Answer to Question #242 Submitted to "Ask the Experts"

Q

The Reuter Stokes High Pressure Ionization Chamber (HPIC) consists of a stainless steel sphere of roughly one foot diameter filled with ultra pure Ar-40 at a pressure of several hundred psi and is designed to measure ambient gamma radiation. As might be expected, when the Ar-40 is subjected to thermal neutrons, Ar-41 is formed via an n, gamma reaction. As the Ar-41(t 1/2 = 109 minutes) decays, the HPIC reports the energy from the decay product beta (1.198 MeV) and gamma (1293 keV) as energy of environmental gamma radiation received; i.e., it does not allow the discernment of the energy from the argon decay from the environmental gamma. My question is simply, how may one determine the neutron flux that causes the formation of the quantity of Ar-41 whose decay is observed from the HPIC?

A

The HPIC is not very sensitive to thermal neutrons. One approach to evaluate the thermal neutron sensitivity of a HPIC is the following: First, the response of the detector must be estimated for thermal neutron exposure. Since no calibration data exists for HPIC exposure to thermal neutrons, a coefficient is estimated from other empirical data. Pertinent assumptions and information include:

1. A high-range HPIC model (0 to 10 R/h) with a nominal response of 14 fA/uR/h.
    
2. Thermal cross section for Ar-40 of 0.61 barns.
    
3. The chamber volume is 4.2 liters, and the pressure is 25 atm (abs).
    
4. Ar-41 decay: Beta average energy of 464 keV @ 100 percent branch. Single gamma at 1.29 MeV @ 99 percent branch.

The detailed calculations are too long to be easily included in this response, but the following gives a brief summary of the key steps and the resulting values:

1. With a STP density of 1.78 g/L, the number of Ar-40 atoms inside the chamber at a pressure of 25 atm (absolute) will be 2.8E24.
    
2. Given a microscopic cross section of 0.61 barns, the total absorption capacity for thermal neutrons will be 1.7 per neutron/cm².
    
3. At saturation following exposure for 14 hours or more to l n/cm², the activity of Ar-41 inside the chamber will be 1.7 Bq. This amount of activity will generate an anode current due to beta decay alone of about 4.8E-15 A.
    
4. Considering the known response of the detector, the observed HPIC response will be 3.4E-01 uR/h per n/cm².
    
5. If the Ar-41gamma decay component is also considered, the relative contribution is about 0.001 of that generated by the beta decay and is, therefore, not significant.

The response of a high-range (0-10 R/h) HPIC is therefore estimated to be 0.34 uR/h per thermal neutron /cm² per second after the detector has been exposed for at least 14 hours. Estimates for shorter periods can be determined assuming asymptotic production of Ar-41. As an additional note, the production of Ar-42 will be trivial considering the small number of Ar-41 atoms available at saturation. Therefore, possible contributions from this isotope have been ignored. As for actually measuring thermal neutron flux, sensitive methods would use an instrument designed for this purpose such as a BF3 tube. However, if it is necessary to attempt measurement with a HPIC, then the only way would be to collect measuements before, during, and after exposure with subsequent extrapolation of the differences based on the half-life of Ar-41. Bobby Coleman ORNL