Submitted to: Journal of Radioanalytical and Nuclear Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/13/2000
Publication Date: 8/1/2001
Citation: SHYPAILO, R.J., ELLIS, K.J. ESTIMATION OF BACKGROUND INTERFERENCE IN PROMPT-GAMMA NEUTRON ACTIVATION USING MCNP. JOURNAL OF RADIOANALYTICAL AND NUCLEAR CHEMISTRY, 249(2): 407-412. 2001. Interpretive Summary: Prompt-gamma neutron activation (PGNA) is a method of measuring the total amount of nitrogen in a person's body, which is converted into protein, giving an idea of the proportion of lean tissue as a health-related yardstick. We ran a modeling program that tracked the nuclear particle activity in a PGNA because we wanted to understand the source of unacceptably high background activity, and how to reduce or eliminate it, in order to get a more accurate reading. The program we used was a Monte Carlo simulation program called MCNP. This proved to be effective in measuring the amount of interference that was coming from sources outside the body being measured. The results helped us to design a better shield and develop standard corrections that we can use in order to improve measurement precision. Many other researchers can use this same program to achieve greater accuracy, which in turn will benefit medical doctors who want to use this device to better clinically diagnose and treat certain types of patients.
Technical Abstract: Prompt-gamma neutron activation (PGNA) is used to measure total-body nitrogen (TBN) and hydrogen (TBH) in humans. Background interference in the gamma spectra arises from both the subject and the instrument shielding. The magnitude of this interference is difficult to ascertain directly, since the absence of a subject on the scanner alters the conditions for neutron scattering and thermalization. We attempted to improve measurement precision by modeling experimental conditions to account for background interference. A Monte Carlo simulation program (MCNP4B2; Los Alamos National Laboratory) was used to examine the neutron and gamma background signals in the PGNA system, which uses a single 241AmBe source. Hydrogen and nitrogen peak regions were assessed in the presence and absence of phantoms containing various nitrogen and nitrogen-free solutions. Gamma counts reaching the detectors were categorized as originating from shielding materials or subject tissues, or produced internally within the detector crystals. The simulations suggested extracorporeal H peak contributions of up to 30%, depending on subject body habitus. The majority of the N background could be attributed to detector pileup events. Variations in background with subject size must be taken into account in order to improve the accuracy of PGNA measurements. The MCNP results allowed us to improve shielding design and develop background correction algorithms based on body habitus in order to improve measurement precision.