Few-Body Syst (2013) 54:1599–1602
DOI 10.1007/s00601-012-0524-x
A. D. Bacher · D. T. Casey · J. A. Frenje · M. J. Gatu Johnson
M. Manuel · N. Sinenian · A. B. Zylstra ·
F. H. Séguin · C. K. Li · R. D. Petrasso · V. Yu Glebov ·
P. B. Radha · D. D. Meyerhofer · T. C. Sangster ·
D. P. McNabb · P. A. Amendt · R. N. Boyd · J. A. Caggiano ·
S. P. Hatchett · J. E. Pino · S. Quaglioni · J. R. Rygg ·
I. J. Thompson · H. W. Herrmann · Y. H. Kim
T–T Neutron Spectrum from Inertial Confinement
Implosions
Received: 23 October 2012 / Accepted: 4 December 2012 / Published online: 23 December 2012
© Springer-Verlag Wien 2012
Abstract A new technique that uses inertial confinement implosions for measuring low-energy nuclear reac-
tions important to nuclear astrophysics is described. Simultaneous measurements of n–D and n–T elastic scat-
tering at 14.1 MeV using deuterium–tritium gas-filled capsules provide a proof of principle for this technique.
Measurements have been made of D(d,p)T (dd) and T(t,2n)4He (tt) reaction yields relative to the D(t,n)4He
(dt) reaction yield for deuterium–tritium mixtures with f T /f D between 0.62 and 0.75 and for a wide range
of ion temperatures to test our understanding of the implosion processes. Measurements of the shape of the
neutron spectrum from the T(t,2n)4He reaction have been made for each of these target configurations.
1 Introduction
A new technique is being developed for measuring low-energy nuclear reactions important to nuclear astro-
physics using laser-driven inertial confinement fusion (ICF) experiments at the University of Rochester’s
OMEGA Laser Facility [1] and Lawrence Livermore National Laboratory’s National Ignition Facility (NIF)
[2]. This technique produces thermal plasma environments that are very different from accelerator experiments
with a beam and target. Since the reactants are ionized and the electrons are in continuum states, the plasma
closely resembles the burning core in a star with negligible electron screening corrections. Neutron fluxes are
extremely high and the experiments are very short (1 ps–10 ns) so radioactive backgrounds contribute less. On
the other hand, the plasma environment is complex and not completely understood. And because the duration
is so short, there is no possibility to measure traditional time-domain coincidence events.
A. D. Bacher (B)
Department of Physics, Indiana University, Bloomington, IN 47405, USA
E-mail: bacher@indiana.edu
D. T. Casey · J. A. Frenje · M. J. Gatu Johnson · M. Manuel · N. Sinenian · A. B. Zylstra · F. H. Séguin · C. K. Li · R. D. Petrasso
Plasma Science and Fusion Center, MIT, Cambridge, MA 02139, USA
V. Y. Glebov · P. B. Radha · D. D. Meyerhofer · T. C. Sangster
Laboratory for Laser Energetics, University of Rochester, Rochester, NY 14623, USA
D. P. McNabb · P. A. Amendt · R. N. Boyd · J. A. Caggiano · S. P. Hatchett · J. E. Pino · S. Quaglioni · J. R. Rygg · I. J. Thompson
Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
H. W. Herrmann · Y. H. Kim
Los Alamos National Laboratory, Los Alamos, NM 87545, USA
1600 A. D. Bacher et al.
With deuterium–tritium (DT) capsules, the primary fusion reaction is D(t,n)4He (or dt). A fusion reaction of
special interest is T(t,2n)4He (or tt), the charge symmetric reaction to the stellar 3He(3He,2p)4He (or 3He3He)
reaction, the dominant energy-producing step in the solar proton-proton chain. Information about the tt three-
body reaction mechanism is contained in the spectral shapes of the final-state particles, neutrons and alphas. For
example, the tt neutron spectrum is a broad continuum modified by the n-n and n-4He final-state interactions
corresponding to the di-neutron+4He and n+5He reaction channels.
2 Experimental Considerations
All of the measurements were carried out at the University of Rochester’s OMEGA Laser System. This 60-
beam, 30-kJ facility is ideal for developing new techniques since it can fire about a dozen shots each day. In
these experiments, deuterium–tritium (DT) gas-filled, spherical capsules were symmetrically irradiated with
powerful lasers to compress and heat the fuel to high enough temperatures and densities for significant nuclear
fusion reactions to occur. To study the ICF plasma conditions, the spectral features of the reactant products
were measured using magnet-based nuclear diagnostics consisting of a permanent magnet for dispersion and
pieces of CR-39, a plastic polymer insensitive to the electromagnetic transients in an implosion, positioned
in the focal plane to detect and identify momentum-analyzed particles. The more compact charged-particle
spectrometer (CPS) [3] was used in the 14.1 MeV neutron scattering measurements and the larger magnetic
recoil spectrometer (MRS) [4] was used with a recoil foil for absolute neutron spectrum measurements.
For DT capsules, the spectra of reactant particles contain both discrete lines and continua. At thermal
energies the primary fusion reaction D(t,n)4He produces a 14.1 MeV neutron and a 3.5 MeV 4He. The position
of this neutron peak is an energy calibration and its width is related to the ion temperature of the DT burn. The
mid-energy (10–13 MeV) portion of the neutron spectrum is dominated by down scattering of the 14 MeV
neutrons due to elastic and inelastic scattering or (n,2n) reactions on the capsule shell or on H, D or T ions in
the plasma. At OMEGA, the mean-free-path for neutrons in the capsule is long enough that a neutron has about
a 1 % chance of scattering before escape. The spectrum can be approximated by a single scattering model and
the amount of down-scattered neutrons is a direct measure of areal density < ρR > in the capsule, a measure
of implosion performance [5,6].
Simultaneous measurements of n–D and n–T elastic scattering were made using these 14.1 MeV dt fusion
neutrons scattering from D and T ions in the ICF plasma to make the first nuclear cross section measurement
using an ICF implosion [7]. The spectra of knock-on deuteron (d ′) and triton (t ′) ions were detected in a CPS
and the ratio of the CR-39 track-identified (d ′) and (t ′) spectra has been determined. The differential cross
sections were obtained by deconvolving the CPS-spectrometer response and the Doppler-broadened 14.1 MeV
neutron spectrum. One can then use accurate (in this case Faddeev) calculations of the reference n–D elastic
scattering at 14.1 MeV [8], to extract precise values for the n–T elastic scattering at 14.1 MeV from the ratio.
3 Experimental Results
For experiments with D–T capsules at OMEGA, the spectra of the fusion products from the three fusion
reactions [D(d,p)T (dd), T(t,2n)4He (tt), and D(t,n)4He (dt)], carry information about the implosion core and
can be used to diagnose the underlying implosion physics [9]. For the dd reaction, the reaction yield (Ydd ) and
the absolute dd proton spectrum were measured by two charge particle spectrometers (CPSs). For the more
numerous and more energetic neutrons from the dt reaction, the reaction yield (Ydt ), the absolute dt neutron
spectrum, and the ion temperature were measured by a suite of neutron time-of-flight (nTOF) detectors. For
the tt reaction [10], the reaction yield (Ytt ) and the absolute tt neutron spectrum were measured by the MRS by
converting the neutrons incident on a CD2 foil positioned close to the implosion into elastically scattered recoil
deuterons. The CR-39 array recorded the deuteron spectrum which was then used to determine the original
neutron spectrum.
This neutron spectrum is dominated by the large peak of dt neutrons near 14 MeV and their associated
down scattered neutron (DSn) spectrum visible only in a narrow window just below the dt peak. In the recoil
deuteron spectrum, counts from the tt reaction rise above a DSn background with a signal to noise ratio between
1.4 and 3.7. In the fitting process [10], a modeled neutron spectrum is folded with the MRS-response function
and the magnitude of the DSn spectrum is used as a fit parameter, while the shape of the DSn spectrum is
defined by the fuel and shell (mainly D, T, and C). The tt neutron spectrum is obtained by subtracting the
T–T Neutron Spectrum from Inertial Confinement Implosions 1601
best-fit contribution from DSn and then converting the recoil-deuteron energy to neutron energy through the
MRS-response function. To improve the statistical accuracy, the final tt neutron spectrum is the average of six
different series of implosions.
In this combined ICF spectrum at an average Ec.m. of 23 keV, within the MRS resolution and other
uncertainties, there is no sign of a peak near a neutron energy of 8.5 MeV corresponding to the tt reaction
channel n + 5He(g.s.). This is in sharp contrast with accelerator measurements at higher energies, Ec.m. of
110 keV and 250 keV, where the size of the n + 5He(g.s.) reaction channel is about 5 and 20 %, respectively.
This indicates that the tt reaction mechanism is changing as the reaction energy is lowered. To improve upon
these measurements at OMEGA, implosions of pure T2 gas will greatly reduce the dt yield and the DSn
background. Understanding the reaction mechanism is important for the tt reaction because it impacts the
areal density measurement, a result of immediate relevance at NIF. For the charge symmetric 3He3He reaction,
a changing reaction mechanism at energies near the Gamow peak in the sun is of great interest.
The extensive measurements that have been made of dd and tt reaction yields relative to the dt reaction yield
for deuterium–tritium mixtures with f T /f D between 0.62 and 0.75 and for a wide range of ion temperatures
are providing an important test of our understanding of the implosion processes [9]. Using the reaction yields
described above and our measurements of the ion temperature Ti , the yield ratios (Ydd )/(Ydt ) and (Ytt )/(Ydt )
have been plotted as a function of Ti . For the measurements described here, the ion temperatures range from
Ti = 9–18 keV for dd/dt and from Ti = 2–15 keV for t t /dt . For (Ydd )/(Ydt ), the yield ratios are suppressed
relative to the expected value by a factor of 1.5–2, possibly indicating a lower deuterium fraction in the core
than expected. For (Ytt )/(Ydt ), the yield ratios are 3–6 times higher than the expected value, also possibly
indicating a lower deuterium fraction in the core than expected. It appears that these anomalous ratios could
be induced by deuterium leaving the center of the implosion, an effect referred to as stratification of the
fuel [9].
4 Summary and Conclusions
A new technique is reported for measuring low-energy nuclear reactions important to nuclear astrophysics. The
first application measured n–D and n–T elastic scattering at a neutron energy of 14.1 MeV and the n–T results
are as good as our ability to calculate the n–D reference reaction using modern techniques. In the future, the
plasma environments at NIF will allow us to probe a number of new degrees of freedom in nuclear reactions
and nuclear-atomic interactions.
Measurements of the shape of the neutron spectrum for the tt reaction have been used to estimate the role
of the n + 5He(g.s.) reaction channel in both accelerator experiments and ICF implosions. It is apparent that
the tt reaction mechanism is changing as the reaction energy is lowered, but this needs to be verified by using
implosions of pure T2 gas filled capsules. Measurements of dd and tt reaction yields relative to the dt reaction
yield suggest new and exciting plasma physics phenomena with direct relevance to ICF. The most important
impact of this work may be the ways in which we can use known nuclear physics to help us sort out the complex
behavior occurring during the ICF implosions.
Acknowledgements The authors thank the OMEGA operations, engineering, and scientific staff who supported this work at every
level. We also thank G. Hale for valuable discussions. This work was supported in part by the US DOE (DE-FG52-09NA29553),
NLUF (NA0000877), FSC (Rochester Subaward PO No. 415023-G, UR Account No. 5-24431), LLE (412160-001G), LLNNL
(B580243), and prepared in part by LLNL (under DE-AC52-07NA27344).
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