Department of Physics of Radiation Shielding and Radioprotection
Veritas Medical Solutions, Harleysville, PA USA
by El Hassane Bentefour, PhD.
Director of Physics
Robert J. Farrell
The decommissioning of a proton therapy facility is challenging due to neutron-induced activation of the facility shielding, structure, and equipment. Activation is caused by a series of nuclear reactions that produce radioactive isotopes with half-lives ranging from hours to many years. Long-lived isotopes, such as 152Eu, 133Ba, 134Cs, 60Co, and 22Na, commonly present in concrete, are particularly problematic because, in addition to their long half-life, they require very low Clearance Index (CI) to qualify as non-nuclear waste.
To review the neutron shielding attenuation characteristics of VeriShield, a high-density, engineered shielding material. Then, to explore the extent of neutron activation for these modules when made with different proportions of cement, reduced to just 2%.
Review of VeriShield® Neutron Attenuation:
We have studied more than a half dozen different Veritas shielding materials utilizing a thin, copper target as a source of neutrons as it was bombarded by 230 MeV protons. A WENDI neutron detector, operated in integration mode, was shielded from 5 sides by a specially constructed, shielded cave with one side open to detect neutrons from the source at a set angle as they passed through the shielding stack. Some of the materials in the study featured both high-density elements in combination with special additives designed to increase their attenuation efficiency.
Addressing Proton Therapy’s Neutron Activation Issue:
To minimize the proton therapy’s neutron activation issue, we experimented with two approaches:
(i) Creating a new generation of VeriShield that uses low cement content
and alternative binding agents.
(ii) Exploring the practicality of the company’s sacrificial layer concept to
increase the longevity of proton therapy facilities.
Figure 1: (a) Sketch of the test set-up, (b) signature of clinical proton beam on a copper target, (c) Neutron detector shielded from bouncing neutrons on vault’s walls, (d) Typical structure of VeriShield barrier.
VeriShield shielding compared to concrete
The experimental results, supported by Monte Carlo calculations; as illustrated in Fig. 2a, 2b, 2c, and 2d, show that the VeriShield barrier has a higher cumulative shielding efficiency compared to concrete. This cumulative efficiency, along with the shielding depth increases as the proton beam energy (i.e., neutron energy) increases. Per Figure 2d, One can see that for a 230 MeV clinical proton beam, one would need 6.4 feet of VeriShield® V300 to reduce the neutron dose down to 0.01% compared to 10.7 feet of concrete. Thus, a savings of about 40% of space would otherwise be covered by mass concrete if used for the shielding.
VeriShield and Neutron activation
A new generation of VeriShield using an alternative binding agent with minimal content of cement, only 3% (77% less than in the current VeriShield has been developed. Fig. 3a shows a few samples with cement content varying from 5% down to 0%. With these new modules, the long-term activation problem is significantly reduced as the presence of long half-life elements such as 152Eu, 133Ba, 134Cs, 60Co and 22Na is practically nonexistent.
Monte Carlo activation studies have been conducted on VeriShield modules and concrete. The preliminary results, in Fig. 3b, show similar neutron activation to concrete, but with two advantages:
-The current VeriShield has the advantage to be used as a sacrificial layer as it is a dry build that enables barriers to be selectively removed and replaced.
-The new VeriShield module presents the promise of reduced long-term activation issues. Additional studies are warranted.
The Quantitative Evaluation of VeriShield is a means to save space and time when designing a proton therapy facility as it offers a higher degree of shielding efficiency against neutrons. In contrast to mass concrete, it allows for better management of neutron activation as it can be used as a shielding sacrificial layer. A new prospective generation of VeriShield which uses an alternative binding agent to cement is likely to improve its actual performance and suitability for proton therapy.
1. Experimental data from Veritas-MIT VeriShield® testing report, V. Nazaryan et al. Unpublished.
2. Tomaž Žagar, Matjaž Ravnik, “MEASUREMENT OF NEUTRON ACTIVATION IN CONCRETE SAMPLES”. Int. conference, Nuclear Energy in Central Europe 2000