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09 February 2012

Answer to Question #6675 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment/Shielding — Shielding

The following question was answered by an expert in the appropriate field:

What are the generally accepted half-value layer (HVLs) constants for "high-density" concrete (i.e., the addition of iron aggregates) for both ¹⁹²Ir and ⁶⁰Co?

There are no "generally accepted half-value layers (HVLs) for high-density concrete" for the following reasons:

Any concrete with a density of greater than 2.35 g cm^-3 (147 lb ft^-3) can be considered high-density concrete. The increased density is obtained by the addition of various high-Z aggregates such as iron, etc., to increase photon attenuation. The transmission of photons will depend upon the composition, homogeneity, and density of the concrete.
NCRP Report No. 49, Appendix D, Fig. 12 (1) shows transmission curves for ¹⁹²Ir and ⁶⁰Co for standard concrete of density 2.35 g cm^-3. A study of these transmission curves shows that the HVL changes with increasing thickness of shielding and then reaches an equilibrium value. Therefore technically there is no single HVL that can be used to characterize the concrete. The HVLs listed in Table 28 of NCRP Report No. 49 are approximate values determined at high attenuation (large shielding thicknesses) and without build up (initial increase in dose due to scattered radiation). The equilibrium HVLs for ⁶⁰Co and ¹⁹²Ir are approximately 6.2 cm and 4.3 cm, respectively. Other data for concrete can also be found in the references provided.

Having said that, one could obtain a rather approximate value for the equilibrium HVL by multiplying the equilibrium HVL for concrete by the ratio of the concrete density to the density of the heavy concrete. Ideally it would be best to determine these values by Monte Carlo calculations and/or experiment.

Please note the following caveat when using high-density concrete. Careful attention should be paid in the pouring of high-density concretes since the high-Z aggregates could settle to the bottom if proper mixing procedures etc. are not used. Therefore it is important to work with contractors who are familiar in the handling of such materials. Alternatively, prefabricated interlocking blocks of high-density concrete can be used. There are several providers of such blocks.

References

National Council on Radiation Protection and Measurements. Structural shielding design and evaluation for medical use of x rays and gamma rays of energies up to 10 MeV. Bethesda, MD: NCRP Report No. 49; 1976.
Boutroux-Jaffre F. Photon emitting sources. In: Pierquin B, Marinell G, eds. A practical manual of brachytherapy. Madison, WI: Medical Physics Publishing; 1997: 3-21.
Lymperopoulou G, Papagiannis P, Sakelliou L, Georgiou E, Hourdakis CJ, Baltas D. Comparison of radiation shielding requirements for HDR brachytherapy using ¹⁶⁹Yb and ¹⁹²Ir sources. Med Phys 33:2541-2547; 2006.

Nisy E. Ipe, PhD, CHP

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