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Need to better understand the relationship between Be as used in:

  • Neutron reflector
  • Neutron moderator
  • Neutron producer

Alpha production

Small scale neutron production, ie for detector demonstration

Am-Be summary

  • Am-241:
      • Rooms Require Posting When There Is Greater Than: 0.01 μCi
      • Seems pretty stringent given smoke detectors have 1 uCi
    • 5.48 MeV α
    • Gamma
      • 26 keV: 2.4%
      • 320 keV: 0.017%
      • 33 keV: 0.126%
    • Dose
      • Oral 1 uC: 5 rem / yr whole body
        • Dose info: 5-20: Possible late effects; possible chromosomal damage
      • 1 μCi over 10 cm2 of skin: 8.87 mrad/hr gamma
  • QSA global: Primary Energy: 2-10 MeV
    • Average: 4 MeV
    • Max: 11 MeV
  • Seal neutron source: “average neutron energy of ~4 MeV”
    • 4α2 + 9Be4 (target) → 12C6 + neutron + 5.71 MeV
  • YouTube video: “Mean energy is 5-6 MeV; the maximum energy something like 12 MeV”

Resulting neutrons:

Easy polonium-beryllium neutron source: assembly, disassembly, and discussion

  • Watch it
  • Neutrons
    • “Mean energy is 5-6 MeV; the maximum energy something like 12 MeV”
  • Moderator/reflector: (high density) polyethylene
  • Beryllium
    • “Minimum Be thickness from a nuclear perspective would be on the order of 40 micrometers, which is the range of alpha particles in the beryllium. Practically, it could be much thicker”
    • ~$50
    • Used a thick slab (0.25“ or so)
  • Detector: He-3 proportional tube
    • Model?
    • Background: 1 CPS
  • Meter: Ludlum 12 ratemeter
  • Source: 5 mCi Po-210
    • Nuclespot (amstat?)
    • $175/yr
    • Polonium from (Myak?) Russian plant
    • “Tried it with AM241 source (a matrix of 6 of them – not as strong as yours) and a He3 detector tube (running at about 2400V). At first, nothing happened. I found that I needed to carefully adjust the gain on the input amplifier on the Ludlum 12. Finally I got about 60 neutrons as background count. When I put the Be on top of the Am241 source the counts went up to about 120 -140 CPM”


  • “Neutron oven”
  • “5 mC AmBe source made with many parts of old fashioned smoke detectors and Be foil”
  • Slowed down with polyethylene
  • Sealed with “plastic plug”

241 AmBe Sealed Neutron Source Assessment Studies for the Fissile Mass Flow Monitor

  • “standard size U.S. Am-241 Be source (3 Ci in a 0.75-in.-diam, 2-in.-long double-sealed stainless steel capsule)…provides ~6.6 × 10 6 neutron/s”
  • 1 uC: 1e-6 / 3 * 6.6e6 = 2.2 n/s
  • 80 uC:176 n/s
  • 800 uC: 1760 n/s
  • High end hobbyist fusors are well above this
  • TBe?


He-4 + Be-9 ⇒ C-12 + n

Scattering SW



  • Has a bunch of background on more conventional sources
  • U-232 is usually a highly undesirable byproduct of making U-233
  • “It was found early on that beryllium had the best neutron yields of the light elements. Therefore nearly all isotopic neutron sources after the 1950’s were a combination of an alpha emitter and beryllium. However some isotopic neutron sources used fluorine, boron or lithium instead of beryllium.”
  • “The yields of Table 3 assumes a spherical ( γ ,n) source 2.38 cm in diameter, and a 3.2 mm thick blanket of target (Be or D) material.”
    • Was afraid a thick Be slap would completely moderate/stop most neutrons. Probably not?


  • graphite, beryllium, steel, tungsten carbide, or other materials
  • A reflector made of a light material like graphite or beryllium will also serve as a neutron moderator reducing neutron kinetic energy, while a heavy material like lead or lead-bismuth eutectic will have less effect on neutron velocity

PANDA: interaction with matter

  • The physical cross-sectional area s of a heavy nucleus is about 2e-24 cm2. Interaction cross sections for most nuclei are typically between 1e-27 and 1e-21 cm2. To avoid the inconvenience of working with such smaller numbers, a different unit of area is used: the barn (b). It is defined to be 1e-24 cm**2 so that the physical cross sectional area of a heavy nuclear is about 2 b


Good resources:

  • LANL Passive Nondestructive Assay (PANDA) Manual: Link
    • One of the few places that has practical numbers
  • Understand PMTs

Gas tubes

Typically either He-3 or BF3 as the main fill gas. In more detail: “Gas-filled detectors typically employ He-3 He-4, BF3, or CH4 as the primary constituent, at pressures less than 1 to about 20 atm…A gas such as argon can be used to … [improve] the output pulse-height resolution…” [PANDA 384]

“A polyatomic gas may also be added to proportional counters to serve as a quench gas. …Gases such as BF3 and CH4 are already polyatomic gases and require no additional quench gas. Tubes filled with He-3 and He-4 often have a small quantity of CH4 or CO2 added.” [PANDA 384]

“If little or no voltage is applied to the tube, most of the ions will recombine and no electrical output signal is produced. If a positive voltage is applied to the central wire (anode) the electronics will move toward it and the positively charged ions will move toward the tube wall (cathode). An electrical output signal will be produced whose magnitude depends on the applied voltage, the geometry of the counter, and the fill gas.” [PANDA 381]

Why can't you switch positive and negative? Wouldn't the electrons just hit the walls instead?

Important point above: the strikes cause create charge, canceling out some of the electric field. This results in a decrease in current (negative pulse). If the applied voltage was very high though it would breakdown and increase current?


neutron.txt · Last modified: 2015/12/30 00:29 by mcmaster