Answer to Question #3457 Submitted to "Ask the Experts"

Please let me know how to measure simultaneously neutron-dose equivalent and gamma-dose equivalent behind different thicknesses of various shield materials using a ²⁵²Cf source and Harshaw TLD card (type-7776). Since the quality factor for neutron and gamma is different, which calibration source should I use? Is it the same calibration procedure for both two-element card (type-0110) and four-element card (type-7776)? How do I calculate the relaxation length or removal cross section for the neutron and absorption coefficients for the gamma of the ²⁵²Cf source? I have known that ANSI N13.11, which gives the information on TLD calibration requirements, is available only for Health Physics Society (HPS) members. How can I read ANSI N13.11 and NCRP Report 122 if I'm not a member of HPS?

First, you do not have to be a member of the Health Physics Society (HPS) to order American National Standards Institute (ANSI) standards or National Council on Radiation Protection and Measurements (NCRP) reports. However, many of the N13 and N43 ANSI standards are provided to HPS members at no charge as a membership benefit.

My approach to estimating the neutron dose equivalent and gamma dose equivalent from a ²⁵²Cf source, behind different shield materials and using various amounts of thickness, would be to first calculate it theoretically.

Californium-252 undergoes an alpha decay about 96 percent of the time. The balance, or ~3 percent, of the nuclear transitions undergo spontaneous fission. Thus, depending on the geometry of the source, moderation before or within the shield, (neutron, gamma) reactions within the shield, gamma scattering before the photons exit the shield, etc., this can be a complicated problem. The relaxation length of the shield is dependent on the particle, its energy, material, thickness, etc. I'd start by reviewing a classic text, Principles of Radiation Protection, edited by K.Z. Morgan and J.E. Turner, for a detailed discussion of these subjects. (See 1973 reprint with corrections from Robert E. Krieger Publishing Company, Malabar, Florida.)

Your next step might be to measure these radiation fields with photon and neutron instruments, as energy independent as possible. For a review of health physics instrumentation calibration, the NCRP Report No. 112 on "Calibration of Survey Instruments Used in Radiation Protection for the Assessment of Ionizing Radiation Fields and Radioactive Surface Contamination" is a very good reference.

Once you have the neutron and gamma fields characterized, then you could calibrate the two- and four-chip TLD system. I would consult the manufacturer for proper calibration procedures. I'm not familiar with these particular TLD systems, but you should examine the energy response of the TLD elements and be sure it is flat over the range of energies expected exiting the shield. An important consideration is also the "cross-talk" sensitivity between TLD elements. For example, the neutron sensitive element may be affected by the gamma radiation, and the neutron insensitive response may need to be subtracted before applying a correction factor. You might start with TLD calibrations to unshielded sources moderated to produce the expected spectrum of neutrons. NCRP Report No. 122 on "Use of Personal Monitors to Estimate Effective Dose Equivalent to Workers for Exposure to Low-LET Radiation" will help you if you have nonuniform fields and need to perform multiple badging to determine effective dose equivalent.

David J. Allard, CHP