Sources of γ rays sterilize objects, measure thicknesses without contact, display the contents of closed containers, etc. But radionuclides like 60
Co are also a danger. A switchable γ source
could be useful. As usual, I didn't check what is already done or abandoned. Nor am I reliable on the topic.Bombardment by protons or deuterons
lets some targets emit γ.
- It often produces β+ emitters. When the 511keV photons from positron-electron annihilation are a drawback, shielding them away lets waste many 2MeV photons. Very few nuclides disintegrate by electron capture without β+.
- Some reactions create β- emitters, typically by deuteron absorption and proton emission. The β braked by surrounding matter (Bremsstrahlung) creates photons rarely useful that can be minimized. The disintegration often emits γ rays of single or several energies by internal transition. If the β- emitter is short-lived, the γ emission stops some time after the protons or deuterons beam. But seconds would be better than minutes.
- A beam could first produce neutrons whose absorption creates β- emitters. But neutrons take decimetres to brake, and a wide source makes fuzzy γ images. Neutrons tend to activate all materials, possibly for a long time. And they need strong protons but the double conversion is inefficient. I didn't insist.
- A few reactions just absorb a weak proton or deuteron and emit a strong photon, immediately at human scale. Most promising.
Among these fourth-type reactions, in this message I consider 19F(p,γ)20Ne
. For chemists: 19
F + proton 20
Ne + γ
Data is gratefully taken from the Janis books protons
(8MB) page 62 - deuterons
Natural fluorine is pure 19
Ne is stable. I understand the γ carries 12844keV reaction energy plus 19/20 of the incident proton contribution: an energy not available from radioactivity, twice as penetrating as 1.33MeV from 60
Co, for instance to measure thicker red-hot steel plates when rolled.
The first competing reaction is on p63 in the Janis book, with similar sections over the energy range 19
The product is stable and the α stops within the source. If emitting no additional γ, this reaction looks harmless and acceptable.
The next competing reaction is on p59 19
which is 4MeV endothermal, so a smaller proton energy prevents it and the others.
The useful reaction has a measured section around 40mb from 1.4 to 0.3MeV
. Target fluorine can be AlF3
since the Janis book lists no reaction of Al active at these energies. NaF and MgF2
seem clean too. 13
N from (CF2
would emit 1/1000 γ at 511keV with 10min half-life. Very pure 11
B is less convenient. Li and Be emit other radiations.
32%wt Al and 68%wt F brake 1.4MeV protons over 44g/m2
, that is 30g/m2
F, as deduced from physics.nist.gov
and then 40mb section let 3.8ppm of all protons create a γ, so a 1.4MeV 1.6mA 2.2kW proton beam emits 1Ci
. Maybe protons up to 4MeV improve the yield but the Janis book lacks experimental data, and 1.4MeV accepts a small cyclotron
, or maybe a linear accelerator with radiofrequency quadrupoles (RFQ).
Aluminium or other, possibly cooled by water, can support AlF3
of which 10-20µm stop the protons. The two transmutations deplete F in few years of continuous 1Ci operation, faster degradation processes are plausible.
Marc Schaefer, aka Enthalpy