Establishment of a simple neutron calibration field using a moderated 252Cf source: Part II. Experimental characterization of simple neutron calibration field

This paper describes the development and experimental standardization of neutron fields simply arranged for detector calibrations used for radiation control and environmental measurement. These fields are the following: (1) bare ²⁵²Cf fission field, (2) iron-moderated ²⁵²Cf field, (3) carbone-moderated Cf field, and (4) polyethylene-moderated ²⁵²Cf field. These fields are most suitable for calibrating the detectors used in and around nuclear and radiation facilities, since the fields are designed to simulate the typical neutron fields in and around the facilities.The direct neutron components of these fields have been standardized by the following two methods: (1) calculation by the ANISN code, and (2) measurements with and without a shadow shield by detectors standardized in the national standard field at the Electrotechnical Laboratory (ETL). The neutron emission rates of the ²⁵²Cf source have been calibrated also at ETL. We have standardized only direct components because of their independence of room size and peripheral structures. The standardized values are energy spectra and dose equivalent rates of the direct neutron components; the accuracies have also been evaluated to be 20% below 100 keV, 15% at 1 MeV, and 50% above 5 MeV. These fields including room scattered components have also been characterized especially to calibrate neutron detectors having sensitivity to low energy room scattered neutrons, because of large errors caused by shadow shield subtraction.     

Two miniaturised TEPCS in a single detector for BNCT microdosimetry

Microdosimetry with tissue-equivalent proportional counters (TEPC) has proven to be an ideal dosimetry technique for mixed radiation fields as those ones used in boron neutron capture therapy (BNCT). A new counter, composed of two twin cylindrical mini TEPCs inserted in a slim titanium sleeve of 2.7 mm external diameter, has been constructed. The detector has been designed to perform dosimetry and microdosimetry in intense radiation fields. The two mini TEPCs work in gas flow mode. They have right cylinder sensitive volumes of 0.9 mm. In spite of gas line tiny sizes, the gas pressure inside the two counters is well established with <1% of uncertainty. The counter has been calibrated in a secondary standard photon fields. The mean of the effective sensitive volume sizes has been measured to be 0.86 mm. The twin TEPC acquisition system processes properly the signals up to about 30 kHz of counting rate. Therefore, twin TEPC can perform dosimetric measurements in photon field with intensities of some tens of Gy h(-1).     

Neutron measurements at nuclear power reactors [55]

Staff from the Pacific Northwest National Laboratory (operated by Battelle Memorial Institute), have performed neutron measurements at a number of commercial nuclear power plants in the United States. Neutron radiation fields at light water reactor (LWR) power plants are typically characterized by low-energy distributions due to the presence of large amounts of scattering material such as water and concrete. These low-energy distributions make it difficult to accurately monitor personnel exposures, since most survey meters and dosimeters are calibrated to higher-energy fields such as those produced by bare or D₂O-moderated ²⁵²Cf sources. Commercial plants typically use thermoluminescent dosimeters in an albedo configuration for personnel dosimetry and survey meters based on a thermal-neutron detector inside a cylindrical or spherical moderator for dose rate assessment, so their methods of routine monitoring are highly dependent on the energy of the neutron fields.Battelle has participated in neutron assessments at a number of LWR facilities to characterize neutron radiation fields and to evaluate the responses of plant dosimeters and survey instruments. In these studies, the tissue equivalent proportional counter was used for measuring neutron dose and dose equivalent rates, and multisphere spectrometers were used to measure energy distributions. The use of these instruments in LWR work locations is usually difficult because of extreme environmental conditions such as high temperatures.These studies have confirmed the presence of low-energy neutron fields in most work locations. The studies have also found that albedo dosimeters used at power plants typically overrespond significantly when using a calibration based on californium exposures. Survey instruments also respond highly in typical LWR environments.     

Field calibration studies for ionisation chambers in mixed high-energy radiation fields

The monitoring of ambient doses at work places around high-energy accelerators is a challenging task due the complexity of the mixed stray radiation fields encountered. At CERN, mainly Centronics IG5 high-pressure ionisation chambers are used to monitor radiation exposure in mixed fields. The monitors are calibrated in the operational quantity ambient dose equivalent H*(10) using standard, source-generated photon- and neutron fields. However, the relationship between ionisation chamber reading and ambient dose equivalent in a mixed high-energy radiation field can only be assessed if the spectral response to every component and the field composition is known. Therefore, comprehensive studies were performed at the CERN-EU high-energy reference field facility where the spectral fluence for each particle type has been assessed with Monte Carlo simulations. Moreover, studies have been performed in an accessible controlled radiation area in the vicinity of a beam loss point of CERN's proton synchrotron. The comparison of measurements and calculations has shown reasonable agreement for most exposure conditions. The results indicate that conventionally calibrated ionisation chambers can give satisfactory response in terms of ambient dose equivalent in stray radiation fields at high-energy accelerators in many cases. These studies are one step towards establishing a method of 'field calibration' of radiation protection instruments in which Monte Carlo simulations will be used to establish a correct correlation between the response of specific detectors to a given high-energy radiation field.     

Heavy ion RBE and microdosimetric spectra

Reliable values for the relative biological effectiveness (RBE) of the complex field of heavy ions is essential for radiation therapy with carbon beams. Clinical experience with this novel form of therapy is still quite narrow and it is, therefore, desirable to compare and combine relevant clinical findings and theoretical approaches. One major source of available information comes from neutron therapy. The approach towards the determination of RBE for neutron therapy is, therefore, tentatively applied to the heavy ion therapy. This includes the determination of microdosimetric spectra and their numerical evaluation towards the determination of RBE. A microdosimetric detector on the basis of a tissue-equivalent proportional counter (TEPC) was developed for measurements in the heavy ion fields of the GSI, Darmstadt. Measurements were performed first near the perspex phantom surface with carbon ion energies between 89 MeV.amu-1 and 430 MeV.amu-1. Subsequently, measurements were taken at various depths in the neighbourhood of the Bragg region. The numerical techniques that were developed for neutron therapy lead to tentative values of the RBE that are compared to the RBE values from the biophysical model developed at the GSI. The comparison is still preliminary but can be helpful in modifying the microdosimetric approach for its application in heavy ion therapy.     

Neutron field measurements for alara purposes around a Van de Graaff accelerator building

The Institute for Reference Materials and Measurements operates a 7.0 MV Van de Graaff accelerator to generate monoenergetic neutron radiation for experimental applications. Owing to increased intensities of generated neutron fields and the more stringent regulation related to the maximum dose for the public, a concrete shielding wall surrounding the experimental building was constructed. This paper presents a study aiming at evaluating the effect of the shielding on the neutron field outside the wall. For this purpose, the following measurements were carried out around the building: (1) cartography of the neutron field for different experimental conditions; (2) measurement of neutron spectra using multiple Bonner spheres; (3) activation measurements using gold discs followed by low-level gamma spectrometry. From the measurements, it can be concluded that the wall fulfils its purpose to reduce the neutron dose rate to the surrounding area to an acceptable level.     

The CERN-EU high-energy reference field (CERF) facility for dosimetry at commercial flight altitudes and in space

A reference facility for the calibration and intercomparison of active and passive detectors in broad neutron fields has been available at CERN since 1992. A positively charged hadron beam (a mixture of protons and pions) with momentum of 120 GeV/c hits a copper target, 50 cm thick and 7 cm in diameter. The secondary particles produced in the interaction traverse a shield, at 90 degrees with respect to the direction of the incoming beam. made of either 80 to 160 cm of concrete or 40 cm of iron. Behind the iron shield, the resulting neutron spectrum has a maximum at about 1 MeV, with an additional high-energy component. Behind the 80 cm concrete shield, the neutron spectrum has a second pronounced maximum at about 70 MeV and resembles the high-energy component of the radiation field created by cosmic rays at commercial flight altitudes. This paper describes the facility, reports on the latest neutron spectral measurements, gives an overview of the most important experiments performed by the various collaborating institutions over recent years and briefly addresses the possible application of the facility to measurements related to the space programme.     

Microdosimetry of epithermal neutron field at the Kyoto University reactor

Microdosimetric spectra were measured in order to gain the microdosimetric parameters of some epithermal neutron fields. Changes in dose mean lineal energy YD as a function of depth of heavy water showed a trend of softening with heavy water of the beam. The neutron absorbed dose was obtained by using the frequency mean lineal energy. Results show good agreement with measurements with the activation method using gold foil. This study demonstrated how microdosimetric parameters change in radiation quality as a function of heavy water depth.     

Characterization of neutron reference fields at US Department of Energy calibration fields

The Health Physics Measurements Group at the Los Alamos National Laboratory (LANL) has initiated a study of neutron reference fields at selected US Department of Energy (DOE) calibration facilities. To date, field characterisation has been completed at five facilities. These fields are traceable to the National Institute for Standards and Technology (NIST) through either a primary calibration of the source emission rate or through the use of a secondary standard. However, neutron spectral variation is caused by factors such as room return, scatter from positioning tables and fixtures, source anisotropy and spectral degradation due to source rabbits and guide tubes. Perturbations from the ideal isotropic point source field may impact the accuracy of instrument calibrations. In particular, the thermal neutron component of the spectrum, while contributing only a small fraction of the conventionally true dose, can contribute a significant fraction of a dosemeter's response with the result that the calibration becomes facility-specific. A protocol has been developed to characterise neutron fields that relies primarily on spectral measurements with the Bubble Technology Industries (BTI) rotating neutron spectrometer (ROSPEC) and the LANL Bonner sphere spectrometer. The ROSPEC measurements were supplemented at several sites by the BTI Simple Scintillation Spectrometer probe, which is designed to extend the ROSPEC upper energy range from 5 to 15 MeV. In addition, measurements were performed with several rem meters and neutron dosemeters. Detailed simulations were performed using the LANL MCNPX Monte Carlo code to calculate the magnitude of source anisotropy and scatter factors.     

Measurements of the neutron dose near a 15 Mv medical linear accelerator

In this work, simplified recombination methods for routine estimation of dose equivalent in mixed (gamma and neutrons) radiation field outside the irradiation field of linear medical accelerators is considered. The author's earlier reported method of H(10) measurements, involving determination of the recombination index of radiation quality, Q(4) by tissue-equivalent recombination chamber was combined with the new method for determination of the photon to neutron dose ratio D(X)/D(n) from the ratio of ion collection efficiencies measured in the investigated radiation field and in two reference fields of gamma and neutron radiations. The method is suitable when the neutron contribution to the total absorbed dose, D(n)/D, is >3%.     

Neutron spectra around medical treatment facilities

The neutron field condition in and around the treatment room of the neutron therapy facility at the research reactor FRM in Garching-München was studied for radiation protection purposes. Neutron spectrometry was conducted by using a 15-channel multisphere spectrometer. From the unfolded spectral shapes, averaged dose conversion coefficients were derived and correction factors for the reading of two neutron monitors obtained. These results were compared with further measurements done at a medical linear accelerator where the neutrons are produced by (γ, n) processes in the irradiation head.     

Numerical and experimental results of the operational neutron dosemeter 'Saphydose-N'

Since 1993, the Institute for Radiological Protection and Nuclear Safety (IRSN) has lead, in association with Electricité de France (EDF), a R&D study of a neutron personal electronic dosemeter. This dosemeter, called 'Saphydose-N', is manufactured by the SAPHYMO company. This paper presents first the optimisation of some detector components using Monte Carlo calculations, and second the test of the manufactured Saphydose-N under radiation following the IEC 1323 standard's recommendations for active personal neutron dosemeters. The measurements with the manufactured dosemeter were performed on the one hand at PTB (Physikalisch-Technische Bundesanstalt) in mono-energetic neutron fields and, on the other hand at IRSN in neutron fields generated by a thermal facility (SIGMA), radionuclide ISO sources and a realistic spectrum (CANEL/T400). The manufactured dosemeter Saphydose-N was also tested during measurement campaigns of the European programme EVIDOS ('Evaluation of Individual Dosimetry in Mixed Neutron and Photon Radiation Fields') at different nuclear workplaces. The study showed that Saphydose-N complies with the recommendations of standard IEC 1323 and can be used at any workplace with no previous knowledge of the neutron field characteristics.     

Neutron measurements in a Varian 2,100C LINAC facility using a Bonner sphere system based on passive gold activation detectors

The use of high-energy linear electron accelerators (LINACs) for medical cancer treatments is widespread on an international scale. The associated bremsstrahlung X rays may produce neutrons as a result of subsequent photonuclear reactions with the different materials constituting the accelerator head. The generated neutron field is highly variable and depends strongly on the beam energy, on the accelerator shielding, on the flattering filter as well as on the movable collimators (jaws) design and on the irradiation field geometry. An estimate of this photoneutron component is, thus, of practical interest to quantify the radiological risk for the working staff and patients. Due to high frequency electromagnetic fields, and also to the presence of abundant leaked and scattered photons in these installations, measurements of the corresponding neutron fields by active dosemeters are extremely difficult. A modified version of the Bonner sphere system, based on passive gold activation detectors, has been used to perform neutron measurements at two points in a Varian 2,100C LINAC facility. A home-made unfolding procedure (CDM) has been utilised to determine the neutron spectra present at the measurement points. Results indicate that the giant dipole resonance process is the most adequate model to explain neutron production in the LINAC and that a thermal component is present at the measurement points.     

Measurement of energy and directional distribution of neutron fluence inside a nuclear power plant

A detailed analysis of the neutron radiation field was performed at selected positions inside the Krümmel boiling water reactor. The measurements were performed with the PTB Bonner sphere spectrometer and with a newly developed directional neutron spectrometer based on silicon detectors mounted onto the surface of a 30 cm diameter polyethylene sphere. Both, angle-integrated and angle-differential spectral neutron fluences were determined in this way.     

Application of TEPC microdosimetry to boron neutron capture therapy

Boron neutron capture therapy (BNCT) is a bimodal radiation therapy used primarily for highly malignant gliomas. Tissue-equivalent proportional counter (TEPC) microdosimetry has proven an ideal dosimetry technique for BNCT, facilitating accurate separation of the photon and neutron absorbed dose components, assessment of radiation quality and measurement of the BNC dose. A miniature dual-TEPC system has been constructed to facilitate microdosimetry measurements with excellent spatial resolution in high-flux clinical neutron capture therapy beams. A 10B-loaded TEPC allows direct measurement of the secondary charged particle spectrum resulting from the BNC reaction. A matching TEPC fabricated from brain-tissue-equivalent plastic allows evaluation of secondary charged particle spectra from photon and neutron interactions in normal brain tissue. Microdosimetric measurements performed in clinical BNCT beams using these novel miniature TEPCs are presented, and the advantages of this technique for such applications are discussed.     

Self-shielding effects in neutron spectra measurements for neutron capture therapy by means of activation foils

The design and optimisation of a neutron beam for neutron capture therapy (NCT) is accompanied by the neutron spectra measurements at the target position. The method of activation detectors was applied for the neutron spectra measurements. Epithermal neutron energy region imposes the resonance structure of activation cross sections resulting in strong self-shielding effects. The neutron self-shielding correction factor was calculated using a simple analytical model of a single absorption event. Such a procedure has been applied to individual cross sections from pointwise ENDF/B-VI library and new corrected activation cross sections were introduced to a spectra unfolding algorithm. The method has been verified experimentally both for isotropic and for parallel neutron beams. Two sets of diluted and non-diluted activation foils covered with cadmium were irradiated in the neutron field. The comparison of activation rates of diluted and non-diluted foils has demonstrated the correctness of the applied self-shielding model.     

Investigating the TEPC radiation quality factor response for low energy accelerator based clinical applications

The use of a tissue equivalent proportional counter (TEPC) filled with propane based tissue equivalent gas simulating a 2 microm diameter tissue sphere has been investigated to estimate the radiation quality factor of the neutron fields used in in vivo neutron activation measurements at the McMaster University 3 MV Van de Graaff accelerator. The counter response to estimate the effective quality factor based on the definitions of Q(L) provided in ICRP 26 and 60 as a function of neutron energy has been examined experimentally using monoenergetic and continuous neutron spectra in the energy range of 35 to 600 keV. In agreement with other studies, the counter failed to provide a flat R(Q) response and showed a sharp drop below 200 keV neutron energy. Development of an algorithm to evaluate the quality factors using measured dose-mean lineal energy, yD, and comparison of the algorithm with other reported algorithms and analytical methods developed for the improvement in TEPC dose equivalent response has been reported.     

A novel ultra-light structure for radiation shielding

A new ultra-light structure based on the application of open-cell metal foams has been designed and investigated to determine its ability for attenuation of γ-rays and thermal neutrons. Open-cell metal foam, a unique class of material, has been employed in the structure and is studied in this work where radiation attenuation abilities of foams and foams filled with water and borated water have been compared with bulk Aluminum. The γ-ray attenuation measurements were performed using γ-ray at 0.662, 1.173 and 1.332 MeV photon energies and thermal neutron attenuation measurements were conducted using a polyenergetic thermal neutron beam. The results show that the metallic foam by itself attenuates less γ-ray as compared to bulk material, while the mass attenuation coefficients of foams filled with water is higher than that of bulk metals. The thermal neutron experiment, on the other hand, has shown a dramatic attenuation improvement in foams filled with water and particularly with borated water as compared to bulk metal and foam.     

Using a Lithium Glass Scintillator to Record Neutron Radiation

Measurement Techniques - Studies have been performed to improve the accuracy of energy measurements of neutron fields and to improve the spectrometric analysis of neutron radiation fields using...     

Individual neutron monitoring in workplaces with mixed neutron/photon radiation

EVIDOS ('evaluation of individual dosimetry in mixed neutron and photon radiation fields') is an European Commission (EC)-sponsored project that aims at a significant improvement of radiation protection dosimetry in mixed neutron/photon fields via spectrometric and dosimetric investigations in representative workplaces of the nuclear industry. In particular, new spectrometry methods are developed that provide the energy and direction distribution of the neutron fluence from which the reference dosimetric quantities are derived and compared to the readings of dosemeters. The final results of the project will be a comprehensive set of spectrometric and dosimetric data for the workplaces and an analysis of the performance of dosemeters, including novel electronic dosemeters. This paper gives an overview of the project and focuses on the results from measurements performed in calibration fields with broad energy distributions (simulated workplace fields) and on the first results from workplaces in the nuclear industry, inside a boiling water reactor and around a spent fuel transport cask.