Neutron Dose Equivalent Rates Research Articles

Analyses and Development of Effective Compensation Shields in a Multilegged Duct Streaming System

The 14-MeV neutron streaming experiment with three straight ducts is analyzed with the cell flagging technique of the MCNP code. The contributions of neutrons passing through the flagging cell located at the duct inlet and entering the detectors located in the duct are identified quantitatively. Furthermore, the streaming paths of neutrons entering in the duct inlet are cleared by the analysis.As an application of the cell flagging technique, a neutron streaming system with two-bend cylindrical duct in a thick concrete shield is prepared, and the flagging cells are located around the duct. The contributions of neutrons passing the flagging cells to the detector located at the duct outlet are cleared, and effective compensation shields to reduce the neutron dose-equivalent rate at the duct outlet are obtained by replacing some of the flagging cells with the NS-4-FR shield. Moreover, it is expected that the equilibrating contribution from each flagging cell to the dose-equivalent rate at the duct outlet is the essential function to make an effective compensation shielding system with neutron streaming.

Neutron dose equivalent rate in intermediate energy heavy ion target area

The fluence rate distributions of neutrons emitted in the reactions of 50 MeV/u 18O-ion on thick Be, Cu, Au targets were measured with an activation method of threshold detectors and the neutron dose equivalent rate distributions at 1 m from the targets in intermediate energy heavy ion target area were obtained using the conversion coefficients for neutron fluence rate to ambient dose equivalent rate.

Measurement of Dose-Equivalent Rates around a Cask and Monte Carlo Analysis with Actual Configuration of Fuel Basket

Gamma ray and neutron dose-equivalent rate distributions are measured with an ionization-type gamma-ray survey meter and a moderator-type neutron survey meter respectively around a TN-12A spent fuel transport cask, and the measured dose-equivalent rate distributions are analyzed by the Monte Carlo method. Two models for the aluminum-alloy fuel basket are considered in the Monte Carlo code MCNP 4B. In one case the configuration of the basket with 12 spent fuel assemblies is modeled in detail and the other is the homogenized basket as employed in the Sn code DOT 3.5. In addition, the burn-up distribution is taken into account to generate source neutrons and gamma rays in the z-axis of the spent fuel assemblies in both cases. The essential difference in the dose-equivalent rates is obtained from the Monte Carlo calculations employing the homogenized model and the actual configuration of the basket. Due to employing the actual configuration, the gamma ray and neutron dose-equivalent rates reduce to 67% and 80%, respectively as compared with the homogenized model on the surface of the TN-12A cask. As the results, the good C/Es are obtained: for the neutron it is approximately 1.0 and 1.25 for the gamma ray it is at the center of the cask surface, respectively The effect of the burn-up distribution appears clearly at the off-center cask surface, and in particular, the neutron dose-equivalent rates come close to the measured ones.

Experimental Analyses on Radiation Streaming through a Labyrinth in a Proton Accelerator Facility of Several Tens MeV

In order to estimate the accuracy of calculation methods such as empirical formulas and Monte Carlo codes for neutron and gamma ray fluxes through a labyrinth connected with a room having a Cu target irradiated by 68-MeV-proton, experimental analyses are carried out. As results of the analyses, the followings are obtained. (l)The MCNP calculation is applicable to estimation of neutron and gamma ray dose equivalent rates in the labyrinth, although cross sections in the process of neutron thermalization and gamma ray production cross section should be modified for more precise estimation. (2)The empirical formulas are useful for estimation of neutron dose equivalent rate. However, an additional safety factor of 2 should be required to apply to the estimation of total dose equivalent rates, because the contribution of gamma ray dose equivalent rate increases with the distance in the labyrinth and becomes comparable to that of neutron dose equivalent rate at the distance longer than 20 m.

Active Neutron Personal Dosimetry - Concepts and Recent Approaches

Active personal neutron dosemeters aim at obtaining the immediate readout of neutron personal dose equivalent or dose equivalent rate. Fulfillment of basic physical requirements like the correct dependence of the response of a dosemeter on energy and angle of the incident neutrons is a difficult task; other requirements like high sensitivity to neutrons because of the desired immediate information and insensitivity to photon radiation add to these difficulties. Among other efforts, the European Commission presently sponsors a research project within which various different approaches are investigated, aiming at the development of active personal dosemeters with improved sensitivity and energy dependence. Basic information on the various concepts is summarised in this paper.

Neutrons around thick target bombarded by 50MeV/u -ion beam

Abstract Neutron energy, fluence rate, angular distributions and dose equivalent rate distributions around thick Be, Cu, Au targets bombarded by a 50 MeV/ u 18 O -ion beam were measured by using a threshold detector activation method. The neutron yields and emission rates in the forward direction were obtained.

Radiation measurements on the flight of IML-2

The second flight of the International Microgravity Laboratory (IML-2) on Space Shuttle flight STS-65 provided a unique opportunity for the intercomparison of a wide variety of radiation measurement techniques. Although this was not a coordinated or planned campaign, by shear chance, a number of space radiation experiments from several countries were flown on this mission. There were active radiation measuring instruments from Japan and US, and passive detectors from US, Russia, Japan, and Germany. These detectors were distributed throughout the Space Shuttle volume: payload bay, middeck, flight deck, and Spacelab. STS-65 was launched on July 8, 1994, in a 28.45° × 306 km orbit for a duration of 14 d 17 hr and 55 min. The crew doses varied from 0.935 mGy to 1.235 mGy. A factor of two variation was observed between various passive detectors mounted inside the habitable Shuttle volume. There is reasonable agreement between the galactic cosmic ray dose, dose equivalent and LET spectra measured by the tissue equivalent proportional counter flown in the payload bay with model calculations. There are significant differences in the measurements of LET spectra measured by different groups. The neutron spectrum in the 1–20 MeV region was measured. Using fluence-dose conversion factors, the neutron dose and dose equivalent rates were 11±2.7 μGy/day and 95±23.5 μSv/day respectively. The average east-west asymmetry of trapped proton (>30MeV) and (>60 MeV) dose rate was 3.3 and 1.9 respectively.

Cosmic‐ray‐induced neutron intensity in Taiwan and its variations during a typhoon

Cosmic neutrons as a secondary cosmic radiation, strike the ground with intensity which varies at different latitude, altitude, and solar activity. In a 3‐year extensive survey period, the cosmic neutron intensity was measured over Taiwan islandwide; the average neutron dose equivalent rate is 0.65 nSv h−1 near sea level, up to 14.5 nSv h−1 at the highest mountaintop (altitude of 3952 m). In a rare event, the cosmic neutron intensity at a monitoring station was recorded during the entire period that Typhoon Herb swept across the island in 1996. The anomalous reading of cosmic neutron intensity in the typhoon eye, 2.17±0.40 nSv h−1 (or 4 times higher than the normal reading) is reported here for the first time. The onset of cosmic neutron intensity in the typhoon is closely associated with the steady, rapid air pressure drop as well as the 6×106 km3 spiral cloud; the dips in the onset are correlated with sustained gale winds and heavy precipitation. It is believed that the air pressure drop provides thinner air within the typhoon, and the spiral cloud containing dense moisture further slows down high energy neutrons from the ground, where we observed the neutrons showering down from the stratosphere where the cosmic neutrons have the maximum density. These combined effects result in more medium‐energy neutrons penetrating down to the ground through the typhoon; while the gale wind and heavy rain deflect, attenuate, and, to a lesser extent, absorb the cosmic neutrons from the atmosphere.

Transmission of neutron and gamma radiation fields along the maze of a cyclotron vault

The neutron and gamma dose equivalent rates at various locations along the central plane of the four legged maze leading to the concrete vault of a 30 MeV Medical Cyclotron were measured. These measurements were performed during the bombardment of a water cooled aluminium target with a 30 MeV proton beam. The experimentally estimated neutron dose equivalent rates along the distance of penetration into the maze were fitted explicitly with a set of exponential functions. These functions were utilised to optimise the maze design calculations of similar cyclotron target caves.

Neutron and Photon Spectra and Dose Rates Around a Shielding Cask Placed in a Salt Mine to Simulate a Nuclear Waste Package

The dose equivalent rate around a POLLUX storage container with radioactive waste is increased by neutrons and photons backscattered by the host rock when the waste package is transported to its final position in a repository. This was simulated by a smaller shielding cask with a 252Cf line source. Neutron spectral fluence and dose equivalent rates were measured with Bonner spheres above ground and underground and compared with MCNP calculations. There may be the same quantity of backscattered neutrons as of the unscattered component. The photon spectra were determined with an NE213 scintillation spectrometer above ground. Photon dose equivalent rates are derived and compared with calculated ones and dosemeter readings.

Neutron Dose Equivalent Rates Calculated from Measured Neutron Angular and Energy Distributions in Working Environments

The estimation of non-isotropic neutron dose equivalent quantities in multidirectional fields is complicated. The estimation must be based on the neutron fluence rate as a function of both energy and angle. There are several well developed techniques for measuring neutron energy spectra. However, the directional characteristics have not usually been investigated in detail. The personal dose equivalent inside the PWR stations studied varies by a factor of 4 between AP and ISO irradiation geometries, but only by a factor of 2 between different locations for AP irradiation. This indicates the importance of knowing the directional distribution, in order to interrelate personal dose equivalent, effective dose equivalent (effective dose) and ambient dose equivalent, and to interpret dosemeter and instrument readings. A purpose built phantom has been constructed and used for the separation of the neutron fluence into 18 incident directions. Measurements with the phantom have been made at workplaces at Ringhals Nuclear Power Plant. The phantom consists of a 19 cm diameter boron doped paraffin sphere. Boron was added to decrease the transmission of neutrons through the sphere and to decrease the photon contribution from (n;gamma) reactions inside the phantom to the measurement of dose equivalent from incident photons. The phantom has proved to be capable of separating the incident neutrons into 18 directions and effective in reducing (n;gamma) reactions. Measured neutron angular distributions with coarse energy information were used with previously measured neutron spectra, to make better estimates of the neutron personal dose equivalent rates at these workplaces in Swedish nuclear facilities. The values of personal dose equivalent calculated with separation into 18 incident directions are slightly lower than those calculated for 2 or 3 directional distributions (AP, ISO and/or ROT). Results of calculations of neutron personal dose equivalent using the new (ICRP Publication 60) Q(L) function and new stopping powers, are also given, with and without resolution into 18 incident directions.

Laminated primary ceiling barriers for medical accelerator rooms

In this work the performance of a number of laminated shields used as the primary ceiling barrier for high-energy accelerator rooms was investigated. The neutron and photon dose equivalent rates were measured outside each shield. Based on the measured photon level and the calculated X-ray leakage level the dose equivalent rate due to photons produced by neutron interactions with the shield materials was estimated. Shielding parameters for polyethylene were established for photons and neutrons. It was found that the barriers designed using the techniques given in an NCRP Report suffered from excessive radiation leakage.

Characterisation of the Mixed Neutron-Gamma Fields Inside the Swiss Nuclear Power Plants by Different Active Systems

Measurements have been made with different active dosimetric systems in four Swiss nuclear power plants (2 BWR and 2 PWR) to characterise the mixed n-γ fields. They were performed in 22 positions covering a wide variety of situations inside the containments, in a fuel storage facility and around spent fuel transport flasks. The spectrometric and dosimetric results are presented, discussed and compared. A classification of the neutron spectra in four categories according to their hardness is proposed. The spectra inside the containments are rather soft; they are much harder in the fuel storage facilities and around spent fuel transport flasks. Neutron dose equivalent rates in free access areas are low compared to those due to gammas, whereas in some limited access areas the neutron contribution to the dose equivalent may be appreciable

Superheated drop detector for determination of neutron dose equivalent to patients undergoing high-energy x-ray and electron radiotherapy.

The superheated drop detector (SDD) consists of thousands of superheated drops dispersed in a small vial of gel, which vaporize upon exposure to high LET radiation, thereby providing a directly observable indication of neutron dose. This detector possesses high sensitivity to neutrons and insensitivity to high-energy photons and electrons, making it suitable for the determination of neutron dose equivalent rates around high-energy photon and electron radiotherapy beams. In the present work, the SDD was used to measure the neutron dose equivalent in and around the radiotherapy beams produced by a 32-MeV linear accelerator. For both x-ray and electron beams, the neutron dose profiles were observed to follow the photon/electron radiotherapy beam profiles. For 25-MV x rays, the neutron dose equivalent per photon dose on the central axis increased by a factor of about 3 as field size increased from 5 x 5 to 30 x 30 cm. However, the neutron dose equivalent rate at 50 cm off-axis in the patient plane was essentially independent of field size. The neutron dose equivalent per electron dose was essentially zero for electron beams with energies below 15 MeV, but increased rapidly above 15 MeV. For 25-MeV electrons, neutron dose equivalent on the central axis was about 1/5 that for 25-MV x rays. Analogous to the data for 25-MV x rays, the neutron dose equivalent rate on the central axis of a 25-MeV electron beam exhibited a similar field size dependence and outside the beam it was essentially independent of field size.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative behaviour of different personal neutron dosimetry systems

The results of measurements with different personal neutron dosimetry systems are presented. This study is part of a broad programme of neutron and gamma dose measurements carried out at four Swiss nuclear power plants. The measurements were made simultaneously with various active and passive systems. The results reported here are those obtained by poly-allyl-diglycol-carbonate (PADC/CR-39), fission-track and bubble detectors. The irradiation positions covered a variety of neutron spectra, neutron-to-photon ratios, and dose equivalent rates. All the measurements were made with a water phantom. Different types/thicknesses of track detector materials were used. The agreement between the results of the dose equivalent rates obtained by the fission-track, bubble and CR-39 detectors was found to be within a statistical uncertainty of ≈± 30%.

Low-level neutron monitoring by use of germanium detectors

The detection sensitivity of various size germanium detectors to low-level neutron dose equivalent rate is described. Six high-purity germanium detectors with a crystal volume of 60 to 120 cm 3 were irradiated by neutrons produced from a 0.76 μg 252Cf isotopic neutron source. The counting rate of the prompt gamma rays of 596 keV was used to determine the detecting sensitivities of the detectors. The results show that the detecting sensitivity of the HPGe detector decreases with increasing detector size, and approximately proportional to the crystal volume.

Photoneutron production in the primary barriers of medical accelerator rooms.

Several radiation surveys, at medical linear accelerator facilities where lead or steel had been used with concrete to fabricate the primary barriers, revealed the existence of a sizable neutron field outside the shielding. This neutron field is produced by photoneutrons generated in the metal portion of the shield when the primary x-ray beam is aimed at the barrier. A method was developed to calculate the neutron dose-equivalent rate expected outside a primary shield when it is irradiated by a high-energy x-ray beam. It was found that the minimum photoneutron dose was produced when the metal part of the shield was positioned inside the treatment room in front of the concrete and also by using steel in place of lead. A thickness of less than or equal to 17 cm of metal on the inner surface of the shield produced only a slight increase in the neutron dose equivalent outside the barrier.

Neutronenfeldmessungen während der Bestrahlung mit hochenergetischen Photonen eines Linearbeschleunigers

Measurements of neutron dose equivalent rate with a LiJ scintillation counter and polyethylene moderator spheres (7.5, 30 and 40 cm diameter) have been performed in the treatment room of the linear accelerator Saturne-25 for 23 and 8 MV photon radiation. All values were normalized upon an absorbed dose rate of 0.8 Gy/min at 3 cm depth in a water phantom. At 50 cm central beam distance, at the level of the accelerator rotation axis, the neutron dose equivalent rate for 23 MV corresponds to a ratio of 0.0011 Sv/Gy; it is 4 times lower than on the beam axis, 4 times higher than in the room corners, and 80 times higher than for 8 MV. Only a small dependence upon photon beam direction and field size was found, probably due to a compensation between changes of leaking and scattered radiation. The neutron spectrum, determined by measurements without and with three moderator spheres, primarily consists of fractions of fast and thermal neutrons. The neutron fluence distribution in a 30 cm moderator sphere, measured with LiF-TLD, is in agreement with the neutron spectrum.

医用電子加速装置施設からの漏洩放射線 中性子の測定，計算との比較

Neutron dose equivalent rates are measured in the irradiation room and the maze of a medical electron accelerator facility. An m-counter used for measurement is associated with a gate circuit to reject piled up signals caused by pulsed X-rays, so that it is possible to measure the neutrons without obstruction by the piled up signals even in the irradiation room. Counting loss of neutrons is mathematically corrected, and the correction factor comes nearly equal to 1.5 at dose points close to the entrance to the irradiation room when 14 MV X-rays are generated. The measurement in the maze agrees with the calculation reported in the preceding report in a factor less than 1.5 except for dose points close to the entrance to the irradiation room.

医用電子加速装置施設からの漏洩放射線 Ｘ線，捕獲ガンマ線

Absorbed dose rates of photons are measured in the maze of a medical electron accelerator facility. Radiations in the maze include high energy recoiled electrons other than low energy reflected X-rays. An ionization chamber for low energy X-rays is used for measurement, and is put in a build-up cap thick enough to obstruct the recoiled electrons of which contribution to the chamber output comes sometimes equal to that of X-rays. Dose rates of capture gamma-rays in the maze are estimated applying the calculation of neutron dose equivalent rates reported in the preceding paper. It is proved that capture gamma-rays are dominant in the dose rates measured in the maze at 14 MV X-ray generation. The estimated absorbed dose rates of total photons including capture gamma-rays and reflected X-rays agree with the measured ones in the range from +10% to -27%.