Difference between revisions of "Forest He-3 Tubes"

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Thermal neutron capture of He-3 may be represented by the reaction below
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<math>n + He^3 \rightarrow p + H^3</math>
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About 764 keV of energy is liberated in this nuclear reaction and distributed between the final products according to their masses. 
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;Cons Momentum <math>\Rightarrow</math>
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: <math>m_p v_p = m_{H^3} v_{H^3} \Rightarrow v_p = 3v_{H^3}</math>
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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
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<math>p+He^3 \rightarrow p + He^3(+) + e^-</math>
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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.
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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)
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<math>H^3 \rightarrow He^3 + e^- + \bar{\nu_e}</math>
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reference: J. W. Leake, "Nuclear Instruments and Methods", Vol. 63, page 329, 1968).
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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 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>).
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;Total cross section is defined as
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:<math>\sigma \equiv \frac{\# particles\; scattered} {\frac{ \# incident \; particles}{Area}}</math>
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: <math>\# protons = \sigma \frac{\# incident particles}{Area}</math>
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: <math>= \sigma \rho \times L</math>
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: <math>= ( 3 \times 10^{-24} cm^2) (10 Atm) \left ( 2.7 \times 10^{19} \frac{atoms}{cm^3} \right ) (76 cm) = 0.06 partices \Rightarrow \epsilon = 6</math> %
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where
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:<math>\rho</math> = density of the He-3 target
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: L = length of the target.
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10-atm He-3, 2.54 cm diameter tube, 76 cm long, poly moderator, cadmium metal, Boron loaded shielding.
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[[Image:He-3Tube_DetectorDrawing.jpg]]
 
[[Image:He-3Tube_DetectorDrawing.jpg]]
  
 
[[Image:He-3Tube_PPND_Efficiency.jpg | 200 px]]
 
[[Image:He-3Tube_PPND_Efficiency.jpg | 200 px]]
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[http://www.iac.isu.edu/mediawiki/index.php/Forest_Detectors Go Back]

Latest revision as of 23:33, 22 June 2009

Thermal neutron capture of He-3 may be represented by the reaction below

[math]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.

Cons Momentum [math]\Rightarrow[/math]
[math]m_p v_p = m_{H^3} v_{H^3} \Rightarrow v_p = 3v_{H^3}[/math]

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]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]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]\sigma[/math]) is [math]3.10 \pm 0.13[/math] barns ( 1 barn = [math]10^{-24} cm^2[/math]).

Total cross section is defined as
[math]\sigma \equiv \frac{\# particles\; scattered} {\frac{ \# incident \; particles}{Area}}[/math]
[math]\# protons = \sigma \frac{\# incident particles}{Area}[/math]
[math]= \sigma \rho \times L[/math]
[math]= ( 3 \times 10^{-24} cm^2) (10 Atm) \left ( 2.7 \times 10^{19} \frac{atoms}{cm^3} \right ) (76 cm) = 0.06 partices \Rightarrow \epsilon = 6[/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.

File:He-3Tube DetectorDrawing.jpg

He-3Tube PPND Efficiency.jpg

Go Back

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