-ray counts of a specific energy, 𝐸
𝛾
,
measured in the HPGe detector; 𝜆 is the decay constant of the nuclide
responsible for emission of that gamma ray; 𝛥𝑡
𝑗
is the time between the
end of irradiation and the start of the counting period; 𝛥𝑡
𝑐
is the total
count time; 𝜖
𝑑
(𝐸
𝛾
) is the efficiency of the detector at a given gamma-ray
energy at a given distance, 𝑑, from the detector; 𝑓
𝑙
is the detector live-
time fraction; and 𝐼
𝛾
(𝐸
𝛾
) is the intensity of gamma rays at that energy.
The source and ETA foil sets were both measured at a distance of
1 cm from the face of the detector, so the counting efficiency for these
foils at 1 cm, 𝜖
1
(𝐸
𝛾
), is needed. However, at this distance correction
factors must be applied to account for the geometry of the source
and coincident summing from gamma cascades [38,39]. The measured
absolute efficiency can be expressed as:
𝜖
𝑑
(𝐸
𝛾
) = 𝐹
𝑑,𝛾
𝐺
𝑑
(𝐸
𝛾
)
𝑑
(𝐸
𝛾
), (4)
where 𝐹
𝑑,𝛾
is the peak summing correction factor for a gamma ray
characteristic of a given nuclide at some distance 𝑑 from the detector,
𝐺
𝑑
(𝐸
𝛾
) is the energy dependent geometric correction factor accounting
for the efficiency difference between a volume source and a point source,
and
𝑑
(𝐸
𝛾
) is the intrinsic energy-dependent efficiency of the detector
at a distance 𝑑.
At the counting location 1 cm from the detector, there can be
large uncertainties in the calculated coincident summing and geometric
correction factors. Therefore, it would be beneficial to express the initial
activities in terms of the absolute efficiency 18 cm from the detector,
𝜖
18
(𝐸
𝛾
), instead of the absolute efficiency 1 cm from the detector,
𝜖
1
(𝐸
𝛾
). At large source-to-detector distances, the absolute efficiency is
dominated by the
𝑑
(𝐸
𝛾
) term since the foils approximate a point source,
𝐺
18
(𝐸
𝛾
) 1, and coincidence summing is negligible 𝐹
𝑑
(𝛾) 1 [39]. At
18 cm from the detector, the absolute efficiency is approximately only
a function of the gamma energy, with a minor geometric correction,
𝐺
18
(𝐸
𝛾
), required for the difference between a point source and the foil
geometry. The latter correction was determined using MCNP [40].
Using a set of highly activated normalization foils corresponding to
the same composition and geometry as the foils used to measure the
source and ETA spectra, 𝐴
0
for a given decay can be calculated using
Eq. (3) at a distance of 1 cm and 18 cm. Setting the initial activity
equations at the two different distances equal to one another gives:
𝐶
1,norm
𝜆𝑒
𝜆𝛥𝑡
𝑗(1 cm)
(1 𝑒
𝜆𝛥𝑡
𝑐(1 cm)
)𝜖
1
(𝐸
𝛾
)𝑓
𝑙
𝐼
𝛾
=
𝐶
18,norm
𝜆𝑒
𝜆𝛥𝑡
𝑗(18 cm)
(1 𝑒
𝜆𝛥𝑡
𝑐(18 cm)
)𝜖
18
(𝐸
𝛾
)𝑓
𝑙
𝐼
𝛾
, (5)
which can be simplified to yield
𝐶
1,norm
𝐶
18,norm
=
𝜖
1
(𝐸
𝛾
)
𝜖
18
(𝐸
𝛾
)
𝜅
norm
, (6)
where
𝜅
𝑛𝑜𝑟𝑚
=
𝑒
𝜆𝛥𝑡
𝑗(18 cm)
(1 𝑒
𝜆𝛥𝑡
𝑐(1 cm)
)
𝑒
𝜆𝛥𝑡
𝑗(1 cm)
(1 𝑒
𝜆𝛥𝑡
𝑐(18 cm)
)
𝑓
𝑙(1 cm)
𝑓
𝑙(18 cm)
(7)
= 𝑒
𝜆(𝛥𝑡
𝑗(18 cm)
𝛥𝑡
𝑗(1 cm)
)
(1 𝑒
𝜆𝛥𝑡
𝑐(1 cm)
)
(1 𝑒
𝜆𝛥𝑡
𝑐(18 cm)
)
𝑓
𝑙(1 cm)
𝑓
𝑙(18 cm)
.
The total detector efficiency at 1 cm for each reaction channel can be
related to the efficiency at 18 cm through the ratios of the counts for
that reaction from the normalization data set by:
𝜖
1
(𝐸
𝛾
) =
1
𝜅
𝑛𝑜𝑟𝑚
𝐶
1,norm
𝐶
18,norm
𝜖
18
(𝐸
𝛾
)𝐺
18
(𝐸
𝛾
). (8)
Eq. (3) can be modified using Eq. (8) to express the initial activity from
the sources and ETA irradiation data sets as a function of the efficiency
82

Citations
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Journal ArticleDOI
TL;DR: In this article, the nuclear data covariance analysis of an experimental design for a neutron energy-tuning assembly (ETA) created to shape a 14-MeV neutron point source to an objective spectrum is presented.
Abstract: This article describes the nuclear data covariance analysis of an experimental design for a neutron energy-tuning assembly (ETA) created to shape a 14-MeV neutron point source to an objective spectrum. Underlying nuclear data uncertainties play a large role in the radiation transport and reaction rates for the range of responses to be expected from an experiment. The methodology leveraged the Standardized Computer Analysis for Licensing Evaluation (SCALE) Sampler module to determine the uncertainty in the neutron transport. The reaction uncertainty was perturbed with the International Reactor Dosimetry and Fusion File v.1.05 uncertainty, correlation matrix, and reaction cross section through multivariate normal distribution sampling to provide a final response metric. The resultant neutron fluence uncertainty for the ETA ranged from 2.7% to 6.2% in the energy range from 1.28 keV to 14.1 MeV, which contains 99.99% of the neutron fluence. The integrated uncertainties, including statistical and systematic nuclear data uncertainties, for the reaction products analyzed were 2.33% to 4.84% for most reactions, but 55Mn(n, $\gamma $ ), a less well-characterized reaction occurring in an energy domain with high flux uncertainty, was 19.7%. The mean of the reaction distributions was within 1.1% of the unperturbed nuclear data simulation. The experiment is planned for late 2019, where the predicted results will be compared against the experimental outcomes. The methodology presented can be utilized with alternate nuclear libraries in SCALE to develop uncertainty bounds and neutron flux spectra for many radiation-transport problems.

6 citations


Cites background from "Performance evaluation of an energy..."

  • ...Each foil has threshold and nonthreshold reactions that span the range of interest for the TN + PFNS energy spectrum, which can be used to unfold the incident neutron energy spectrum to validate the ETA performance [27], [30], [31]....

    [...]

  • ...1, was previously designed to produce a notional thermonuclear and prompt fission neutron spectrum (TN + PFNS) [26], [27]....

    [...]

Journal ArticleDOI
TL;DR: In this paper, an energy tuning assembly created to modify the National Ignition Facility deuterium-tritium fusion neutron source into a notional thermonuclear and prompt fission neutron spectrum, which has applications in integral measurements, nuclear data benchmarks, and radiation effects on microelectronics.

3 citations

Journal ArticleDOI
TL;DR: The ATHENA platform, an energy tuning assembly, was developed to spectrally shape the National Ignition Facility (NIF) deuterium-tritium fusion neutron source to a thermonuclear plus prompt fission neutron spectrum with a capability to act as a short-pulse neutron source.
Abstract: This paper describes the ATHENA platform, an energy tuning assembly, which was developed to spectrally shape the National Ignition Facility (NIF) deuterium–tritium fusion neutron source to a thermonuclear (fusion) plus prompt fission neutron spectrum with a capability to act as a short-pulse neutron source. This unique, otherwise inaccessible radiation environment complements existing experimental facilities and capabilities. The flexible ATHENA irradiation positions were modeled using an ensemble of Monte Carlo simulations with stochastic sampling of the nuclear cross-sections to characterize the radiation environments and uncertainty for the platform. Validation of the internal neutron spectrum produced from fielding ATHENA at NIF occurred through neutron flux unfolding with 20 measured activation products. The STAYSL unfolded neutron flux resulted in a reduced χ 2 value of 1.4 with a larger contribution from 46 Ti(n,2n) reaction channel. The total ionizing dose was measured to be 515 ± 7.9% rad(TLD-400), whereas the modeled values indicated a lower value of 290 ± 4.6% rad(TLD-400). The NIF experiment demonstrated that ATHENA is capable of producing a thermonuclear and prompt fission neutron spectrum with a 50 nanosecond pulse width and a 1-MeV equivalent neutron fluence of 3 . 6 × 1 0 12 n c m 2 with strong radial uniformity over the sample volume. Example case studies of ATHENA for integral experiments and microelectronic device responses are also presented.

3 citations

Journal ArticleDOI
08 Oct 2022
TL;DR: In this paper , the first measurement of the proton light yield of a water-based liquid scintillator (WbLS) formulated from 5% linear alkyl benzene (LAB), at energies below 20 MeV, was performed using a double time-offlight method and a pulsed neutron beam from the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory.
Abstract: Abstract The proton light yield of liquid scintillators is an important property in the context of their use in large-scale neutrino experiments, with direct implications for neutrino-proton scattering measurements and the discrimination of fast neutrons from inverse $$\beta $$ β -decay coincidence signals. This work presents the first measurement of the proton light yield of a water-based liquid scintillator (WbLS) formulated from 5% linear alkyl benzene (LAB), at energies below 20 MeV, as well as a measurement of the proton light yield of a pure LAB + 2 g/L 2,5-diphenyloxazole (PPO) mixture (LABPPO). The measurements were performed using a double time-of-flight method and a pulsed neutron beam from the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory. The proton light yields were measured relative to that of a 477 keV electron. The relative proton light yield of WbLS was approximately 3.8% lower than that of LABPPO, itself exhibiting a relative proton light yield 15–20% higher than previous measurements of an analogous anoxic sample. The observed quenching is not compatible with the Birks model for either material, but is well described with the addition of Chou’s bimolecular quenching term.

2 citations

Journal ArticleDOI
TL;DR: In this paper , a combination of experimental neutron and gamma-ray measurements coupled with GEometry ANd Tracking (Geant4) Monte Carlo radiation transport simulations was used to determine the pulse-integral response calibration, resolution, efficiency, and heavy charged particle light yield functions of boron-loaded deuterated liquid scintillators.
Abstract: This research presents the characterization methods and results for determining the pulse-integral response calibration, resolution, efficiency, and heavy charged particle light yield functions of boron-loaded deuterated liquid scintillators (BLDLS) through a combination of experimental neutron and gamma-ray measurements coupled with GEometry ANd Tracking (Geant4) Monte Carlo radiation transport simulations. These methods were used to characterize the scintillator light output, pulse-shape discrimination performance, and response matrix for a boron-loaded deuterated liquid scintillator to be used for future neutron spectrum unfolding measurements. These results were compared against an un-loaded deuterated liquid scintillator and a commercially-available hydrogen-based liquid scintillator, EJ-309, to benchmark the performance of the methods and scintillators to comparable measurements in literature. EJ-309 was shown to have only a 3.5 ± 2.3% and 8.1 ± 1.4% higher relative light output at 477 keV compared to Tol-D 8 and Tol-D 8 − 10 B, respectively, a significant increase in performance over previous BLDLS results. Additionally, Tol-D 8 and Tol-D 8 − 10 B have similar PSD performance, both exceeding that of EJ-309 as a function of light yield. Finally, full detector response matrices, used for neutron spectrum unfolding, were generated for each scintillator, and the condition of these response matrices were evaluated. The deuterated scintillators resulted in worse conditioned response matrices compared to EJ-309 for neutron energies ranging between 0.5 MeV to 6 MeV, primarily due to having a lower pulse-integral resolution at the smaller detector sizes used in this work. The boron-loaded scintillator shows a similarly conditioned response matrix to the un-loaded variant, thereby lowering the characterizable neutron energy threshold to thermal energies while also enhanced the neutron detection efficiency.
References
More filters
01 Jan 1993
TL;DR: In this article, the authors present a practical guide for the use of general-purpose Monte Carlo code MCNP, including several examples and a discussion of the particular techniques and the Monte Carlo method itself.
Abstract: This manual is a practical guide for the use of our general-purpose Monte Carlo code MCNP. The first chapter is a primer for the novice user. The second chapter describes the mathematics, data, physics, and Monte Carlo simulation found in MCNP. This discussion is not meant to be exhaustive---details of the particular techniques and of the Monte Carlo method itself will have to be found elsewhere. The third chapter shows the user how to prepare input for the code. The fourth chapter contains several examples, and the fifth chapter explains the output. The appendices show how to use MCNP on various computer systems and also give details about some of the code internals.

6,481 citations

01 Oct 1987

706 citations

Journal ArticleDOI
TL;DR: In this paper, the correction factors for the same geometries for the most abundant lines of 18 radionuclides were calculated experimentally, and the correction factor was found to vary between 1.0 and 1.5 depending on the particular decay scheme.

287 citations


"Performance evaluation of an energy..." refers background in this paper

  • ...At large source-to-detector distances, the absolute efficiency is dominated by theEd (Eγ ) term since the foils approximate a point source, G18(Eγ ) ≈ 1, and coincidence summing is negligible Fd (γ) ≈ 1 [39]....

    [...]

  • ...However, at this distance correction factors must be applied to account for the geometry of the source and coincident summing from gamma cascades [38,39]....

    [...]

Journal ArticleDOI
TL;DR: The National Ignition Facility (NIF) successfully completed its first inertial confinement fusion (ICF) campaign in 2009 and a neutron time-of-flight (nTOF) system was part of the nuclear diagnostics used in this campaign.
Abstract: The National Ignition Facility (NIF) successfully completed its first inertial confinement fusion (ICF) campaign in 2009. A neutron time-of-flight (nTOF) system was part of the nuclear diagnostics used in this campaign. The nTOF technique has been used for decades on ICF facilities to infer the ion temperature of hot deuterium (D(2)) and deuterium-tritium (DT) plasmas based on the temporal Doppler broadening of the primary neutron peak. Once calibrated for absolute neutron sensitivity, the nTOF detectors can be used to measure the yield with high accuracy. The NIF nTOF system is designed to measure neutron yield and ion temperature over 11 orders of magnitude (from 10(8) to 10(19)), neutron bang time in DT implosions between 10(12) and 10(16), and to infer areal density for DT yields above 10(12). During the 2009 campaign, the three most sensitive neutron time-of-flight detectors were installed and used to measure the primary neutron yield and ion temperature from 25 high-convergence implosions using D(2) fuel. The OMEGA yield calibration of these detectors was successfully transferred to the NIF.

127 citations


"Performance evaluation of an energy..." refers methods in this paper

  • ...While typical techniques such as time-of-flight [18], proton recoil spectrometers [19], pulse height spectrum unfolding with organic scintillators [20], and capture gated spectrometers [21] will not work, foil activation analysis and unfolding is a viable technique to measure the neutron energy spectrum in the ETA experimental cavity....

    [...]

Journal ArticleDOI
TL;DR: Neutron production from targets of Be, C, Mo, Cu, Ta and Au bombarded with deuterons of 16, 33 and 50 MeV has been studied at the isochronous cyclotron at Louvain-la-Neuve.
Abstract: Neutron production from targets of Be, C, Mo, Cu, Ta and Au has been studied at the isochronous cyclotron at Louvain-la-Neuve. Neutron spectra were measured by the time of flight method. The yields of neutrons and gamma rays were also measured, and the greatest ratio of neutrons to gamma rays in the forward direction was found to occur with 50 MeV deuterons on a Be target. The angular distribution of neutrons from Be was measured at 16, 33 and 50 MeV, and neutron spectra were measured as a function of angle with 50 MeV deuterons on Be.

106 citations


"Performance evaluation of an energy..." refers background or methods in this paper

  • ...While the beam does vary in energy and intensity as a function of angle, previous research has shown the first five degrees to be roughly uniform [32]....

    [...]

  • ...Published neutron spectra for 33 MeV deuteron breakup on tantalum (or similar energies) do not address the low energy component (<3 MeV) necessary for this measurement [30,32], so the measured deuteronbreakup source spectrum from this work was used as the initial source term for the ETA performance simulation....

    [...]

  • ...The neutron distribution is narrows with increasing Z due to the higher dE∕dZ of the heavier target nucleus and the increasing relative fraction of the elastic channels [29,32,33]....

    [...]

Frequently Asked Questions (1)
Q1. What are the contributions in "Performance evaluation of an energy tuning assembly for neutron spectral shaping" ?

The ETA was characterized at the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory to measure the shaped spectrum from an incident deuteron breakup neutron source, test the proposed neutron spectroscopy techniques used to inform the flux measurements at NIF, and validate the ability to predict ETA performance using a Monte Carlo Neutral Particle ( MCNP ) simulation. This work demonstrates shaping of the NIF neutron spectrum via the ETA to be a viable path forward for tailored neutron beams at NIF.