WEBVTT

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In the laser direct
drive ICF experiments performed

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at the OMEGA laser neutrons are
generated at the center of the

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target chamber and are measured
by several nTOF detectors

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fielded along multiple lines of
sight as shown in the figure

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below. These detectors have been
built along multiple quasi

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orthogonal lines of sight to
provide unique measurements of

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the neutron energy spectrum
along different directions.

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These detectors are useful in
diagnosing 3D asymmetries that

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might be present in the ICF
experiment, which can lead to

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asymmetries in the apparent ion
temperature or areal. Density of

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the implosion. To energy
spectrum emitted from an ICF

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target contains information on
the fusion yield the hotspot and

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temperature and the compressed
fuel areal density. In

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particular, the thermonuclear
neutrons that are generated

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through the primary DT fusion
reactions occurring within the

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hotspot generates a narrow
spectrum of neutrons centered

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near 14 MeV. The area under the
DT fusion peak can be used to

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infer the number of dt fusion
reactions while the width of the

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spectrum can be used to infer
the ion temperature of the

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hotspot. There are

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also thermonuclear neutrons that
are generated through the

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primary DD fusion reactions.
Similarly, by measuring the area

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under the DD peak and the width
of the DD peak, the total number

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of DD fusion reactions and the
hotspot ion temperature can be

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inferred.

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Additionally, a fraction of the
neutrons born within the hotspot

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will scatter off the dense fuel
layer and produce a broad

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scattered neutron spectrum. By
measuring the number of

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scattered neutrons in an
experiment, the areal density of

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the target can be inferred.

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nTOF detectors work by
converting the time flight

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spectrum into a neutron energy
spectrum. This movie shows the

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neutron flux in space at
different snapshots in time.

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Initially, all neutrons are
emitted from the target which is

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labeled as distance equal to
zero on the plot. As time

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proceeds the fast DT neutrons
begin to outrun the slower DD

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and scatter neutrons. The DT
neutrons arrived first at our

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detector which is indicated by
the red band and are followed by

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the slower lower energy
scattered and DD neutrons. And

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nTOF detectors measure the flux
of neutrons arriving at the

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detector as a function of time.
When the neutrons reach our

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detector. They interact with a
scintillating material which is

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coupled to a photomultiplier
tube also known as a PMT. The

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PMT converts the simulation
photons into an electrical

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signal that can be measured with
a digitizer. The digitizer

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creates the resulting time of
flight spectrum. From the time

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of flight spectrum shown on the
right hand side we again see

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that the DT neutrons arrive at
the detector earliest in time

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followed by the slower scattered
and DD neutrons. Because the

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distance to the detector is
known, the velocity and

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therefore the energy of the
neutrons can be inferred, thus

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converting the time of flight
spectrum into the neutron energy

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spectrum. There are many
different neutron detection

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technologies that can be used as
nTOF detectors including

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semiconductor based Cherenkov
radiators or scintillators. By

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far, the most common neutron
detector technology used in ICF

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experiments rely on
scintillators made of a

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hydrocarbon. One such detector
on OMEGA is a xylene based

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simulator coupled to four
photomultiplier tubes. This

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detector is paired with oxygen,
which suppresses the long

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scintillation decay that
naturally occurs in xylene and

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results in a rapid instrument
response. The large volume of

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this detector enables the
measurement of low yield neutron

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signals. Having for PMTs allows
for a high dynamic range of

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measurement. This detector can
measure the entirety of the

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neutron energy spectrum.
Conversely, the pedal detector

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is a quench plastic scintillator
that is only half a centimeter

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thick. By using such a thin
scintillator the instrument

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response function is
significantly faster than xylene

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making it best for measuring the
narrow DT peak. The last

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detector is a turned off
radiator coupled to a PMT. The

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fast response of the radiator
has a superior instrument

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response and is compared to
simulator based detectors.

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However, this detector requires
sufficiently high yields to

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produce a measurable signal.

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To minimize neutron backgrounds
in the nTOF signals

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shielded column aided lines of
sight are required. On OMEGA

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there are two shielded column
added lines of sight located

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along the P seven and H 10
diagnostic ports.

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After an implosion neutrons are
generated at the center of the

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target chamber and are emitted
in all directions. The first

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column made a neutron beam goes
through port P seven. The

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detectors along this line of
sight are located 13 meters away

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from the target. Once the
neutrons exit the target chamber

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they pass through the column as
shown here. This collimator is

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made of high density
polyethylene. Additional column

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ation is provided by a hole that
has been bored through thick

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cement floor. The detectors are
located on the other side

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beneath the floor of the target
Bay. Finally the neutrons hit

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the detectors and data is
recorded on a digitizer

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the second collimator neutron
beam goes through Port H 10. The

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detectors along this line of
sight are located 22 meters away

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from the target chamber. A hole
has been born through the cement

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wall before reaching the final
collimator shown here is the

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final collimator and the
detectors at the end of the line

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of sight. Important ICF
quantities that can be inferred

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from the data are shown on this
digitizer including the fusion

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yield hotspot ion temperature
and the compressed fuel areal

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density