Experimental results: an update

This article is a summary of experimental results from highest energy cosmic ray measurements, focusing on data and analyses that became available after 1999. To cite this article: R. Engel, H. Klages, C. R. Physique 5 (2004).     

Double pair production by ultra-high-energy cosmic ray photons

Using the CompHEP package, we provide a detailed estimate of the influence of double e ⁺ e ⁻ pair production (DPP) by photons on the propagation of ultra-high-energy electromagnetic cascades. We show that in the models where the cosmic ray photon energy reaches a few 10³ EeV, a refined DPP analysis may lead to a substantial difference in the predicted photon spectrum compared to the previous rough estimates. The article is published in the original.     

Gafchromic® film dosimetry for low energy X radiation

Recent developments have produced low energy X ray systems capable of providing a radiation dose to adequate volumes suitable for sterile inset programmes. To support the adoption of these new systems, the performance of the Gafchromic^® HD-810 dosimetry system previously used for gamma irradiation needed to be better understood at the lower photon energies. For low energy photons, the optical density of the irradiated Gafchromic HD-810 film dosimeters significantly depends on the material surrounding them. For example, if paper, Mylar^® or PVC is used to house the dosimeter during irradiation, the optical density can vary by as much as a factor of three or more for the same dose. This paper is an attempt to elucidate the performance of the Gafchromic HD-810 film dosimeters for such low energy X radiation (∼150 keV). Our data show that this behaviour can be explained by the fact that these materials have significantly different photon mass attenuation coefficient. This conclusion was reinforced with mathematical simulation using Monte Carlo modelling. We also show that with the different structure of another Gafchromic film dosimeter (MD-V2-55) this effect is virtually non-existent. An understanding of the behaviour of thin film dosimeters like Gafchromic HD-810 under radiation is crucial for reliable dosimetry. We hope that this work can also provide guidance in the use of other thin film dosimeters at similar low photon energies.     

Fiber-optic-coupled RbMgF3:Eu for remote radiation dosimetry

A portable fiber optic dosimeter has been developed that incorporates RbMgF₃:Eu²⁺ at the end of a multimode polymer fiber. It uses two stimulation sources and takes advantage of the range of stimulable traps in this compound. We observe radioluminescence (RL) during gamma ray irradiation and show that the low-dose RL increases linearly with increasing dose rate where the minimum detectible dose rate is ∼0.015 μSv/s. We show that pulsed infrared-stimulated (940 nm) optically stimulated luminescence (OSL) can be used for real time dose monitoring. The cumulative dose can be readout after irradiation where a linear OSL dose response was observed when stimulating at 505 nm and the minimum detectable dose is ∼50 μSv.     

Occupational doses of medical staff and their relation to patient exposure incurred in coronary angiography and intervention

Patient kerma–area product P_KA, cumulative kerma in the air K_IRP, fluoroscopic time t, personal dose equivalent (in terms of Hp(10), Hp(0.07) and Hp(3)) for most common interventional cardiology procedures were measured. P_KA and K_IRP measurements were used for patients and thermoluminescent dosimetry for the personnel. Dosemeters for personal doses measurements containing MCP-N (LiF: Mg, Cu, P) type thermoluminescence detectors (TLDs), were read out at the Institute of Nuclear Physics Polish Academy of Science (IFJ PAN) dosimetry service.The patient and personal doses were similar to those reported by other authors. The mean values of total kerma–area product (P_KA) were 22.7 (7.3–50.9) Gy·cm² for coronary angiography (CA) and 43.1 (3.2–86.4) Gy·cm² for percutaneous coronary intervention (PCI). In general, doses received by the staff performing PCI procedures were found to be systematically higher than those after CA procedures, by some 30% or more. Within the medical team, operators always received the highest doses, followed by nurses and technicians. Maximum eye lens doses, skin doses and whole body doses were 165 μSv, 962 μSv and 30 μSv per procedure, respectively. Annual eye lens doses received by the operators, especially in PCI procedures, may well exceed the value of the recently recommended annual dose limit of 20 mSv and should be monitored.No meaningful correlation could be established between occupational doses and patient exposure, however some degree of correlation was observed between values of dose to the eye lens and whole body dose.     

Directional reconstruction and anisotropy studies

The incident directions of cosmic ray particles at the highest energies are readily measured by modern detectors. Provided that there are sufficient data, the interpretation of such directions should give strong information on the origin of cosmic rays. However, the choice of statistical methods used in such an analysis can be of paramount importance in the reliability of the interpretation. This article summarizes the conclusions which may be drawn from the present data, and proposes methods needed to interpret statistically the large quantities of data which will become available in the near future. To cite this article: R.W. Clay et al., C. R. Physique 5 (2004).     

The influence of field size and off-axis distance on photoneutron spectra of the 18 MV Siemens Oncor linear accelerator beam

At present, high energy electron linear accelerators (LINACs) producing photons with energies higher than 10 MeV have a wide use in radiotherapy (RT). However, in these beams fast neutrons could be generated, which results in undesired contamination of the therapeutic beams. These neutrons affect the shielding requirements in RT rooms and also increase the out-of-field radiation dose to patients. The neutron flux becomes even more important when high numbers of monitor units are used, as in the intensity modulated radiotherapy. Herein, to evaluate the exposure of patients and medical personnel, it is important to determine the full radiation field correctly. A model of the dual photon beam medical LINAC, Siemens ONCOR, used at the University Hospital Centre of Osijek was built using the MCNP611 code. We tuned the model according to measured photon percentage depth dose curves and profiles. Only 18 MV photon beams were modeled. The dependence of neutron dose equivalent and energy spectrum on field size and off-axis distance in the patient plane was analyzed. The neutron source strength (Q) defined as a number of neutrons coming from the head of the treatment unit per x-ray dose (Gy) delivered at the isocenter was calculated and found to be 1.12 × 10¹² neutrons per photon Gy at isocenter. The simulation showed that the neutron flux increases with increasing field size but field size has almost no effect on the shape of neutron dose profiles. The calculated neutron dose equivalent of different field sizes was between 1 and 3 mSv per photon Gy at isocenter. The mean energy changed from 0.21 MeV to 0.63 MeV with collimator opening from 0 × 0 cm² to 40 × 40 cm². At the 50 cm off-axis the change was less pronounced. According to the results, it is reasonable to conclude that the neutron dose equivalent to the patient is proportional to the photon beam-on time as suggested before. Since the beam-on time is much higher when advanced radiotherapy techniques are used to fulfill high conformity demands, this makes the neutron flux determination even more important. We also showed that the neutron energy in the patient plane significantly changes with field size. This can introduce significant uncertainty in dosimetry of neutrons due to strong dependence of the neutron detector response on the neutron energy in the interval 0.1–5 MeV.     

Commercial pharmaceutical glass containers as probes for the post-sterilization dosimetry of liquid drugs

Drug sterilization with ionizing radiation is a well-established technology, which is gaining ground the last decades since it allows the adequate sterilization of heat-sensitive pharmaceutical preparations. In a previous study (Kazakis et al., 2015a), the possibility to identify irradiated liquid-state drugs by means of TL measurements on their glass containers was explored with very promising findings.The present work constitutes a continuation and extension of the previous work, employing additional TL measurements, along with new OSL measurements, on the same glass containers of two widely used liquid drugs, (Hexalen^® and Voltaren^®), for beta-doses up to 30 kGy, while an investigation of the presence of very deep traps (VDT), i.e., traps with their peak maximum temperature beyond the 500 °C, also took place.Results indicate that dose estimation, after the ionizing sterilization of a liquid drug, using the glass containers is possible in many ways. Both direct OSL and TL dose response can be fitted with a linear function for doses up to 6 kGy and 14 kGy for Hexalen and Voltaren respectively. For higher doses, up to 30 kGy, the intensity continues to increase, though in a lower rate, and the response can be fitted with a linear function as well, indicating that no saturation is reached. Presence of VDT is evident in both glasses with their thermally assisted OSL (TA-OSL) and subsequent photo-transferred residual TL (RTL) dose response exhibiting linear behavior in two distinctive dose areas. In any case, no saturation of the VDT is observed for doses up to 25 kGy. The above is very important, since it would allow the estimation of the sterilization dose even if the glass container has been exposed to light or heated to temperatures up to 500 °C.Thus, all findings are very promising and support the idea of using the glass containers of commercial liquid drugs as probes for the post-sterilization dosimetry of these drugs and for normal and/or accidental dosimetry.     

Estimation of cosmic radiation doses in Punjab,Pakistan

Cosmic radiation is one of the most important sources of human exposure to natural radiation. Data regarding the level of cosmic radiation in different areas of Pakistan are lacking. The aim of the present study was therefore to estimate the annual outdoor effective doses from cosmic radiation in the Punjab province of Pakistan. The Punjab province is located between geodetic latitudes 28° and 34° N, and longitudes 69° and 75° E. The mean value of the outdoor annual effective dose was found to be 333 μ Sv, which generates a radiation dose of 0.33 mSv y^?1 per caput. This is 87% of radiation dose received by the population in the region from terrestrial sources reported elsewhere. The results of this study will contribute to regional as well as world data regarding the exposure of the population to cosmic radiation.     

Energy absorption and exposure build-up factors in hydroxyapatite

The effective atomic number (Z_eff) and electron density (N_el) of hydroxyapatite (HA) and cortical bone have been computed for total photon interaction in the wide energy range of 1 keV–100 GeV using WinXCom. The variations of effective atomic number and electron density with energy of HA are compared with that of cortical bone. GP. fitting method has been used to compute energy absorption and exposure build-up factor of HA for wide energy range (0.015 MeV–15 MeV) up to the penetration depth of 40mean free path. The computed absorption build-up factor is used to estimate specific absorbed fraction of energy (Ф) and relative dose of photon in HA. Build-up factor increases with increase of penetration depth. The results of the present paper will also help in estimating safe dose levels for radiotherapy patients and also will be useful in dosimetry and diagnostics.     

Fabry–Pérot cavities based on chemical etching for high temperature and strain measurement

In this paper, two hybrid multimode/single mode fiber Fabry–Pérot (FP) cavities were compared. The cavities fabricated by chemical etching are presented as high temperature and strain sensors. In order to produce this FP cavity a single mode fiber was spliced to a graded index multimode fiber with 62.5 μm core diameter. The Fabry–Pérot cavities were tested as a high temperature sensor in the range between room temperature and 700 °C and as strain sensors. A reversible shift of the interferometric peaks with temperature allowed to estimate a sensitivity of 0.75 ± 0.03 pm/°C and 0.98 ± 0.04 pm/°C for the sensor A and B respectively. For strain measurement sensor A demonstrated a sensitivity of 1.85 ± 0.07 pm/μ? and sensor B showed a sensitivity of 3.14 ± 0.05 pm/μ?. The sensors demonstrated the feasibility of low cost fiber optic sensors for high temperature and strain.     

Evaluation of epitaxial silicon diodes as dosimeters in X-ray mammography

In this work we report on results obtained with 2 rad-hard n-type epitaxial silicon diodes in mammography X-ray dosimetry. One sample was not irradiated before using as a dosimeter, while the other received a ⁶⁰Co gamma-ray pre-dose of 200 kGy. Both unbiased devices operated in a short-circuit mode as on-line radiation dosimeters for quality assurance in medical imaging dosimetry. The irradiation was performed using 28 kV and 35 kV X-ray beams from a Pantak/Seifert generator, previously calibrated by standardized ionization chamber. The dosimetric response of these devices was investigated with respect to the repeatability, long term stability, sensitivity dependence on energy and dose-rate, charge-dose linearity and directional response. The calibration coefficients of each diode, in terms of air kerma, were also determined. These dosimetric parameters of both diodes fully meet the requirements of IEC 61674 norm, confirming their use as a reliable alternative choice for mammography photon dosimetry within the dose range of 60 μGy-10 Gy (unirradiated EPI diode); for the pre-irradiated EPI diode upper limit of dose was not reached up to now. Nevertheless, it still remains to be investigated whether or not the pre-irradiation procedure influences on the response long-term stability of EPI devices. These studies are under way.     

Measurement of LET distribution and dose equivalent on board the space shuttle STS-65

Space radiation dosimetry measurements have been made on board the Space Shuttle STS-65 in the Second International Microgravity Laboratory (IML-2). In these measurements, three kinds of detectors were used; one is a newly developed active detector telescope called “Real-time Radiation Monitoring Device (RRMD)” utilizing silicon semi-conductor detectors and others are conventional detectors of thermoluminescence dosimeters (TLDs) and CR-39 plastic track detectors. Using the RRMD detector, the first attempt of real-time monitoring of space radiation has been achieved successfully for a continuous period of 251.3 h, giving the temporal variations of LET distribution, particle count rates, and rates of absorbed dose and dose equivalent. The RRMD results indicate that a clear enhancement of the number of trapped particles is seen at the South Atlantic Anomaly (SAA) without clear enhancement of dose equivalent, while some daily periodic enhancements of dose equivalent due to high LET particles are seen at the lower geomagnetic cutoff regions for galactic cosmic ray particles (GCRs). Therefore, the main contribution to dose equivalent is seen to be due to GCRs in this low altitude mission (300 km). Also, the dose equivalent rates obtained by TLDs and CR-39 ranged from 146.9 to 165.2 μSv/day and the average quality factors from 1.45 to 1.57 depending on the locations and directions of detectors inside the Space-lab at this highly protected orbit for space radiation with a small inclination (28.5°) and a low altitude (300 km). The LET distributions obtained by two different detectors, RRMD and CR-39, are in good agreement in the region of 15–200 keV/mm and difference of these distributions in the regions of LET < 15 keV/mm and LET > 200 keV/mm can be explained by considering characteristics of CR-39 etched track formation especially for the low LET tracks.     

Environmental dosimetry with the BeOSL personal dosimeter – State of the Art

In the course of this work, the possibility of the measurement of ambient dose equivalent H*(10) with the BeOSL dosimetry system was evaluated. Calculations for the energy response of the 2-element BeOSL dosimeter for irradiation with H*(10) were performed. The response doesn't fulfil the requirements of IEC 62387-1. Especially the response for photon energies of 60–100 keV is to low. It is possible to correct this under response using a modified BeOSL 2-element dosimeter and a linear algorithm. So the national requirements for a H*(10) dosimeter in Germany can be fulfilled. An incidence angle independent measurement is not possible because for several angles of incidences (>60°) the filters of the 2-element dosimeter doesn't shield the correct element. Another material which is more suitable for the H*(10) measurement was tested. So the doping of BeO with Lanthanum leads to an enhanced energy response for measurement of H*(10). Furthermore a higher OSL sensitivity was found for this material. Further tests on the influence of Lanthanum concentration on the dosimetric properties are necessary.     

Small field output factors: Comparison of measurements with various detectors and effects of detector orientation with primary jaw setting

Accurate dosimetry in small photon fields used in modern radiotherapy is a challenging task due to electronic disequilibrium, steep dose gradients, source occlusion and size of the sensitive volume of the detector. These challenging effects and the lack of metrological dosimetric reference instigated an investigation on the acquisition of output factor with various detectors in parallel and perpendicular orientations. Small field output factor measurements of tertiary collimators such as BrainLab circular cones, BrainLab mMLC and Millennium MLC were carried out in this study. The data acquired show the differences between output factor results with different detectors for all collimating systems. Good agreement in output values was observed in field sizes greater than ∼2 × 2 cm² for all detectors and all tertiary collimators. For smaller fields when compared to electron field diode (EFD), 0.125 cm³ ion chamber underestimates the output by up to −11.1% and −20.4% and pinpoint ion chamber underestimates the output by up to −1.5% and −6.1% in their parallel and perpendicular orientation, respectively. In contrast, PTW SRS diode and photon field diode (PFD) overestimate the output factor by up to 2.5% and 6.9% respectively in its parallel orientation. The investigated data for the effect of jaw position (0.25 × 0.25 cm², 0.5 × 0.5 cm² and 1 × 1 cm²) away from the field edge generated by different tertiary collimating systems inferred that the opening of X–Y jaw highly influences the small field output factors. The orientation of the detectors and the position of the jaws could influence the output factors considerably in small fields.     

Highlights from the Pierre Auger Observatory

The Pierre Auger Observatory is the world’s largest cosmic ray observatory. Our current exposure reaches nearly 40,000 km ²sr and provides us with an unprecedented quality data set. The performance and stability of the detectors and their enhancements are described. Data analyses have led to a number of major breakthroughs. Among these, we discuss the energy spectrum and the searches for large-scale anisotropies. We present analyses of our X _max data and show how it can be interpreted in terms of mass composition. We also describe some new analyses that extract mass-sensitive parameters from the 100 % duty cycle surface detector (SD) data. A coherent interpretation of all these recent results opens new directions. The consequences regarding the cosmic ray composition and the properties of ultrahigh-energy cosmic ray (UHECR) sources are briefly discussed.     

Atmospheric gamma rays in the energy region 40–1000 GeV

A large stack of lead-emulsion sandwich detector assembly was flown over Hyderabad, India. High energy gamma rays at the float altitude were unambiguously identified from the cascades they induced, and their energies reliably determined by improved methods. From an analysis of 163 gamma rays of energy ≳ 30 GeV, it is found that the differential energy spectrum is represented by the power lawJ _r (E)= 129·4E ^{−2·62±0·12} photons m⁻² sr⁻¹sec⁻¹ GeV⁻¹ at an effective atmospheric depth of 14·3 g cm⁻²; this is the first reliable balloon measurement of atmospheric gamma rays in the energy range 40–1000 GeV. After correcting for the gamma rays radiated by the primary cosmic ray electrons, the production spectrum of gamma rays, resulting from the collisions of cosmic ray nuclei with air nuclei, at the top of the atmosphere isP _r (E, 0)=8·2 × 10⁻⁴ E^2.60±0.09 photons g⁻¹sr⁻¹sec⁻¹ GeV⁻¹. The atmospheric propagation of the electromagnetic component due to the cascade process is also derived from the gamma ray production spectrum.     

The LAPEX experiment for observation of the supernova SN 1987A in hard X-rays

Summary The LAPEX payload is described in the configuration that will be launched to observe the supernova SN 1987A. The balloon flight will be performed from Alice Springs (Australia) in April 1989 as a part of a NASA balloon campaign. The experiment will allow simultaneous observations of source and background in order to minimize systematic errors due to background variations. The limiting flux sensitivity of the instrument in the (20÷200) keV energy band is 4·10⁻⁶ ph/(cm²s keV) or ∼2.5 mCrab for a 10⁴ s observation of SN 1987A. The presence of a⁵⁷Co emission line in the SN 1987A photon spectrum could be detected down to an intensity level of ∼8·10⁻⁵ photons/cm²s as at 99.7% confidence level. The high resolution of the event timing (<0.1 ms) will make it possible to detect millisecond pulsations with amplitudes down to ∼4% of the expected average flux from SN 1987A in the (20÷60) keV band. To speed up publication, the proofs were not sent to the authors and were supervised by the Scientific Committee.     

Intercomparison of the response of different photon and neutron detectors around a spent fuel cask

An intercomparison of the response of different photon and neutron detectors was performed in several measurement positions around a spent fuel cask (type TN 12/2B) filled with 4 MOX and 8 UO₂ 15 × 15 PWR fuel assemblies at the nuclear power plant Gösgen (KKG) in Switzerland. The instruments used in the study were both active and passive, photon and neutron detectors calibrated either for ambient or personal dose equivalent.The aim of the measurement campaign was to compare the responses of the radiation instruments to routinely used detectors.It has been shown that especially the indications of the neutron detectors are strongly dependent on the neutron spectra around the cask due to their different energy responses. However, routinely used active photon and neutron detectors were shown to be reliable instruments.     

Ultrahigh-energy cosmic rays: Possible origin and spectrum

The complicated shape of the cosmic ray spectrum recorded by giant arrays in the energy range 10¹⁷–10²⁰ eV is analyzed. It is shown that in the energy region ∼10¹⁸–10¹⁹ eV the spectrum probably coincides with the injection spectrum whose exponent is equal approximately to 3.2–3.3. The flatter component in the energy region (3.2–5.0)×10¹⁹ eV is due to braking of extragalactic protons on primordial photons (the cosmic background radiation). At energies exceeding 3.2×10¹⁹ eV the spectrum does not have a blackbody cutoff. The possibility of determining the distances at which cosmic rays originate and investigating the evolution of their sources on the basis of ultrahigh-energy cosmic ray data is discussed. Zh. éksp. Teor. Fiz. 113, 12–20 (January 1998)