Question

5. Explain how an Am-Be neutron source works. Write down the complete reaction equation, calculate the Q-value and maximum neutron energy for the (a, n) reaction involved. Is there a threshold energy for this (a, n) reaction, and if so what is it?

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Answer 1

part 1- explain how an Am-Be neutron source works?

Answer-

A neutron source is any device that emits neutrons, irrespective of the mechanism used to produce the neutrons. Alpha neutron sources are the most commonly encountered type of neutron source. An alpha emitter is intimately mixed with a low Z material, usually Be-9. Alpha emitters used in neutron sources include Am-241, Pu-238, Pu-239, Po-210, Ra-226. For an AmBe neutron source, we use Am-241 and Be-9. the process for AmBe neutron source to work is as follow-

The alpha emitter(Am-241)and beryllium(Be-9) must be in intimate contact, e.g., by mixing powdered beryllium metal with an oxide of the alpha emitter. This mixture is then compressed into a cylindrical Alpha Neutron Sources ​ (figure1), shape for encapsulation. Another approach is to employ a metallic alloy of the beryllium and the alpha emitting actinide.fig1

The source is doubly encapsulated. The inner and outer capsules are usually fabricated of stainless steel Alpha Neutron Sources fabricated of stainless steel and the end caps are TIG welded. Space is left within the inner capsule to allow for the gradual buildup of helium that results from the alpha emissions.

When the alpha particle pass through the air, its energy decreases, so, in certain energy and distance, the cross section of above reaction increases and the resonance happens, therefore technical restriction and facilities, the distance is opimised to get high alpha radiation flux. In short, the physical mechanism of this is that an energetic alpha particle (produced by an emitter nuclide) breaks through the Coulomb barrier of the nucleus of a second nuclide (called the converter nuclide) and is absorbed. Subsequently, the compound nucleus emits a neutron. in this way AmBe neutron source works.

part2- write down the complete reaction equation, calculate the Q-value and maximum neutron energy for the (alpha,neutron) reaction involved.

answer-

the reaction for AmBe neutron source- Be-9 + alpha -> C-12 + neutron + 4.44 MeV (gamma)

Q-value for the reaction is( from the above reaction)=4.44MeV

maximum newtron energy is 11MeV.

Part3- Is there a threshold energy for this (alpha, neutron) reaction?

Answer- yes, there is a threshold energy. It is 2.6 MeV.