Calculations of neutron shielding data for 10-100 MeV proton accelerators

The characteristics of neutron sources and their attenuation in concrete were investigated in detail for protons with energies ranging from 10 to 100 MeV striking on target materials of C, N, Al, Fe, Cu and W. A two-step approach was adopted: thick-target double-differential neutron yields were first calculated from the (p, xn) cross sections recommended in the ICRU Report 63; further, transport simulations of those neutrons in concrete were performed by using the FLUKA Monte Carlo code. The purpose of this study is to provide reasonably accurate parameters for shielding design for 10-100 MeV proton accelerators. Source terms and the corresponding attenuation lengths in concrete for several target materials are given as a function of proton energies and neutron emission angles.     

Experimental and computational determination of neutron dose equivalent around radiotherapy accelerators

An experimental and computational investigation of the neutron dose equivalent in the treatment room and maze of various radiotherapy accelerators in Canada was completed. A bubble detector was selected for the bulk of the measurements due to its relative insensitivity to gamma radiation. The rooms and accelerator heads were modelled using the MCNP4B Monte Carlo radiation transport code, modified with a photoneutron patch. The results of the investigation showed generally good agreement between the experiments. Monte Carlo and analytical approximations to the neutron dose equivalent, and suggest that if the analytical approximations are used carefully they may be substituted for more costly experimental or Monte Carlo determinations.     

Certifying and utilizing monoenergetic neutron sources of accelerators

Conclusions At present the problems of certifying monoenergetic neutron sources by means of an accelerator and their utilization for certifying various measurement means has not as yet been fully solved, since the metrological organizations have encountered this problem in relatively recent times. It was noted at the 1st All-Union Conference on the Metrology of Neutron Radiations, which was held in 1971 in Moscow, that reference and standard monoenergetic neutron sources are being produced in the USSR on the basis of ÉG-2M and ÉG-2.5 accelerators, and that the standard techniques in direct measurements in pulsed neutron fields are being developed [2]. Experience gained in carrying out this work has shown that the successful solution of the certification problems requires joint efforts of metrologists, measuring equipment developers, and customers.Translated from Izmeritel'naya Tekhnika, No. 10, pp. 54–56, October, 1973.     

Target station shielding issues at the spallation neutron source

Recent spallation neutron source shielding activities in support of the neutron beam shutters and the hot cell walls are presented. Existing neutron beam shutters can be replaced with concrete at low power or with concrete and steel at approximately 500 kW of beam power. Potential voids in the hot cell walls are analysed to determine the impact on dose rates as a function of void size. A change in the type of shielding work is noted as the project moved from the early design stages as a 'green field' site to the current stage as a construction project nearing completion, where issues to be addressed are approaching retrofit-type analyses.     

Overview of secondary neutron production relevant to shielding in space

An overview of experimental secondary neutron measurements relevant to space-related activities is presented. Stopping target yields and cross section measurements conducted at particle accelerators using heavy ions with energies > 100 MeV per nucleon are discussed.     

中子及伽马射线复合屏蔽材料的研究进展

原子能工业的日益发展带来了核泄漏、核污染等安全隐患,人们必须采取严密的防护措施来保障涉核人员的身体健康和环境安全.众多核辐射类型中,中子和γ射线穿透性最强,屏蔽难度最大.然而,传统的中子屏蔽材料如硼、水、聚乙烯和伽马屏蔽材料如铅、铁、钨等屏蔽功能单一,屏蔽性能有限,有的热力学性能不佳,难以满足现代辐射防护的要求.在这方...     

A review of irradiation stability of lithium hydride neutron shielding material

Lithium hydride remains theoretically the most efficient neutron shield material, showing immense application prospects. The properties of Lithium hydride neutron shielding material, such as use temperature, mechanical properties, thermal conductivity and irradiation swelling behaviour, were reviewed. Factors that affect the irradiation swelling of Lithium hydride were also discussed in this paper. The development trends of lithium hydride neutron shielding material were proposed.     

Characterisation of neutron fields around high-energy x-ray radiotherapy machines

Photoneutron spectra around the treatment bed of a Varian Clinac 2100C machine were measured using a Bonner sphere spectrometer. To overcome problems with pulse pile-up and detection of non-neutron-induced events, the active detector of thermal neutrons normally used at the centre of the spheres was replaced by a sandwich of four CR-39 track detectors interleaved with 10B radiators. Track densities measured for the CR-39 detectors in Bonner spheres were used for the unfolding of neutron spectra. Neutron fluence and ambient dose equivalent for the whole energy range and partial energy intervals were derived from the neutron spectra.     

Development of Self-TOF neutron detector and its application to concrete and iron shielding experiments

A new type detector, called ‘Self-TOF detector’, has been developed for high energy neutron spectrometry behind a shield. The detector consists of a veto counter, a set of radiators with 20 thin detectors, a start counter and a stop counter of nine segments. The measurement of the detector response function for high energy neutrons and the concrete and iron shielding experiments were done at the Heavy-Ion Medical Accelerator in Chiba (HIMAC) of National Institute of Radiological Sciences (NIRS), Japan. By using the response functions, neutron spectra behind shield were obtained by unfolding and the results were compared with the LAHET Code System (LCS).     

Magnetic shielding of a large volume for an experiment on quasi free neutron oscillation

Results of the shielding against the Earth's magnetic field on a large volume are given. The experimental procedure is based on a multilayer μ-metal shield and a series of correcting electric coils. A final average value of (8.1±4.4) mG is obtained in a volume of about 20 m³.     

Induced radioactive nuclides of 10-MeV radiotherapy accelerators detected by using a portable HP-Ge survey meter

The radioactivation of linear accelerator components for radiation therapy is interest for radiation protection in general, and particularly, when decommissioning these structures. The energy spectra of gamma rays emitted from the heads of two accelerator models, EXL-15SP and Clinac iX, after 10-MeV X-ray irradiation, were measured using a high-purity germanium semiconductor survey meter. After spectrum analyses, activities of (24)Na, (28)Al, (54)Mn, (56)Mn, (57)Ni, (58)Co, (60)Co, (64)Cu, (65)Zn, (122)Sb, (124)Sb, (181)W, (187)W, (196)Au, and (198)Au were detected. One centimetre deep dose-equivalent rate of the heads of the linear accelerator was measured using the survey meter. The dose rate decreased to ～10 % of its initial rate after 1 week. Long-term activations were few, the radioactivity level was low, and a cooling time of several days was effective for reducing dose rate to an acceptable level for decommissioning.     

Arrangement of high-energy neutron irradiation field and shielding experiment using 4 m concrete at KENS

An irradiation field of high-energy neutrons produced in the forward direction from a thick tungsten target bombarded by 500 MeV protons was arranged at the KENS spallation neutron source facility. In this facility, shielding experiment was performed with an ordinary concrete shield of 4 m thickness assembled in the irradiation room, 2.5 m downstream from the target centre. Activation detectors of bismuth, aluminium, indium and gold were inserted into eight slots inside the shield and attenuations of neutron reaction rates were obtained by measurements of gamma-rays from the activation detectors. A MARS14 Monte Carlo simulation was also performed down to thermal energy, and comparisons between the calculations and measurements show agreements within a factor of 3. This neutron field is useful for studies of shielding, activation and radiation damage of materials for high-energy neutrons, and experimental data are useful to check the accuracies of the transmission and activation calculation codes.     

Three-dimensional shielding calculations for the IFMIF neutron source using a coupled Monte Carlo/Deterministic computational scheme

Shielding calculations for the International Fusion Materials Irradiation Facility (IFMIF) are complicated due to the geometrical complexity of the target system and the large-scale bulk shields around the source target. Three-dimensional shielding calculations were performed by using a newly developed Monte Carlo/Deterministic computational scheme. The neutron-photon fluxes and dose rate distributions in the back wall of the Test Cell and the access/maintenance room are presented and compared with previous shielding calculations. The results demonstrate that this coupled scheme is an useful computational tool for three-dimensional shielding analyses of complex and large nuclear facilities.     

Peripheral neutron and gamma doses in radiotherapy with an 18 MV linear accelerator

More and more attention is being given in radiotherapy to the doses received by organs other than the target organ. With increasing survival time of the patients, the risks of secondary malignancies need to be lowered as much as possible. So total body doses are worth estimating in radiotherapy. The introduction of intensity modulated radiotherapy (IMRT) needs an increase in the number of monitor units given to the patient. So there is a risk of increasing the peripheral doses using this technique. Another aspect, mostly neglected, is the neutron peripheral dose that occurs when LINAC energies above 8 MeV are used. We did measurements for both gammas and neutrons with an 18 MV Varian accelerator for a prostate cancer treatment. The measurements were done both free-in-air, at different depths in a plexi-phantom, and using a Rando-Alderson phantom. Effective doses for the total body outside the treatment area are estimated using these measurements.     

粘结碳化硼材料中子屏蔽性能及制备工艺研究

中子准直器在中国散裂中子源(CSNS)的谱仪上起着限制束流截面与发散度的作用,准直器的结构设计中通过在中子飞行管上间隔设置B4C阻挡块可以起到阻挡、吸收屏蔽杂散中子的作用,为此研制了粘结碳化硼中子阻挡块,并对其成型工艺进行了研究,确定了成型最佳粉体尺寸、粘结剂含量和固化温度.对该中子屏蔽材料的中子衰减系数计算结果表明,波长为0.1×10-10、1×10-10和15×10-10m中子通过该材料的线性衰减系数分别为3.42、30.4和449.9 cm-1.力学冲击和热重实验结果也表明该材料符合在CSNS谱仪中子束线屏蔽使用中的性能要求.     

Radiological assessment of the structural shielding adequacy of the radiotherapy facility at Korle-Bu Teaching Hospital, Accra, Ghana

A (60)Co radiotherapy source with an initial activity of 185 TBq has exhausted its useful half-life and has been replaced with a 222 TBq (60)Co source at the Korle-Bu Teaching Hospital; a radiological assessment was performed to ascertain the shielding integrity of the facility. Dose rate at selected critical positions were calculated for the old and new sources. Dose rate measurements were also performed at these critical locations for the new source. The dose rates at all the critical locations of the public and staff access areas were within the recommended dose rate limit of 0.5 and 7.5 μSv h(-1) for the public and controlled area, respectively. The concrete biological shielding at the facility at the moment is adequate enough to attenuate the gamma photons from the new 222 TBq (60)Co source. High dose rates were recorded at the entrance to the treatment room, it is therefore recommended that optimisation of procedures should be encouraged to restrict activities in this area.     

Applications of particle accelerators in medicine

There are nearly 20,000 particle accelerators in operation worldwide, about half of them employed for biomedical uses. This paper focuses on some recent advances in the two main medical domains where accelerators find their use, radionuclide production and radiation therapy. The paper first discusses the use of high-energy electron and proton accelerators for the potential, future production of (99)Mo, which is presently provided by fission reactors. Next, it reviews the rationale for the use of protons and carbon ions in cancer therapy, discussing the requirements imposed on accelerator technology and looking at some recent developments.     

Application of particle accelerators in research

Since the beginning of the past century, accelerators have started to play a fundamental role as powerful tools to discover the world around us, how the universe has evolved since the big bang and to develop fundamental instruments for everyday life. Although more than 15 000 accelerators are operating around the world only a very few of them are dedicated to fundamental research. An overview of the present high energy physics (HEP) accelerator status and prospectives is presented.     

电磁屏蔽材料中聚苯胺对屏蔽效能的影响及机理

金属材料与导电聚苯胺复合形成屏蔽组分有效提高了复合电磁屏蔽材料的高频电磁屏蔽效能,从而扩展了屏蔽材料的屏蔽带宽.以金属Ni粉/导电聚苯胺复合屏蔽组分为例,分析了在复合电磁屏蔽材料中聚苯胺的对屏蔽效能的贡献及作用机理.结果表明聚苯胺的加入与基体树脂相溶形成基体导电网络,有效阻碍了高频电磁波的穿透,可能是提高高频效能的主要原因.