Forest He-3 Tubes: Difference between revisions
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The probability of neutron capture is measured in terms of a cross section. There is a nuclear data base for neutron capture located at [http://ie.lbl.gov/ng.html LBL]. The "free" neutron thermal cross section is [http://ie.lbl.gov/ngdata/sig.htm <math>3.10 \pm 0.13</math>] barns ( 1 barn = <math>10^{-24} cm^2</math>). | The probability of neutron capture is measured in terms of a cross section. There is a nuclear data base for neutron capture located at [http://ie.lbl.gov/ng.html LBL]. The "free" neutron thermal total cross section (<math>\sigma</math>) is [http://ie.lbl.gov/ngdata/sig.htm <math>3.10 \pm 0.13</math>] barns ( 1 barn = <math>10^{-24} cm^2</math>). | ||
;Total cross section is defined as | ;Total cross section is defined as | ||
:<math>\sigma \equiv \frac{\# particles\; scattered} {\frac{ \# incident \; particles}{Area}}</math> | :<math>\sigma \equiv \frac{\# particles\; scattered} {\frac{ \# incident \; particles}{Area}}</math> | ||
: <math># protons = \sigma \frac{\# incident particles}{Area}</math> | |||
: <math>= \sigma \rho \time L</math> | |||
where | |||
:<math>\rho</math> = density of the He-3 target | |||
: L = length of the target. | |||
10-atm He-3, 2.54 cm diameter tube, 76 cm long, poly moderator, cadmium metal, Boron loaded shielding. | 10-atm He-3, 2.54 cm diameter tube, 76 cm long, poly moderator, cadmium metal, Boron loaded shielding. | ||
Revision as of 20:53, 7 June 2008
Thermal neutron capture of He-3 may be represented by the reaction below
[math]\displaystyle{ n + He^3 \rightarrow p + H^3 }[/math]
About 764 keV of energy is liberated in this nuclear reaction and distributed between the final products according to their masses. Because the proton is about a factor of 3 lighter than Tritium (H^3) , it will have more kinetic energy by about a factor of 3 (about 573 keV). This liberated proton can ionize other He-3 atoms via the reaction
[math]\displaystyle{ p+He^3 \rightarrow p + He^3(+) + e^- }[/math]
The same proton will ionize several He-3 atoms when dissipating the 573 keV kinetic energy. Once you have the creation of ions, you can construct detectors to collect and measure the electrons.
The Tritium (H^3) can also ionize the gas but due to its higher mass it does not travel as far (shorter range) as the proton and makes a smaller contribution to the ionization signal. Tritium decays to He-3 after about 12 years ( neutron is converted to proton)
[math]\displaystyle{ H^3 \rightarrow He^3 + e^- + \bar{\nu_e} }[/math]
reference: J. W. Leake, "Nuclear Instruments and Methods", Vol. 63, page 329, 1968).
The probability of neutron capture is measured in terms of a cross section. There is a nuclear data base for neutron capture located at LBL. The "free" neutron thermal total cross section ([math]\displaystyle{ \sigma }[/math]) is [math]\displaystyle{ 3.10 \pm 0.13 }[/math] barns ( 1 barn = [math]\displaystyle{ 10^{-24} cm^2 }[/math]).
- Total cross section is defined as
- [math]\displaystyle{ \sigma \equiv \frac{\# particles\; scattered} {\frac{ \# incident \; particles}{Area}} }[/math]
- [math]\displaystyle{ # protons = \sigma \frac{\# incident particles}{Area} }[/math]
- [math]\displaystyle{ = \sigma \rho \time L }[/math]
where
- [math]\displaystyle{ \rho }[/math] = density of the He-3 target
- L = length of the target.
10-atm He-3, 2.54 cm diameter tube, 76 cm long, poly moderator, cadmium metal, Boron loaded shielding.
File:He-3Tube DetectorDrawing.jpg
