|
||||||||||||||||
Answer to Question #5218 Submitted to "Ask the Experts"Category: Accelerators — Radiation fields The following question was answered by an expert in the appropriate field: Q
We are performing digital x ray using a 9 or 15 MeV Linatron. The
camera is not in the direct beam path. To shield the digital camera we
used 2.5-inch thick tungsten. We now find we have a very high neutron
dose within the shielded container. Would we have seen this same
problem using lead or is tungsten worse? What can we do to eliminate
the neutron activation in the shielded container? Would it be enough to
add boron plastic or powder coat the container? Even without the
neutron problem we still need to shield the camera from the x rays.
A
The linatron is an industrial linear accelerator manufactured by Varian Medical Systems1.
In a linatron, the electron beam strikes a target, producing
bremsstrahlung which is then collimated with internal collimators and
sometimes with external collimators. The target and internal collimator
are typically made of some high-Z material such as tungsten. The
accelerator head is usually shielded with lead or tungsten. Neutron
production will take place in any material struck by an electron or
bremsstrahlung beam above a threshold energy (Eth). The
minimum threshold for photoneutron production in tungsten and lead is
6.19 MeV and 6.11 MeV, respectively (NCRP 1984). Thus a linatron
operating at 9 MV will produce neutrons. The photoneutron spectrum from
the accelerator head resembles that of a fission spectrum. The spectrum
changes after penetration through the head shielding. Since the
linatron is usually operated in a concrete-shielded room,
room-scattered neutrons will further soften the spectrum. Neutrons are
classified as: National Bureau of Standards. Shielding for high-energy electron accelerator installation. Washington, DC: U.S. Government Printing Press Office. NBS Handbook No. 97; 1964. National Council on Radiation Protection and Measurements. Neutron
contamination from medical electron accelerators. Bethesda, MD: NCRP;
Report No. 79; 1984. Footnote:
Answer posted on 22 March 2006. The information and material posted on this Web site is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of materials and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Web site. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.
|
||||||||||||||||
| This page last updated 18 December 2009. Ask Question | Search ATE | Site Map | Privacy Statement | Disclaimer | Security Notice | Webmaster | ||||||||||||||||