»Ê¹Ú²©²ÊÍøÖ·

Nuclear Diagnostic Experiments and Calibrations

A Proposal Submitted to the Lawrence Livermore National Laboratory by Stephen Padalino of the Nuclear Physics Laboratory At the State University of New York »Ê¹Ú²©²ÊÍøÖ·, New York May 11, 1999. For the period June 1, 1999 to June 1, 2001 LLNL Collaborator: Craig Sangster

Purpose

The purpose for this proposal is to establish an ongoing collaborative effort between our research group and LLNL scientists and to perform ICF research at Livermore on a regular basis. In order to support this collaboration we are requesting funds for travel, lodging and student stipends. It should be noted that no indirect or overhead costs are associated with this grant and that the full funded amount will be used directly for the project.

Introduction

During the past year faculty and students at the Nuclear Physics Laboratory, (NPL) at the State University of New York in »Ê¹Ú²©²ÊÍøÖ·, worked with collaborators at LLNL and LLE to develop nuclear diagnostic measurements and calibrations at Omega and NPL. Below is a list of a few of these projects, which are currently underway.

• Tertiary neutron yield measurements using carbon activation

SUNY/»Ê¹Ú²©²ÊÍøÖ·: Stephen Padalino, Heather Oliver and Joel Nyquis
LLE Collaborators: Stan Skupsky, Vladimir Smalyuk, and Rahda Bahukutumbi
LLNL Collaborator: Craig Sangster

• CPS Burn Time Evolution Detector

SUNY/»Ê¹Ú²©²ÊÍøÖ·: Stephen Padalino, Kurt Fletcher, Sarah Thompson and Brook Schwartze
LLE-MIT Collaborators: John Soures, Richard Petrasso, Chuck Sorce,  and Sam Roberts
LLNL Collaborator: Craig Sangster

• MEDUSA neutron detector elements calibrations ( DD neutrons)

SUNY/»Ê¹Ú²©²ÊÍøÖ·: Stephen Padalino, Heather Oliver and Joel Nyquist
LLE Collaborators: Vladimir Smalyuk
LLNL Collaborator: Craig Sangster

It is further proposed to perform additional measurements at LLNL and LBNL and advance the work of carbon activation and MEDUSA detector calibrations.

• Calibration measurement of carbon at RTNS LBNL

SUNY/»Ê¹Ú²©²ÊÍøÖ·: Stephen Padalino, Heather Oliver and Joel Nyquist
LLNL Collaborator: Craig Sangster

• MEDUSA neutron detector elements calibrations ( DT neutrons)

SUNY/»Ê¹Ú²©²ÊÍøÖ·: Stephen Padalino, Heather Oliver and Joel Nyquist
LLE Collaborators: Vladimir Smalyuk
LLNL Collaborator: Craig Sangster

Tertiary neutron yield measurements using carbon activation

SUNY/»Ê¹Ú²©²ÊÍøÖ·: Stephen Padalino, Heather Oliver and Joel Nyquist
LLE Collaborators: Stan Skupsky, Vladimir Smalyuk, and Rahda Bahukutumbi
LLNL Collaborators: Craig Sangster

 

Tertiary neutron yield (TNY) measurements, given their obvious sensitivity to (r r) ², are of great interest to the theorists. TNY measurements have been attempted on LLE shots that had primary DT neutron yields in excess of 10 ¹³ neutrons, with densities great enough to produce tertiary neutrons. Measurements done by our group thus far at LLE using carbon activation has produced inconclusive results and further work needs to be performed. Even though the experimental method is straightforward it is difficult to analyze the data in practice. During an ICF reaction, 14.1 MeV neutrons emitted from the T(d,n) fusion reaction strike fuel deuterons causing them to accelerate. These deuterons (or tritons) then collide with tritium (or deuterium) respectively, to produce tertiary T(d,n) reactions that emit high energy neutrons in the range of 18 to 30 MeV. A pure carbon sample is placed near the reaction where it becomes activated through the ¹²C(n,2n) ¹¹C reaction which has a high neutron threshold (around 22 MeV shown below) and can not be activated by the primary neutrons. The ¹¹C consequently beta decays by emitting positrons. Once activated, the sample is removed from the reaction area. NaI detectors are then used to count, in coincidence, the back to back 511 keV gamma rays emitted from the positron annihilation. The number of gamma rays counted is directly related to the tertiary neutron yield of the fusion reaction.

 

The chief concern of using this method arises from contamination of the graphite with materials that will be activated by the primary 14 MeV neutrons via the (n,2n) reaction. Since there are 10 ⁷ more primaries than tertiaries produced during the burn any contaminant, even at a level of 1 PPM, that produces radioactive products that emit positrons can severely complicate the analysis of the decay curve for ¹¹C. Stable nuclei such as copper, nitrogen and oxygen become activated and then emit positrons thus convoluting the decay spectrum. Many of these contaminants have been investigated through the use of trace elemental analysis at the Cornell Research reactor. Reactor measurements at Cornell showed that high purity graphite obtainable from Bay Graphite Corporation is acceptable for TNY measurements via neutron activation. Many contaminants were identified in the graphite, such as Vanadium.

 

 

After a 2 hour thermal neutron activation of the graphite sample, in the Cornell research reactor, the above decay curve was measured by detecting 1434 keV gamma rays emitted from the decay of Vanadium. Vanadium is found in this sample at levels of less than 1 PPM but greater than .01 PPM. This would be an unacceptable level if Vanadium was a positron emitter however, given that its half life is long and it is an electron instead of positron emitter it does not pose a counting problem when compared to the short lived ¹¹C positron decay. There are other contaminants which are positrons emitters and have half-lives of the same order as ¹¹C. These do present a problem!

 

Recent measurements performed at LLE produced interesting results, which require further study. A high purity graphite disk that was covered in plastic to reduce external-surface contamination was activated with a shot that had a primary D-T yield of 10 ¹³ neutrons. The resulting decay spectrum was obtained for the carbon.

To fit the data a double exponential growth curve was required. The sum of the carbon and nitrogen growth curves used the appropriate decay constants based on the 9.97 min half-life for ¹³N and the 20.39 min half-life for ¹¹C. The nitrogen component was greater than the carbon contribution by at least a factor of 2. This demonstrated that, after background subtraction, approximately 25 +/- 3 counts of the 35 counts measured in 3000 seconds were produced from ¹³N. Nitrogen – 13 can be produced via the ¹⁴N(n,2n) ¹³N reaction (9 mB @ 14 MeV) but was most likely produced by the ¹²C(p,gamma) ¹³N reaction which has a cross-section of ~2.5 barns at 14 MeV. It is believed that recoil protons from elastically scattered 14 MeV neutrons in the shrink-wrap produce fast protons via the p(n,p) reaction. Protons with energies less than or equal to 14 MeV then penetrate the carbon producing ¹²C(p,gamma) ¹³N reactions. Nitrogen –13 has a 9.97 min half-life thus dramatically effecting the growth curve for carbon-11 which has a 20.39 min half-life.

 

A table of possible reactions, which can interfere with this method, is given below. A second attempts to protect the carbon from external-surface contamination also met with failure. Silicon Carbide was used to seal the graphite sample during manufacture. The sample was also placed in shrink-wrap. The combination of shrink-wrap and Silicon produced other nuclear reaction to occur that ultimately generated positrons. These reactions can be seen in the table below.

Reaction	X-sec	Energy range	Half Life
C12(n,2n)C11	5-30 mb	22-35 MeV	20.3 min
C12(p,G)N13	2-2.7 b	5-15 MeV	9.9 min
N14(n,2n)N13	9 mb	14 MeV	9.9 min
Cu63(n,2n)Cu62	400 mb	14 MeV	9.7 min
Si30((p,n)P30	?		2.5 min
Si29(p,G)P30	?		2.5 min
O16(n,2n)O15	1.5 mb	17 MeV	2.01 min
N14(p,G)O15	?		2.01 min

It is proposed that further studies be performed at the RTNS to determine the best manner of protecting the carbon samples from external-surface contamination with out introducing (p,n) reactions in the carbon.

 

CPS Burn Time Evolution Detector

 

SUNY/»Ê¹Ú²©²ÊÍøÖ·: Stephen Padalino, Kurt Fletcher, Sarah Thompson and Brook Schwarze

LLE-MIT Collaborators: John Soures, Richard Petrasso, Chuck Sorce, and Sam Roberts

LLNL Collaborators: Craig Sangster

 

It is proposed that a Burn Time Evolution (BTE) detector be built for the Charged Particle Spectrometer (CPS). The purpose of the BTE would be to count the number of knock on particles emitted as a function of time during the burn. The entire counting process would occur between 500 and 3000 pico-sec.

One design being considered uses PIN diode detectors similar to the setup used on the petawatt laser at Livermore.

 

 

A second possible detector design and electronics setup is shown below. The BTE would be mounted on one of the fingers in the existing CPS. The detector would be constructed of a carbon foil, which would produce electrons when struck by knock on fuel nuclei. A micro-channel plate would amplify the ejected electrons and the resulting electron cascade would be collected by a plate or plates at the back of the detector and amplified with a fast preamp or fast preamp array. The signal would then be recorded with a fast multi channel oscilloscope. This system is analogous to a framing camera. But would detect charged particles instead of x-rays.

MEDUSA neutron detector elements calibrations (DD-neutrons)

Stephen Padalino, Heather Oliver and Joel Nyquist
LLE Collaborators: Vladimir Smalyuk
LLNL Collaborators: Craig Sangster

Over the past year several projects have