Departamento de Radiología y Medicina Física, Facultad de Medicina, Universidad de Málaga, 29010 Málaga, Spain.
Med Phys. 2010 Nov;37(11):6015-21. doi: 10.1118/1.3505919.
The authors want to assess the relevance of the neutron activation of the concrete vault of the PET cyclotron at CIMES (Universidad de Malaga) by predicting specific activities of the main activation products in the vault and their variation profiles as a function of penetration depth into concrete at present and after 10 yr of cyclotron operation.
The dual proton cyclotron is used for PET isotopes production, mainly 18F. During the years 2006 and 2008, the using rate has been 1 h/day at single beam (40 microA). From January 2008, using rate is 4 h/day at dual beam (80 microA). The energy of the cyclotron proton beam is 18 MeV. Four point locations were chosen on the walls of the cyclotron room to assess neutron induced activity concentrations. In each wall point location, neutron induced radionuclide specific activity was assessed from the wall surface to a depth of 120 cm within concrete. Simulations were carried out with the Monte Carlo based radiation transport code MCNPX (v2.6.0).
According to MCNPX calculations, activity depth profiles of activation products studied, except 54Mn, have a maximum at variable depths from the wall surface never beyond 12 cm. 54Mn activity decreases exponentially in all the studied depth ranges within wall concrete. The activity of 152Eu, 154Eu, 60CO, 134Cs, 46Sc, and 65Zn decreases exponentially beyond a 30 cm depth into concrete. 54Mn activity presents the faster decrease within a concrete vault with an attenuation length of 21 cm. According to MCNPX estimations, present activity in the cyclotron vault is mostly due to 46Sc and 60Co, with highest specific activity near the vault surface of 146 +/- 16 and 50 +/- 4.6 Bq/kg, respectively. 46Sc and 60Co activity measurements near the surface wall present an acceptable match with the estimation within the uncertainties, but measured activities of the other radionuclides are quite over the MCNPX estimations. The calculations after 10 yr of cyclotron operation predict a slight increase for short half-life radionuclides (46Sc, 54Mn, and 65Zn). However, long half-life neutron induced radionuclides importantly increase their activities, especially 60Co and 152Eu. These radionuclides and 46Sc give the main contribution to the wall activity in a 10 yr period. Estimated highest 46Sc and 60Co activities in 10 yr of cyclotron operation are in acceptable agreement with published measurements, but MCNPX calculated activities are lower than the measurements for the rest of the radionuclides.
MCNPX estimates acceptably present activity levels of 46Sc and 60Co as confirmed by activity measurements, but underestimates activity for the rest of the neutron induced radionuclides in the wall. Activity measurements have revealed the inhomogeneity of wall concrete atomic composition since large differences in activity values were found in two near wall surface locations with similar neutron flux. Such inhomogeneity cannot be modeled with the program that considers the material composition homogeneous. Highest specific activities estimated in a 10 yr operation are under exemption limits and therefore the cyclotron vault can be discarded as radioactive waste.
作者希望通过预测当前和经过 10 年回旋加速器运行后混凝土穹顶内主要活化产物的比活度及其随混凝土穿透深度的变化情况，评估 CIMES（马拉加大学）PET 回旋加速器混凝土穹顶的中子活化相关性。
双质子回旋加速器用于生产 PET 同位素，主要是 18F。在 2006 年和 2008 年期间，单束（40μA）的使用时间为每天 1 小时。从 2008 年 1 月开始，双束（80μA）的使用时间为每天 4 小时。回旋加速器质子束的能量为 18MeV。在回旋加速器室的四面墙上选择了四个位置来评估中子诱导的活度浓度。在每个墙壁位置，从墙壁表面评估了混凝土内 120cm 深度范围内的中子诱导放射性核素的比活度。使用蒙特卡罗辐射输运代码 MCNPX（v2.6.0）进行了模拟。
根据 MCNPX 计算，研究的活化产物的活性深度分布，除 54Mn 外，在距壁面不同深度处具有最大值，从未超过 12cm。54Mn 活性在壁内所有研究的深度范围内呈指数衰减。在混凝土内超过 30cm 深度处，152Eu、154Eu、60CO、134Cs、46Sc 和 65Zn 的活性呈指数衰减。54Mn 活性在混凝土穹顶内具有较快的衰减，衰减长度为 21cm。根据 MCNPX 的估计，回旋加速器穹顶内的当前活性主要来自 46Sc 和 60Co，其表面附近的比活度分别为 146±16 和 50±4.6Bq/kg。表面壁附近的 46Sc 和 60Co 活性测量值与不确定性范围内的估算值吻合良好，但其他放射性核素的测量活性明显高于 MCNPX 的估算值。回旋加速器运行 10 年后的计算预测短半衰期放射性核素（46Sc、54Mn 和 65Zn）的活性略有增加。然而，长半衰期中子诱导放射性核素的活性显著增加，特别是 60Co 和 152Eu。这些放射性核素和 46Sc 在 10 年内对壁活性的贡献最大。在 10 年回旋加速器运行期间，估计的最高 46Sc 和 60Co 活性与已发表的测量值相符，但 MCNPX 计算的活性低于其余放射性核素的测量值。
MCNPX 估算值可接受地呈现了 46Sc 和 60Co 的活度水平，正如活性测量所证实的，但低估了壁内其余中子诱导放射性核素的活度。活性测量揭示了墙壁混凝土原子组成的不均匀性，因为在具有相似中子通量的两个近壁表面位置发现了活性值的巨大差异。由于该程序认为材料组成均匀，因此无法对这种不均匀性进行建模。在 10 年运行中估计的最高比活度低于豁免限值，因此回旋加速器穹顶可以作为放射性废物丢弃。
