The BF3 counter for neutrons is one of the oldest of neutron counters.
It is still around today and with good reason.
In its early years (1940s) it was a rare and cumbersome bird often weighing in at 200 lbs with the minimal electronic support devices. Needless to say, only the government and research institutions had or could afford them.
The device relies on the large cross sectioned B10(n,a)Li7 reaction for THERMAL neutrons. This is a one for one correspondence device with ideal efficiencies for thermal neuts peaking in the 15% range in higher pressure tubes. the median pressures found in these tubes is one atmosphere. Typical operating voltages vary from 1600 volts for a 3/4 atmosphere tube to 4500 volts for bigger high pressure tubes.
They suffer from a very low output pulse level that is extremely fast. (super circuitry needed to capture, stretch, amplify and count pulses). Modern Integrated circuit electronics makes this one time tough job very easy now. Somehow the mantle of complexity has remained attached to this counter which is not normally considered a main stream device and the costs have remained relatively high with cheapo, new BF3 neutron survey instruments listing in the $3000 range.
Traditionally, the BF3 tube is shielded and stored in a parafin or Polyethylene moderator in order to allow it to yield a relatively flat response for dosages over a wide range of energies. This can be back converted to flux using the reverse "radiation protection guide" (RPG) curve for the calibrated moderator in each instrument.
Most such counters can have the naked tube outside of the moderator to count thermal neuts with superb efficiency (for a neutral particle detector.)
Additionally, there is little counting due to large Gamma background often found in neutron fields.
The net fast neutron efficency for a moderated BF3 system is not much better than the BC-720 special scintillation system (<1.0%).
This is where an honest form of boosting can pump the efficiency for fast neutrons up a bit. The moderator is usually a form of cylinderical blanket or tube around the BF3 detector tube. A simple wax barrier infront of the tube would work at the <1% level. However, due to the kinetic, mechanical, elastic scattering of neutrons in a moderator as they are slowed, if the neutrons source is "shined" axially along the tube throught the moderator, more neutrons enter the sides of the tube along the surrounding moderator than enter the small circular area of the tube end. This gives a boost to the "effective" cross section of the detector.
Please note that we are not recounting multiple neutrons. Remember, that the BF3 is a one to one correspondence system. Every neutron entering the tube and colliding effectively with a B10 atom will issue one and only one alpha. The long counter, as this setup is labeled, is just scooping up more neutrons and funneling them into the tube as they thermalize. This is why in such systems we really need not be concerned as to the area of the long counter's detector face as they are usually calibrated to a known incident flux, averaged to the center of the tube.
It is only with the advent of transistorized electronics that smaller, lighter weight and even portable BF3 systems became available in a price range where they could be afforded by smaller research facilities. (circa 1960)
Modern plastics have effectively replaced the old parafin and borated parafin moderators.
Work done by Bonner at Rice University in the late 50's and others put the instrument on an almost plug and play footing.
In spite of all of this, there is still a misperception that, armed with a neutron counter, you can count neutrons with the ease and simplicity of a geiger counter for the normal charged particle radiation. This is not the case. Neutron metrology requires a much deeper understanding than is had by most people armed with the device. The effort is not mysterious, just filled with pitfalls to the un-informed.
Older used BF3 systems are encountered surplus. If working they must be calibrated professionally at some point or be taken to where there is a known and calibrated neutron source. This is, of course, also true for any neutron counter looking to give quantiative data regarding absolute neutron measurements, especially homebrew neutron counters of any type.
Scintillator systems are deceptively attractive to the homebrew neutron metrologist inspite of their poor efficiency. This is mainly due to the relative ease of obtaining individual components with well understood an documented circutry.
In the real world, however, when the big boys want basic neutron info. They invariably reach for the venerable BF3 counter. The commercial BC720 type fast neutron detector is relatively rare and the old BC-400 straight plastic type neutron detector is long ago extinct.
Richard Hull
Created on Thursday, May 03, 2001 5:17 PM EDT by Richard Hull