I made some measurements recently of the neutron detection yields of radium alpha particles on beryllium, boron, and...Teflon, believe it or not (see photo). The reactions occurring are these:
Be: Be-9(a,n)C-12, average neutron energy around 5 MeV
B: B-11(a,n)N-14, average neutron energy around 3 MeV
Teflon: F-19(a,n)Na-22, average neutron energy around 1.4 MeV
Neutron yields from all three are easy to measure with the superlative He-3 proportional tube and 120-lb paraffin reflector described in my earlier post on spontaneous fission. The radium source used was a 5.6 microcurie Ionotron-type foil strip. Source was simply taped face-down against (a) a piece of beryllium metal, (b) a piece of Teflon, and (c) a shallow tray ~ 2mm deep packed with 98.5% elemental boron, and the whole assembly so formed placed in a 1/8" lead sheet box next to the tube within the reflector. A region of interest windowing the thermal-neutron peak in the He-3 tube spectrum was set up on the MCA, and the following counts were observed in the ROI (rounded to nearest 1/2 significant digit):
Beryllium target: 6820 counts / 6000 sec.
Boron target: 2000 counts / 6000 sec.
Teflon target: 1285 counts / 6000 sec.
No target (i.e. bare source): 675 counts / 6000 sec.
Background: 685 counts / 6000 sec.
Subtracting background, the net counts per 6000 sec. were:
Ra-Be: 6145 +/- 85 counts
Ra-B: 1325 +/- 50 counts
Ra-Teflon: 610 +/- 40 counts
No target: 0 counts
Data on total neutron yield from Glenn Knoll's book for Am-241 neutron sources show a relative yield of 18.6% for B relative to Be, and 5.9% for F relative to Be. To compare, my measured response from boron is 22% that of beryllium, and the response from Teflon, 10%. Note that these are not measurements of total neutron yield; the reactions giving lower-energy neutrons result in higher flux per source neutron near the He-3 tube. However, these measurements are what ultimately matters if one wishes to use such a reflector and proportional tube to investigate low-level matrix reactions.
Note that the Teflon source performs admirably even though it's unit composition is C2F4. In reality, were you to use pure fluorine (god forbid) or BF3 or HF, yield would rise considerably. The same is true with the boron. The isotope responsible for almost all of the neutrons is B-11, and B-10 is actually a nuisance because it absorbs some of these neutrons. So if we use depleted boron rather than natural boron, yields would perhaps almost double!
Lastly, note that the F(n,a) reaction product Na-22 is radioactive, emitting positrons and gammas with a half life of 2.6 years. If I were to store my radium source for the next few years in a Teflon tube, I am certain I'd get a detectable* quantity of this isotope.
Finally, I am making a little paper on the general subject of neutron sources with some more data. However, this won't be a public document, so email if interested.
*On a shielded $30,000 HPGe detector
-Carl
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2006-02-05 21:15 Teflon neutron source 
(Carl Willis) [Latest: 2006-03-05 19:49]