Passive Dosimeters Research Articles

A Passive chemical dosimeter for the determination of N-substituted aromatic amines in air

A passive chemical dosimeter for the determination of a long exposure of N-substituted aromatic amines in air is described. The analytes are accumulated on a sorption filter by the many-hour exposure of a unit using a chemical reaction with a selective reagent. The diffusion coefficients, rate of chemisorption preconcentration, and detection limits of chemical dosimeters are found. The concentrations of N-methyl-aniline, N,N-dimethylaniline, and diphenylamine in air samples are determined.

Small fields output factors measurements and correction factors determination for several detectors for a CyberKnife® and linear accelerators equipped with microMLC and circular cones

The use of small photon fields is now an established practice in stereotactic radiosurgery and radiotherapy. However, due to a lack of lateral electron equilibrium and high dose gradients, it is difficult to accurately measure the dosimetric quantities required for the commissioning of such systems. Moreover, there is still no metrological dosimetric reference for this kind of beam today. In this context, the first objective of this work was to determine and to compare small fields output factors (OF) measured with different types of active detectors and passive dosimeters for three types of facilities: a CyberKnife(®) system, a dedicated medical linear accelerator (Novalis) equipped with m3 microMLC and circular cones, and an adaptive medical linear accelerator (Clinac 2100) equipped with an additional m3 microMLC. The second one was to determine the kQclin,Qmsr (fclin,fmsr) correction factors introduced in a recently proposed small field dosimetry formalism for different active detectors. Small field sizes were defined either by microMLC down to 6 × 6 mm(2) or by circular cones down to 4 mm in diameter. OF measurements were performed with several commercially available active detectors dedicated to measurements in small fields (high resolution diodes: IBA SFD, Sun Nuclear EDGE, PTW 60016, PTW 60017; ionizing chambers: PTW 31014 PinPoint chamber, PTW 31018 microLion liquid chamber, and PTW 60003 natural diamond). Two types of passive dosimeters were used: LiF microcubes and EBT2 radiochromic films. Significant differences between the results obtained by several dosimetric systems were observed, particularly for the smallest field size for which the difference in the measured OF reaches more than 20%. For passive dosimeters, an excellent agreement was observed (better than 2%) between EBT2 and LiF microcubes for all OF measurements. Moreover, it has been shown that these passive dosimeters do not require correction factors and can then be used as reference dosimeters. Correction factors for the active detectors have then been determined from the mean experimental OF measured by the passive dosimeters. Four sets of correction factors needed to apply the new small field dosimetry formalism are provided for several active detectors. A protocol for small photon beams OF determination based on passive dosimeters measurements has been recently proposed to French radiotherapy treatment centers.

Area radiation monitoring on ISS Increments 17 to 22 using PADLES in the Japanese Experiment Module Kibo

The measurement of radiation environmental parameters in space is essential to support radiation risk assessments for astronauts and establish a benchmark for space radiation models for present and future human space activities. The Japan Aerospace Exploration Agency (JAXA) is performing a continuous area radiation monitoring experiment using the “PAssive Dosimeters for Lifescience Experiments in Space” (PADLES) system inside the Japanese Experiment Module Kibo on board the International Space Station (ISS). The PADLES dosimeter consists of thermoluminescent dosimeters (TLDs) and CR-39 plastic nuclear track detectors (PNTDs). JAXA has run the Area PADLES experiment since the Kibo module was attached to the ISS in June 2008, using 17 dosimeters in fixed locations on the Pressurized Module (PM) and the Experiment Logistics Module-Pressurized Section (ELM-PS) of Kibo, which are replaced every 6 months or every Increment, respectively. For three monitoring periods, known as Area PADLES experiment series #1 to #3, of 301, 180, and 232 days in June 2008 to April 2010 over ISS Increments 17 to 22, the average absorbed dose (dose equivalent) rates of 12 positions in the PM of Kibo were 319 ± 30 μGy/day (618 ± 102 μSv/day), 276 ± 30 μGy/day (608 ± 94 μSv/day), and 293 ± 33 μGy/day (588 ± 84 μSv/day), respectively. The radiation measurement in the ELM-PS was conducted in only Area PADLES experiment series #3 from August 2009 to April 2010 (232 days) over ISS Increments 21 to 22, the average absorbed dose (dose equivalent) rates of 5 positions was 297 ± 28 μGy/day (661 ± 65 μSv/day). The directional dependence of the radiation field was also investigated by installing PADLES dosimeters located in the zenith of ELM-PS of Kibo.

SU-E-T-85: Total Body Irradiation (TBI) Optically Stimulated Luminescence In Vivo Dosimetry

Purpose: To adequately monitor the doses for Total Body Irradiation (TBI) patients using opposed fields, we carry out both active and passive in vivo dosimetry, with the passive dosimeter being Optically Stimulated Luminescence (OSL) detectors. The OSLs are embedded in hemispherical brass buildup caps with the OSL/flat surface placed against patient skin. The in vivo dosimetry devices are not repositioned when the patient is rotated, so both entrance and exit dosimetry are carried out, which arguably gives a better midline dose estimate, and improves marrow graft acceptance by minimizing treatment time. The OSLs were calibrated using a 100cm SAD entrance method with brass buildup caps. The buildup caps have a maximum density thickness of 6.6g/cm2; dmax for our 18MV beam is 3.4g/cm2. This method results in an overestimation of exit dose due to the density thickness differences, requiring a correction factor for exit measured doses. Methods: To determine the exit dosimetry correction factor, OSLs were placed on the entrance and exit sides of solid water phantoms under TBI conditions having thicknesses of 10, 30, and 49.2cm. Combinations of OSLs included entrance-only, exit-only and combined entrance and exit measured doses. The entrance and exit dose to the dosimeter position was calculated. Results: The exit dose correction factor calculated from the ratio of calculated to measured values was larger than anticipated (∼17%). The correction factor was applied to the exit dose calculations and compared to the combined measurement for verification. Results were within 1.2% (Table 1).The method was tested on ten patients, giving agreement within 1.1±4% (Table 2). Acceptable TBI tolerance is ±10%. Conclusion: While this is acceptable, we will consider using non-brass buildup caps to obtain smaller correction factors.

TH‐A‐144‐01: Lens of the Eye Dosimetry

International Commission on Radiological Protection (ICRP) issued a statement in 2011 decreasing the threshold for the lens of the eye to 0.5 Gy from earlier figure of 5 Gy for chronic and 0.5–2.0 Gy for acute exposures. Further, ICRP has reduced the dose limit for workers from150 mSv/y to 20 mSv/y, averaged over defined period of 5 years. Interventional radiology and cardiology are areas with high potential for risk to eye lens. If radiation protection is not practiced, the eye lens dose to workers in these areas can reach or exceed the regulatory limit. There are recent reports of eye lens injuries among interventionalists and support professionals in interventional suite. As against well established personal whole‐body dosimetry, eye lens dosimetry practically does not exist. Accurate assessment of eye lens dose is one of the most important aspects of correlating doses with observed lens opacities among workers in interventional suites. There are two approaches for practical dosimetry: a) passive dosimeters; and b) active dosimeters. Besides practical dosimetry, approaches are based either on retrospective dose assessment using scatter radiation dose levels or correlations between patient dose indices and eye doses to the operators. In cases when practical individual monitoring is not established or when it is not reliable, the later approaches are often the only possible. At present, there is no clear consensus in the literature on the correlation between the patient dose and the dose to the eyes of the medical staff. Some authors provided a reasonable good relationship between PKA and dose to staff members, but there are also reports claiming that the establishment of the correlation between PKA and eye lens doses is associated with difficulties. The retrospective assessment needs to be based on the reconstruction of the laboratory workload (types and numbers of procedures), usually with questionnaires and the application of many assumptions about past activity (procedures performed, corresponding doses based on previous dosimetric studies and the use of radiation protection tools). This approach is also related to a “typical” procedure in terms of fluoroscopy time and number of cine series, which may not always reflect the real clinical situation. Accurate dose measurements are a prerequisite for investigation of low dose effects to the lens of the eye. This can be done only if dedicated and suitable calibrated dosimeters are available. With current state of technology and practice, only an approximate retrospective analysis can be accomplished. This is often not adequate for verification of compliance with regulatory dose limits. Future challenges include development of practical methods for regular monitoring of individual eye lens doses and development of better techniques to estimate eye dose from measurements at same reference points. While the situation with dosimetry is gloomy, it is very optimistic with eye protection. “Eye lens radiation dose from brain perfusion CT exams”.CT perfusion imaging requires repeatedly exposing one location of the head to monitor the uptake and washout of iodinated contrast. The accumulated radiation dose to the eye lens can be high, leading to concerns about potential radiation injury from these scans. CTDIvol assumes continuous z coverage and can overestimate eye lens dose in CT perfusion scans where the table do not increment. The radiation dose to the eye lens from clinical CT brain perfusion studies can be estimate using Monte Carlo simulation methods on voxelized patient models. MDCT scanners from four major manufacturers were simulated and the eye lens doses were estimated using the AAPM posted clinical protocols. They were also compared to CTDIvol values to evaluate the overestimation from CTDIvol. The efficacy of eye lens dose reduction techniques such as tilting the gantry and moving the scan location away from the eyelens were also investigated. Eye lens dose ranged from 81 mGy to 279 mGy, depending on the scanner and protocol used. It is between 59% and 63% of the CTDIvol values reported by the scanners. The eye lens dose is significantly reduced when the eye lenses were not directly irradiated. CTDIvol should not be interpreted as patient dose; this study has shown it to overestimate dose to the eye lens. These results may be used to provide more accurate estimates of actual dose to ensure that protocols are operated safely below thresholds. Tilting the gantry or moving the scanning region further away from the eyes are effective for reducing lensdose in clinical practice. These actions should be considered when they are consistent with the clinical task and patient anatomy.Learning Objectives:1. To understand current methods for estimation and measurement of eye lens dose for occupationally exposure.2. To understand correlation between patient dose indies and staff eye dose.3. To become familiar with method of eye dose estimation for patient in specific situation of brain perfusion CT.

Dosimetry for Small Fields in Stereotactic Radiosurgery Using Gafchromic MD-V2-55 Film, TLD-100 and Alanine Dosimeters

This work investigated the suitability of passive dosimeters for reference dosimetry in small fields with acceptable accuracy. Absorbed dose to water rate was determined in nine small radiation fields with diameters between 4 and 35 mm in a Leksell Gamma Knife (LGK) and a modified linear accelerator (linac) for stereotactic radiosurgery treatments. Measurements were made using Gafchromic film (MD-V2-55), alanine and thermoluminescent (TLD-100) dosimeters and compared with conventional dosimetry systems. Detectors were calibrated in terms of absorbed dose to water in 60Co gamma-ray and 6 MV x-ray reference (10×10 cm2) fields using an ionization chamber calibrated at a standards laboratory. Absorbed dose to water rate computed with MD-V2-55 was higher than that obtained with the others dosimeters, possibly due to a smaller volume averaging effect. Ratio between the dose-rates determined with each dosimeter and those obtained with the film was evaluated for both treatment modalities. For the LGK, the ratio decreased as the dosimeter size increased and remained constant for collimator diameters larger than 8 mm. The same behaviour was observed for the linac and the ratio increased with field size, independent of the dosimeter used. These behaviours could be explained as an averaging volume effect due to dose gradient and lack of electronic equilibrium. Evaluation of the output factors for the LGK collimators indicated that, even when agreement was observed between Monte Carlo simulation and measurements with different dosimeters, this does not warrant that the absorbed dose to water rate in the field was properly known and thus, investigation of the reference dosimetry should be an important issue. These results indicated that alanine dosimeter provides a high degree of accuracy but cannot be used in fields smaller than 20 mm diameter. Gafchromic film can be considered as a suitable methodology for reference dosimetry. TLD dosimeters are not appropriate in fields smaller than 10 mm diameters.

Use of a CMOS Image Sensor for an Active Personal Dosimeter in Interventional Radiology

Interventional radiologists and staff members, during all their professional activities, are frequently exposed to protracted and fractionated low doses of ionizing radiation. Due to skin tissues and peripheral blood irradiation, these exposures can result in deterministic effects (radiodermatitis, aged skin, and hand depilation) or stochastic ones (skin and non-solid cancer incidence). The authors present a novel approach to perform online monitoring of the staff during their interventions by using a device based on an Active Pixel Sensor. The performance of the sensor as an X-ray radiation detector has been evaluated with a proper experimental setup: the number of photons and the generated charge have been assessed as dosimetric observables from the frames acquired by the sensor using a two-threshold clustering algorithm, the efficiency of which has been evaluated as well. The correlation of these observables with passive dosimeter dose measurements has been analyzed: a good linearity has been demonstrated, and the response difference between pulsed and continuous operational modes is reduced to less than 10%, marking a distinct improvement with respect to commercial Active Personal Dosimeters.

Radon Concentration in Al-Gazalia City, Baghdad, Iraq

Inhalation of indoor radon has been recognized as one of the health hazards. In the present work a set of indoor radon measurements was carried out, in different Iraqi houses in Al-Gazalia city/ Baghdad, built of the same type of building materials. Radon concentration determined by using time–integrated passive radon dosimeters containing CR-39 solid state nuclear track detector. Measurements were carried out from September to November 2011. The results show that, the radon concentrations varied from (38.7 to 200) Bq m -3 . The mean values of radon concentrations in second floor hall and first floor of living rooms, kitchens and bathrooms, were: (46.56±9.13), (68.32±8.55), (128.03±21.65) and (168.92±33.52) Bq.m -3 respectively. This data shows that, bathrooms and kitchens have significantly higher radon concentrations and the second floor is lower radon concentration in all dwellings.

Study of combinations of TL/OSL single dosimeters for mixed high/low ionization density radiation fields

In this paper we discuss and compare the potential application of combined OSL/TL measurements using 6LiF:Mg,Ti (TLD-600 is enriched of isotope 6Li which has a high cross-section for the reaction with slow neutrons) or 7LiF:Mg,Ti ( TLD-700 is enriched of 7Li isotope) and TLD-100 (natural isotopic composition) detectors. The OSL/TL duel readout of LiF:Mg,Ti as an ionization density discriminator avoids some of the difficulties inherent to the various types of discrimination mixed-field passive dosimeters, and in addition has several advantages. The preferential excitation of OSL compared to TL following high ionization density (HID) alpha irradiation, naturally explained via the identification of OSL with the “two-hit” F2 or F3 center, whereas the major component of composite TL glow peak 5 is believed to arise from a "one-hit" complex defect. This evidence allows near-total discrimination between HID radiation and low-ionization density (LID) radiation.Beta and alpha particle irradiations were carried out with 90Sr/90Y (∼500 keV average energy) and 241Am sources (4.7 MeV) respectively and neutron irradiations were carried out at the PTB (Germany) (En = 5 MeV) and RARAF (Columbia University, USA) (En = 6 MeV) accelerator facilities.The highest values of the FOM obtained was ∼30 for neutron/gamma discrimination and ∼110 for alpha/gamma discrimination using OSL/TL – peak 5 measurements in TLD-700.

Preliminary study on the measurement of background radiation dose at Antarctica during 32 nd expedition

A significant proportion (10%) of the natural background radiation is of cosmic origin. Cosmic ray consists of gamma, protons, electrons, pions, muons, neutrons and low Z nuclei. Due to the geomagnetic effect, cosmic radiation levels at poles are higher. As a consequence, personnel working in Antarctica (or Arctic) are subjected to high level of cosmic radiation. The present study gives the details of the estimation of background radiation (neutrons, gamma and electrons) dose rate around the Indian station at Antartica named Bharati measured during 32 nd Indian scientific expedition to Antarctica (32 nd INSEA). The measurement was carried out by passive dosimeters such as TLDs and CR-39 and active dosimeter such as RadEye G portable gamma survey meter. Gamma and electron components were measured using TLDs and survey meter, whereas CR-39 SSNTDs and neutron sensitive TLDs were used for neutron measurements. These detectors were deployed at few selected locations around Bharati station for about 2΍ months during summer expedition. The neutron detectors used in the study were pre-calibrated with 241 Am-Be fast/thermal neutron source. The fast neutron dose rate measured based on CR-39 detector was found to about 140-420 nSv/h. The gamma dose rate evaluated by TLDs/survey meter are in the range of 290-400 nSv/h.

Environmental Air Dosimetry in Some Locations of Jazan Using Passive Track Detectors

The concentration of total alpha and radon activities in soil was measured by collecting soil samples at different locations in Jazan. In this study the passive dosimeters which were based on solid state nuclear track detectors SSNTD are reviewed. The principle of passive dosimetry and the factors affecting their performance are discussed. The concentrations of radon and alpha emitters in different locations in Jazan were evaluated using CR-39 detector. The method is based on the proxy equilibrium factor K. The alpha emission distributions in volume and radon concentrations at the top of the shelf of a 9.0 cm height diffusion chamber were determined. The radon concentrations are ranging from 55 to 151 Bq/m 3 with an average of 88 Bq/m 3 . The highest dose was measured to be 1.7mSv/y on Cady Mall and the average annual effective dose was 1.42 mSv/y. These values were in the range of allowed international value according to the values of (ICRP

Radon concentration levels in the historical city of Jerash, Jordan

Abstract Radon concentration levels were measured in the soil and indoors of Jerash city using time-integrated passive radon dosimeters enclosing CR-39 solid-state nuclear track detectors (SSNTDs). The city was divided into five sectors over which four hundred dosimeters were distributed randomly; one hundred dosimeters were installed into soil and the rest were distributed indoors. The study revealed that radon concentration in soil ranges from 0.98 to 3.05 kBq/m 3 with an average of 2.29 kBq/m 3 . Indoors radon concentration measurements versus floor elevation showed that the maximum concentration was in the basement and decreases gradually for upper floors. A considerable variation in concentration levels for the same floor in the different sectors was noticed and attributed to the difference in the type of the soil of each sector. Results showed that soil and indoors radon concentration are strongly related for the same area of study. In general, it is found that the average indoor radon concentration in Jerash city is 49.21 Bq/m 3 . The measured radon levels are within the globally accepted values and no unusual concentrations were observed.

A Study of Factors Affecting in Increasing or Decreasing of Radon Levels in Buildings of Suwaylih Town

This research aimed to study the factors affected in radon levels in buildings of Suwaylih town (altitude about 900 to 1300 above the sea level) as areference to Jordan. The study was started from august, 10, 2012 to October, 10, 2012. Suwaylih Town divided into six districts namely, Al-Kamaliah, Al-Rahmaniah, Al-Sharqy, Al-Fadielah, Maysaloon, Al-Bashaaer districts. About 780 Passive dosimeters containing highly pure CR - 39 were distributed randomly in districts of Suwaylih town. The indoor dosimeters were collected after three months. The collected detectors were chemically etched using 30% KOH for 9 hours at 70 ± 0.1 °C. An optical microscope was used to measure the nuclear alpha track density on the detectors surfaces. The research results showed that radon concentration affected by many factors, for example, the average concentration in guest rooms was 83 ± 18 Bq.m -3 and 70 ± 14 Bq.m -3 in bed rooms, while it was 53 ± 10 Bq.m -3 in living rooms. Morever, the concentration was about 97 ± 15 Bq.m -3 in rooms without ventilation while the concentration was about 30 ± 10 Bq.m -3 at rooms ventilated more than 9 hours daily. The study also showed the concentration was relatively high 90 ± 18 Bq.m -3 in buildings made of stones while concentration was low 47 ± 12 Bq.m -3 in the buildings made from blocks. In addition, the concentration was different with increase in age of building, the average were 63 ± 15 Bq.m -3 , 51 ± 13 Bq.m -3 and 39 ± 11 Bq.m -3 in more than 25 years, between 10 - 25 years and less than 10 years, respectively. In general the radon concentration in Suwaylih town was found to be about 62 ± 13 Bq.m -3 .

Analysis of radiation dose variations measured by passive dosimeters onboard the International Space Station during the solar quiet period (2007–2008)

Abstract The average absorbed dose and dose equivalent rates from space radiation were observed using passive dosimeters with same material and configuration at the same location onboard the International Space Station (ISS) over four different occasions (I–IV) between 2007 and 2008. The passive dosimeters consisted of a combination of thermoluminescent detectors (TLDs) and plastic nuclear track detectors (PNTDs). Total average absorbed dose rate increased by 68 ± 9% over two years. The observed increase was due to the incremental increase in the altitude of the ISS over the course of the experiment and the corresponding increase in trapped proton flux encountered during passage of the ISS through the SAA (South Atlantic Anomaly), which was confirmed with the results monitored by DB-8 active dosimeter on the ISS. The PNTD data showed that the average absorbed dose and dose equivalent rates from particles of LET∞H2O ≥ 100 keV/μm were 28 ± 2% and 51 ± 3% of ≥10 keV/μm during Periods I–III, while the dose contributions of particles ≥100 keV/μm during Period IV were 36 ± 5% and 59 ± 10%, respectively. The integral dose equivalent distribution during Period IV shows significant enhancement from particles ≥100 keV/μm. These facts suggest that a significant fraction of the high LET component is due to short-range recoil nuclei produced in target fragmentation reactions between primary protons and the nuclei of the passive dosimeters and surrounding materials.

Characterization of the neutron field from the 241Am-Be isotopic source of the IPHC irradiator

241 Am-Be source abstract A measurement campaign has been carried out recently to provide the source intensity and the reference spectra around a neutron irradiation facility based on 241 Am-Be radionuclide source, using the UAB Bonner Sphere Spectrometer. This facility, which consists of a bunker, a container/shielding for the source and an irradiation device that uses an automated remote-controlled system for the source positioning and rotating during the dosimeter irradiation, is intended to be routinely used to check the response of passive dosimeters, namely those based on photo-stimulated imaging plates and solid-state nuclear track detectors. The measurement results, in terms of neutron spectra and global dosimetric quantities (i.e., fluence and ambient dose equivalent rates) at different distances with respect to the 241 Am-Be source, were compared with Monte Carlo simulations using the MCNPX code and a good agreement was observed. An estimation of the un-scattered neutron spectrum directly emitted from the 241 Am-Be source is given as well.

Mesure de l’exposition externe du personnel du service de médecine nucléaire de l’hôpital Ibn Sina de Rabat au cours de la manipulation des radiopharmaceutiques

The nuclear medicine department of Ibn Sina hospital was created in 1965. Its first location was in radiology department, and then it was transferred in 1977 to the old emergency unit. The conception was not originally dedicated for a nuclear medicine department. To reassure the medical staff about their exposition during radiopharmaceutical manipulation, a dosimetric study was performed in collaboration with the National center of radiation protection (CNRP). Dosimetric monitoring was conducted for 3months with the use of passive thermoluminescent dosimeters, (Harshow, 6600 plus), covering all categories of staff. The administration of 153 Samarium has been also studied. Other cases of people concerned our study: a pregnant woman doctor, whose exposure of the unborn child must be reduced as much as possible, and a woman from a private company, working occasionally in the department. To control the conditions imposed on all activities requiring exposure to ionizing radiation, we evaluated the dose at the extremities of operators with the use of ring dosimeters (Harshow 6600 plus) and the dose on the ambient environment of staff. This experiment has shown exposure levels below legal limits, without been negligible for some workstations. Technicians equivalent doses results justify the wearing of dosimeter rings as a complementary dosimeter in nuclear medicine department. Finally, monitoring ambient dosimetry showed that the environment measurements are in keeping with national and international norms. The Medical staff was very interested by this study, which demonstrates that radiation protection standards are respected through awareness, basic training, continuing education and information. The first author is particularly involved in the practical procedures in order to respect radiation protection recommendations for restraining staff annual chest and extremities dosimetry figures within regulations values.

Doses to skin during dynamic perfusion computed tomography of the liver

Many new computed tomography (CT) techniques have been introduced during the recent years, one of them being CT-assisted dynamic perfusion imaging (perfusion CT, PCT). Many concerns were raised when first cases of deterministic radiation effects were reported. This paper shows how radiochromic films can be utilised as passive dosemeters for use in PCT. Radiochromic dosemeters undergo a colour change directly and do not require chemical processing. Prior to their use, they need to be calibrated. Films are placed on top and on the right side of the patient and exposed during the procedure. Readout is performed using a densitometer. Results show that average local skin doses are 0.51±0.07 and 0.42±0.04 Gy on top and on the lateral side of the patient, respectively. Results of the patient dosimetry (local skin doses) are consistent. This is due to the fact that each patient had the same CT protocol used for imaging (120 kV, 60 mA and C(vol) of 247.75 mGy). Radiochromic films designed for interventional radiology can be effectively used for local skin dose measurements in perfusion CT. Dose values obtained are below the threshold needed for deterministic effects (erythema, hair loss, etc.). These effects might happen if inappropriate CT protocol is used; one that is usually used for routine imaging.

Photon contributions from the 252Cf and 241Am–Be neutron sources at the PSI Calibration Laboratory

Abstract At the accredited PSI Calibration Laboratory neutron reference fields traceable to the national standards of the Physikalisch-Technische Bundesanstalt (PTB) in Germany are available for the calibration of ambient and personal dose equivalent (rate) meters and passive dosimeters. The photon contribution to the ambient dose equivalent in the neutron fields of the 252Cf and 241Am–Be sources was measured using various photon dose rate meters and active and passive dosimeters. Measuring photons from a neutron source usually involves considerable uncertainties due to the presence of neutron induced photons in the room, due to a non-zero neutron sensitivity of the photon detector, and last but not least due to the energy response of the photon detectors. Therefore eight independent detectors and methods were used to obtain a reliable estimate for the photon contribution of the two sources as an average of the individual methods. For the 241Am–Be source a photon contribution of approximately 4.9% was determined and for the 252Cf source a contribution of 3.6%.

Cosmic Radiation Exposure of Biological Test Systems During the EXPOSE-E Mission

In the frame of the EXPOSE-E mission on the Columbus external payload facility EuTEF on board the International Space Station, passive thermoluminescence dosimeters were applied to measure the radiation exposure of biological samples. The detectors were located either as stacks next to biological specimens to determine the depth dose distribution or beneath the sample carriers to determine the dose levels for maximum shielding. The maximum mission dose measured in the upper layer of the depth dose part of the experiment amounted to 238±10 mGy, which relates to an average dose rate of 408±16 μGy/d. In these stacks of about 8 mm height, the dose decreased by 5-12% with depth. The maximum dose measured beneath the sample carriers was 215±16 mGy, which amounts to an average dose rate of 368±27 μGy/d. These values are close to those assessed for the interior of the Columbus module and demonstrate the high shielding of the biological experiments within the EXPOSE-E facility. Besides the shielding by the EXPOSE-E hardware itself, additional shielding was experienced by the external structures adjacent to EXPOSE-E, such as EuTEF and Columbus. This led to a dose gradient over the entire exposure area, from 215±16 mGy for the lowest to 121±6 mGy for maximum shielding. Hence, the doses perceived by the biological samples inside EXPOSE-E varied by 70% (from lowest to highest dose). As a consequence of the high shielding, the biological samples were predominantly exposed to galactic cosmic heavy ions, while electrons and a significant fraction of protons of the radiation belts and solar wind did not reach the samples.

Investigation of the stability of commercial neutron probes

At the Paul Scherrer Institute's Calibration Laboratory, neutron reference fields are provided for the calibration of ambient and personal dose equivalent (rate) metres and passive dosemeters. To ensure traceability to the standards of the Physikalisch-Technische Bundesanstalt (PTB) in Germany, the neutron fields are characterised by means of a PTB-calibrated Berthold LB6411 neutron probe which is used as a secondary standard. The LB6411 detector suffers from an unstable, increasing dose rate reading in the order of up to +5 % (according to the manufacturers, this is due to a charging effect in the (3)He proportional counter). In a calibration, this instability is usually corrected for based on the reading obtained with a test source. In this work, the instability was investigated by means of measurements under irradiation with ambient dose equivalent rates up to 24 mSv h(-1) for up to 20 h and compared with the behaviour of an LB6419 and a Thermo Wendi-2 probe. The reading of the instruments was found to reach a plateau, e.g. it becomes stable after ∼90 min during irradiation with 10 mSv h(-1) neutrons. The plateau is reached faster for higher dose rates. This supports the interpretation as a charging effect in the proportional counter. The effect could also be duplicated in an irradiation with photons from a (137)Cs source. The decay time of the accumulated charge was found to be very long, i.e. the instrument showed a stable reproducible reading for up to 6 h after the plateau was reached. From these observations, a conditioning procedure was derived which ensures a stable operation of the instrument after an irradiation of the instrument preceding its use in the reference measurements.