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At least 343 records · Page 19Linked to original sources
PubMed1979

Responses of retinal rods to single photons.

1. A suction electrode was used to record the membrane current of single rod outer segments in pieces of toad retina. During dim illumination the membrane current showed pronounced fluctuations. 2. Amplitude histograms of responses to dim flashes of fixed intensity exhibited two discrete peaks, one at 0 pA and one near 1 pA, suggesting that the response was quantized. By setting a criterion amplitude level, flash responses could be classed as 'failures' (no response) or as 'successes' (at least one quantal event). 3. The variation of fraction of successes with flash intensity was consistent with the hypothesis that each quantal electrical event resulted from a single photoisomerization. 4. The quantal event had a mean amplitude of about 1 pA (5% of the standing dark current) and a standard deviation of 0.2 pA. Dispersion in the event amplitude prevented identification of histogram peaks corresponding to two or more photoisomerizations. 5. Individual quantal responses exhibited a smooth shape very similar to that of the average quantal response. This suggests that a single photoisomerization releases many particles of transmitter and that radial diffusion of internal transmitter is not a major source of delay in the light response. 6. The 'quantum efficiency' with which an absorbed photon generated an electrical event was measured as 0.5 +/- 0.1 (S.E. of mean, n = 4). This is slightly lower than the quantum efficiency of photoisomerization obtained previously for rhodopsin in solution. 7. At wavelengths between 420 and 700 nm the quantal event was invariant in size, although the cell's sensitivity varied over a range of 10(5). 8. The power spectrum of the fluctuations in dim steady light was predicted by assuming that a random series of quantal events occurred independently. 9. In brighter light the fluctuations were faster, and the response to an incremental flash was reduced in size and duration. The power spectrum could be predicted by assuming random superposition of events with the shape of the incremental flash response.

Animals↗
PubMedDate not supplied

Reduction in extraction efficiency of charged particles from the ion source as the cause of matrix effects in the GC-MS analysis of drugs.

Previous studies have shown that the ion response of a compound can be suppressed by the presence of a large amount of a coeluting substance in a gas chromatographic-mass spectrometric (GC-MS) system. In the present study, the change in the ion current of a constant amount of diazepam-d5 in the presence of a 100-fold amount of diazepam was used to monitor this condition in the Hewlett Packard mass selective detector (MSD). It was observed that a reduced recovery of ions occurred when the potentials of the MSD source elements were established by the autotune algorithm. Increasing the ion focus or the entrance lens potentials or both increased the recovery of the ion current of diazepam-d5 in the presence of large amounts of diazepam. The data suggested that the decreased recovery of ion current observed when the autotune source parameters were used was due to insufficient energy on the focusing lenses to extract a constant fraction of the ions from the source when a high concentration of molecules was present.

Diazepam↗
PubMed1992

Delta-electron emission in fast heavy ion atom collisions.

Biological damages such as mutations, chromosomal aberrations etc. are a consequence of biochemical changes mostly in the DNA. With ionizing radiation, these chemical changes are due to primary ionization events and secondary ionization effects caused by the primarily produced electrons. Differences in the biological response of densely ionizing radiation, like heavy charged particles, in comparison to sparsely ionizing radiation, such as X- or gamma-rays, are mainly due to the differences in the production of the so called delta-electrons. Therefore, the emission process of electrons i.e. the cross section for the primary ionization event as well as the energy and angular distribution of the emitted electrons should be understood in detail. The delta-electron emission processes occuring in fast heavy ion atom collisions are explained qualitatively. The different spectral structures of electron emission arising from either the target or the projectile are explained in terms of simple models of the kinetics of momentum transfer induced by the COULOMB forces. In collisions of very heavy ions with matter, high nuclear COULOMB forces are created. These forces lead to a strong polarization of the electronic states of the participated electrons. The effects of this polarization are discussed.

Argon↗
PubMed1998

Particle beam radiation therapy in prostate cancer: is there an advantage?

Hadron therapy uses heavy particles to deliver therapeutic ionizing energy. Each particle's inherent attributes determine the pattern of energy deposited by its beam, expressed in macro (conformability to a three-dimensional target volume) and micro (radiobiologic properties) distributions. Mass and charge regulate the inherent properties; beam energy provides a controllable, variable characteristic. Generally, heavy charged particles provide superior macrodosimetric properties; heavy particles (charged or not) have microdosimetric characteristics that produce high linear energy transfer (LET). Neutron macrodosimetry is similar to that of photons. Protons and helium ions possess superior macrodosimetric properties, plus microdosimetric characteristics resulting in low LET, yielding beam characteristics that approach the ideal for clinical radiotherapy. Hadron therapy for prostate cancer has been limited by the availability of appropriate treatment facilities. Nonetheless, encouraging results have been obtained. Neutron therapy demonstrated improved overall survival in a multi-institutional randomized trial, and improved local disease control in a subsequent trial. Proton radiation forms the boost component of several conformal dose-escalation studies. A Loma Linda University study demonstrated low treatment-related morbidity despite a prostate dose of 75 CGE; late-morbidity data were superior to published reports from multi-field, conformal photon therapy. A Phase III dose-escalation study of protons for early prostate cancer is proceeding.

Dose-Response Relationship, Radiation↗
PubMed2002

The properties of gamma-radiation and high-energy neutron fluxes in "MIR" station orbit.

The study of radiation background components in the near-Earth space is very important for different branches of space research, in particular for space dosimetry and for the planning of gamma-astronomy experiments. Detailed information on the neutral components (gamma-quanta, neutrons) of background radiation was obtained during the Grif-1 experiment onboard Mir orbital station (OS). The measurements of fluxes of 0.05-50 MeV gamma-quanta and >30 MeV neutrons with a large area instrument (approximately 250 cm2 for gamma-quanta, approximately 30 cm2 for neutrons) as well as corresponding charged particle measurements (0.4-1.5 MeV electrons, 1-200 MeV protons) were made during this experiment. The background components induced by the station's own radiation as well as the albedo gamma-rays from the Earth's atmosphere were revealed as the result of data analysis for about 600 h of observation. A mathematical model describing the latitude and energy dependences of atmospheric albedo gamma-rays as well as of those of gamma-quanta produced in the material of the station due to cosmic ray interactions was developed. An analytical approximation of the spectrum of induced gamma-rays from radioactive isotopes stored in the station and instrument's materials is presented. The dynamics of gamma-quantum background fluxes during the geomagnetic disturbances of January 10-11, 1997 are discussed. An analytical representation of the latitude dependence of the integral flux of neutrons with >30 MeV is given.

Brazil↗
PubMed2001

Predictions of secondary neutrons and their importance to radiation effects inside the International Space Station.

As part of a study funded by NASA MSFC to assess thecontribution of secondary particles in producing radiation damage to optoelectronics devices located on the International Space Station (IS), Monte Carlo calculations have been made to predict secondary spectra vs. shielding inside ISS modules and in electronics boxes attached on the truss (Armstrong and Colborn, 1998). The calculations take into account secondary neutron, proton, and charged pion production from the ambient galactic cosmic-ray (GCR) proton, trapped proton, and neutron albedo environments. Comparisons of the predicted neutron spectra with measurments made on the Mir space station and other spacecraft have also been made (Armstrong and Colborn, 1998). In this paper, some initial results from folding the predicted neutron spectrum inside ISS modules from Armstrong and Colborn (1998) with several types of radiation effects response functions related to electronics damage and astronaut-dose are given. These results provide an estimate of the practical importance of neutrons compared to protons in assessing radiation effects for the ISS. Also, the important neutron energy ranges for producing these effects have been estimated, which provides guidance for onboard neutron measurement requirements.

Cosmic Radiation↗
PubMed2004

The FLUKA code for space applications: recent developments.

The FLUKA Monte Carlo transport code is widely used for fundamental research, radioprotection and dosimetry, hybrid nuclear energy system and cosmic ray calculations. The validity of its physical models has been benchmarked against a variety of experimental data over a wide range of energies, ranging from accelerator data to cosmic ray showers in the earth atmosphere. The code is presently undergoing several developments in order to better fit the needs of space applications. The generation of particle spectra according to up-to-date cosmic ray data as well as the effect of the solar and geomagnetic modulation have been implemented and already successfully applied to a variety of problems. The implementation of suitable models for heavy ion nuclear interactions has reached an operational stage. At medium/high energy FLUKA is using the DPMJET model. The major task of incorporating heavy ion interactions from a few GeV/n down to the threshold for inelastic collisions is also progressing and promising results have been obtained using a modified version of the RQMD-2.4 code. This interim solution is now fully operational, while waiting for the development of new models based on the FLUKA hadron-nucleus interaction code, a newly developed QMD code, and the implementation of the Boltzmann master equation theory for low energy ion interactions.

Computer Simulation↗
PubMed1990

[2-component theory and radiation therapy].

The two-component theory which describes the biological effect of X-ray and particle irradiation divides radiation into densely ionizing radiation (ion density in water greater than four ions per 100 A, i.e. LET greater than 12 keV/microns) and loosely ionizing radiation with low ion densities. In case of densely ionizing radiation, the ions can produce breaks of both cords of DNA thus causing the death of the cell (alpha effect). Lower ion densities will produce only slight damages which are possibly lethal but can be partly repaired (beta effect). If the cell parameters are known (L. Cohen 1983), the number of surviving cells after an irradiation can be calculated. The surviving lung cells and tumor cells (squamous cell carcinoma) have been calculated for a pulmonary irradiation with 30 MeV electrons and 200 keV X-rays (single doses of 2 and 5 Gy), respectively. The electron irradiation with single doses of 5 Gy turned out to be the most favorable therapy sparing the greatest number of lung cells and reducing the tumor cells in the most effective way (down to 1.6 x 10(-10)).

Carcinoma, Squamous Cell↗
PubMedDate not supplied

Production of neutrons from interactions of GCR-like particles.

In order to help assess the risk to astronauts due to the long-term exposure to the natural radiation environment in space, an understanding of how the primary radiation field is changed when passing through shielding and tissue materials must be obtained. One important aspect of the change in the primary radiation field after passing through shielding materials is the production of secondary particles from the breakup of the primary. Neutrons are an important component of the secondary particle field due to their relatively high biological weighting factors, and due to their relative abundance, especially behind thick shielding scenarios. Because of the complexity of the problem, the estimation of the risk from exposure to the secondary neutron field must be handled using calculational techniques. However, those calculations will need an extensive set of neutron cross section and thicktarget neutron yield data in order to make an accurate assessment of the risk. In this paper we briefly survey the existing neutron-production data sets that are applicable to the space radiation transport problem, and we point out how neutron production from protons is different than neutron production from heavy ions. We also make comparisons of one the heavy-ion data sets with Boltzmann-Uehling-Uhlenbeck (BUU) calculations.

Aerospace Medicine↗
PubMed2001

Response of silicon-based linear energy transfer spectrometers: implication for radiation risk assessment in space flights.

There is considerable interest in developing silicon-based telescopes because of their compactness and low power requirements. Three such telescopes have been flown on board the Space Shuttle to measure the linear energy transfer spectra of trapped, galactic cosmic ray, and solar energetic particles. Dosimeters based on single silicon detectors have also been flown on the Mir orbital station. A comparison of the absorbed dose and radiation quality factors calculated from these telescopes with that estimated from measurements made with a tissue equivalent proportional counter show differences which need to be fully understood if these telescopes are to be used for astronaut radiation risk assessments. Instrument performance is complicated by a variety of factors. A Monte Carlo-based technique was developed to model the behavior of both single element detectors in a proton beam, and the performance of a two-element, wide-angle telescope, in the trapped belt proton field inside the Space Shuttle. The technique is based on: (1) radiation transport intranuclear-evaporation model that takes into account the charge and angular distribution of target fragments, (2) Landau-Vavilov distribution of energy deposition allowing for electron escape, (3) true detector geometry of the telescope, (4) coincidence and discriminator settings, (5) spacecraft shielding geometry, and (6) the external space radiation environment, including albedo protons. The value of such detailed modeling and its implications in astronaut risk assessment is addressed.

Astronomy↗
PubMed1982

Kinetics of positron emitters in vivo characterized with a beta probe.

To facilitate characterization of regional myocardial kinetics of positron-emitting tracers in vivo without distortion by activity outside the region of interest, a probe was developed and evaluated for monitoring radioactivity by detection of positrons themselves. These particles (beta particles) have a maximal range in tissue of only few millimeters rather than the larger range of gamma photons emitted as a result of positron annihiliation. Regional myocardial time-activity curves were determined in open-chest dogs after intracoronary injection of 0.5-1.5 mCi [15O]H2O, a tracer used for measurement of myocardial blood flow, or 6.0-8.0 mCi [11C]palmitate, a tracer used for noninvasive assessment of myocardial metabolism. Time-activity curves after [11C]palmitate injection clearly delineated specific components of myocardial tracer clearance previously identified in vitro in isolated perfused hearts. Myocardial washout of [15O]H2O was monoexponential for more than 2 min without distortion induced by recirculating tracer in ventricular blood. Reproducibility of measured tracer clearance rates during monoexponential clearance was high based on duplicate determinations for both tracers. The beta-detector probe developed overcomes several intrinsic limitations of gamma-probe systems or well counting of serial myocardial biopsies for studies of positron-emitting tracers in vivo and should facilitate assessment of factors of influencing tracer kinetics in vivo relevant to positron-emission tomography.

Animals↗
PubMed1990

The medical heavy ion therapy project at the Gesellschaft für Schwerionenforschung facility in Darmstadt.

It could be demonstrated that local tumor control is considerably improved by radiation therapy with charged particles (protons or heavier ions). The advantages of heavy ion therapy compared to conventional photon therapy techniques are due to the better physical dose distributions achievable and the radiobiological characteristics of heavy ions. However, because of the expense and complexity of heavy ion therapy it is only carried out at a few facilities throughout the world. The Radiologische Universitätsklinik (Radiological University Hospital) and the Deutsches Krebsforschungszentrum (German Cancer Research Centre, DKFZ) in Heidelberg, in collaboration with the Gesellschaft für Schwerionenforschung (Laboratory for Heavy Ion, GSI) in Darmstadt, have developed a concept to use the new heavy ion synchrotron (Schwerionen-Synchroton SIS) in Darmstadt for medical-clinical irradiations. Due to the high flexibility of the SIS accelerator the medical program can be performed in addition to the planned physical experiments with minor interference only. The close geographical proximity of the three institutes involved and the accelerator which will be completed by the end of 1989 provide the unique opportunity to carry out relevant clinical, medical-physical, physical-technical, and radiobiological research in a relatively short time and, compared to similar projects in other countries, at low cost.

Costs and Cost Analysis↗
PubMed1978

A heavy particle comparative study. Part III: OER and RBE.

The results of a comparative study of heavy particles of interest in radiotherapy are reported in four parts. In this Part III, cell-survival measurements under aerobic and hypoxic conditions were made for various heavy particle beams. For heavy charged-particle beams, the measurements were made at the beam entrance (plateau), peak centre (10 cm wide peaks), and distal peak (1 cm from dose fall-off). Chinese hamster cells (V79) were used. Metabolic depletion was used to obtain hypoxia. The results indicate that the differences in RBE between the entrance region and peak are not very large when the Bragg peaks are broadened to 10 cm. The RBE for argon ions remains the same at the entrance and peak centre, and the RBE at the distal side of the Bragg peak is significantly reduced compared to the peak centre and entrance region because of saturation effects at high LET. The OER for protons is not significantly different from that for X rays. The OER for helium ions, carbon ions, and negative pions is larger, for neon ions is similar, and for argon ions is smaller when compared with fast neutrons. The OER values for heavy ions are higher than expected and could be due to a large delta-ray penumbra associated with the energy deposited by energetic heavy ions. The oxygen effect may depend upon energy deposition over distances of the order of nanometers.

Argon↗
PubMed1990

Anaplastic astrocytoma and glioblastoma: pion irradiation with the dynamic conformation technique at the Swiss Institute for Nuclear Research (SIN).

Clinical phase I/II studies have been performed at the Swiss Institute for Nuclear Research (SIN) since February 1982. Fifty-two out of 249 patients accepted for pion treatment by the end of 1986 were treated for malignant glioma with high dose pion irradiation. A substantial influence of their radioresistance was expected from increased radiation quality due to the contribution of high LET particles from pion capture, and by the possibility of target volume shaping and dose distribution related to the dynamic spot-scan conformation technique. The patients' treatment followed a dose escalation program with total doses from 2720-3420 cGy, fraction sizes from 170 to 205 cGy (90% isodose, minimum target dose), and treatment times from 4 to 5 weeks. 12/52 patients received an accelerated treatment with 3280 cGy in 14-22 days. 49/52 patients are eligible: 3 with astrocytoma of clinical aggressive behaviour, 14 with anaplastic astrocytoma (median age 42 years), and 32 patients with glioblastoma (median age 52 years). 8/49 patients had total/subtotal tumour resection, 19 patients a stereotactic biopsy. The patients were divided into three groups according to total dose, and a fourth group which received the accelerated treatment. There was no statistically significant difference in the median survival rate between the four groups, which was 13 months for the non-glioblastoma patients and 9 months for the glioblastoma patients. No radiation necrosis and no demyelination was found in 17 patients (6 recraniotomies, 11 autopsies). In 10/17 patients, clearly identifiable tumour cells were not demonstrated. NMR findings showed the tumour-surrounding oedema mostly stimulated by tumour necrosis and tumour progression. From these findings, further dose escalation programs, together with a shaping of the target volume close to the tumour, are not contraindicated.

Adult↗
PubMed1987

The excitation of the far ultraviolet electroglow emissions on Uranus, Saturn, and Jupiter.

We propose that the diffuse FUV emissions of H and H2 in excess of photoelectron excitation observed from the sunlit atmospheres of Uranus, Saturn, and Jupiter are produced by electric field acceleration of photoelectrons and ions locally in the upper atmospheres. This in situ acceleration is required to satisfy the many observational constraints on the altitude distribution, exciting particle energy, and total input energy requirements of the electroglow mechanism. We further suggest that a primary mechanism leading to this acceleration is an ionospheric dynamo, which is created in the same manner as the Earth's dynamo. The calculated altitude of charge separation by the neutral wind drag on ions across magnetic field lines is consistent with the observed peaks in electroglow emissions from the Voyager ultraviolet spectrometer limb scan data on both Saturn (near the homopause) and Uranus (just above the homopause). This dynamo action therefore appears to initiate the acceleration process, which must have the form of field-aligned potentials to accelerate the magnetized electrons. We propose that these field-aligned potentials are due to anomalous resistivity, which results from sufficiently high field-aligned currents in the ionosphere to generate plasma instabilities and therefore runaway electrons and ions above some critical lower initial energy. There are multiple candidate processes for inducing these currents, including polarization in the equivalent F regions and inner magnetospheric convection, and each of these processes should exhibit latitudinal structure. The acceleration of low-energy electrons in an H2 atmosphere preferentially results in FUV radiation and further ionization, whereas electron acceleration in a nitrogen/oxygen atmosphere such as the Earth's is dominated by elastic scattering and thus results in electric currents. Individual electron and proton collisions with H2 molecules will result in excitation, ionization, and heating, so that considerable enhancement of the ionospheric density and heating of the upper atmosphere will accompany the FUV emission.

Electrons↗
PubMed1993

[The hadron therapy project].

The neologism "hadrontherapy" means radiotherapy with hadrons, which are the particles constituted by quarks, such as protons, neutrons and ions. The theoretical considerations about the clinical advantages this treatment modality can yield and the results obtained at the centers where it has already been used justify the proposal to project a center of this kind also in our Country. To this purpose, two of the authors of this paper (U. Amaldi, G. Tosi) founded the TERA Group formed by physicists, engineers and radiotherapists who work in close collaboration on a feasibility study for a hadrontherapy facility. The first aim of the Hadrontherapy Project is to design a center equipped with a synchrotron which, at the beginning, will accelerate negative hydrogen ions (H-) which will first produce 70-250 MeV proton beams and, then accelerate light ions (up to 16O) to 430 MeV/amu. This accelerator will serve four or five treatment rooms where patients can be irradiated simultaneously. Two rooms will be equipped with a fixed horizontal beam for the treatment of eye, head and neck tumors; the others will be equipped with rotating gantries to administer, in any clinical situation, really adequate treatment. Such a unit, when enough experience is fained, will allow at least 1000 patients to be treated yearly. The synchrotron injector will be designed so as to allow, parallel to the radiotherapy activities, other applications of medical and biological interest such as: the production of radioisotopes for diagnostic use (especially positron emitters), the analysis of trace elements through the PIXE technique and the production of thermal and epithermal neutrons for boron neutron capture therapy.

Biophysical Phenomena↗