Waker A J
Faculty of Energy Systems and Nuclear Science, UOIT, 2000 Simcoe St. N., Oshawa, ON, Canada L1H 7K4.
Radiat Prot Dosimetry. 2011 Feb;143(2-4):467-70. doi: 10.1093/rpd/ncq510. Epub 2010 Dec 23.
The accelerator-based in vivo neutron activation facility at McMaster University has been used successfully for the measurement of several minor and trace elements in human hand bones due to their importance to health. Most of these in vivo measurements have been conducted at a proton beam energy (E(p)) of 2.00 MeV to optimise the activation of the selected element of interest with an effective dose of the same order as that received in chest X rays. However, measurement of other elements at the same facility requires beam energies other than 2.00 MeV. The range of energy of neutrons produced at these proton beam energies comes under the region where tissue-equivalent proportional counters (TEPCs) are known to experience difficulty in assessing the quality factor and dose equivalent. In this study, the response of TEPCs was investigated to determine the quality factor of neutron fields generated via the (7)Li(p, n)(7)Be reaction as a function of E(p) in the range 1.884-2.56 MeV at the position of hand irradiation in the facility. An interesting trend has been observed in the quality factor based on ICRP 60, Q(ICRP60), such that the maximum value was observed at E(p)=1.884 MeV (E(n)=33±16 keV) and then continued to decline with increasing E(p) until achieving a minimum value at E(p)=2.0 MeV despite a continuous increase in the mean neutron energy with E(p). This observation is contrary to what has been observed with direct fast neutrons where the quality factor was found to increase continuously with an increase in E(p) (i.e. increasing E(n)). The series of measurements conducted with thermal and fast neutron fields demonstrate that the (14)N(n, p)(14)C produced 580 keV protons in the detector play an important role in the response of the counter under 2.0 MeV proton energy (E(n) ≤ 250 keV). In contrast to the lower response of TEPCs to low-energy neutrons, the quality factor is overestimated in the range 1-2 depending on beam energy <2.0 MeV. This study provides an insight to understanding the response of TEPCs in low-energy neutron fields where the neutrons are moderated using a polyethylene moderator.
麦克马斯特大学基于加速器的体内中子活化设施已成功用于测量人手骨骼中的几种微量元素和痕量元素，因为它们对健康很重要。这些体内测量大多在质子束能量（E(p)）为2.00 MeV的条件下进行，以优化所选感兴趣元素的活化，有效剂量与胸部X光检查时接受的剂量处于同一量级。然而，在同一设施中测量其他元素需要2.00 MeV以外的束能量。在这些质子束能量下产生的中子能量范围处于已知组织等效正比计数器（TEPC）在评估品质因数和剂量当量时会遇到困难的区域。在本研究中，研究了TEPC的响应，以确定通过(7)Li(p, n)(7)Be反应在该设施手部辐照位置产生的中子场的品质因数作为E(p)在1.88～2.56 MeV范围内的函数。基于国际辐射防护委员会（ICRP）第60号出版物的品质因数Q(ICRP60)呈现出一个有趣的趋势，即最大值出现在E(p)=1.884 MeV（E(n)=33±16 keV）处，然后随着E(p)的增加持续下降，直到在E(p)=2.0 MeV时达到最小值，尽管平均中子能量随E(p)持续增加。这一观察结果与直接快中子的情况相反，在直接快中子中，品质因数被发现随E(p)的增加（即E(n)增加）而持续增加。用热中子场和快中子场进行的一系列测量表明，探测器中由(14)N(n, p)(14)C产生的580 keV质子在2.0 MeV质子能量（E(n)≤250 keV）下计数器的响应中起重要作用。与TEPC对低能中子的较低响应相反，根据低于2.0 MeV的束能量，品质因数在1～2的范围内被高估。本研究为理解TEPC在使用聚乙烯慢化剂慢化中子的低能中子场中的响应提供了见解。
