High-resolution NIAR of fissile elements in bone.
Explore the source record for details and available documents.
Animals↗
Biomedical subjects
J L Beach
Publications and source records attributed to J L Beach.
Evaluation of a continuously variable collimator for 14 MeV neutrons.
This paper presents an evaluation of a continuously variable collimator for 14 MeV neutrons. Beam profiles measured in air and a phantom are used to demonstrate the relative importance of patient scatter and collimator throat scatter as they limit the performance characteristics of the collimator. The collimator consists basically of two sets of jaws each composed of 30 cm of steel followed by 20 cm of polyethylene. A solution is presented for the asymmetry in vertical and horizontal collimating characteristics resulting from the use of thick collimators. Beam profiles obtained with the collimator are comparable to published values for a fixed field 14 MeV neutron collimator.
Evaluation Studies as Topic↗
Experimentally determined tissue air ratios and scatter air ratios for collimated beams of 14 mev neutrons.
Tissue air ratios (TAR) and scatter air ratios (SAR) for collimated beams of 14 MeV neutrons were determined from depth dose curves measured in air and water using a tissue equivalent proportional counter. Central axis depth dose data were obtained as a function of field size and depth in a water phantom for an SSD or 125 cm. The collimator was composed of steel and polyethylene and was continuously variable from a 3 times 3 cm to a 20 times 20 cm field size at 125 cm SSD. The field scans for this collimator were flat within plus or minus 2 per cent over 80 per cent of the field dimension in air and were reduced to 20 per cent of the central axis dose at 1-2 field radii in air. Relative depth dose data in air and phantom were measured with an accuracy of plus or minus 2 per cent and were used for the TAR and SAR calculations. The resulting scatter air ratios for 14 MeV neutrons have roughly the same characteristics as 60-Co SAR, but are 60 per cent larger at maximum build-up and reach that maximum at about 60 per cent of the peak depth for 60-Co for all field sizes. The effects of collimator throat scatter on the central axis depth dose characteristics of this particular collimator design are discussed in detail using the measured tissue and scatter air ratios.
Absorption↗
Geometric penumbra calculations for a gas target neutron source.
Explore the source record for details and available documents.
Mathematics↗
Ultrasonic Doppler monitoring of vocal-fold velocity and displacement.
Explore the source record for details and available documents.
Humans↗
Absolute fluence measurement for a prototype neutron radiotherapy source.
An inexpensive recoil-proton counter telescope has been designed for absolute fluence measurements of a gas-target neutron source for radiation therapy. The detector has an absolute efficiency of 1.1 times 10-minus 9 at 20 cm from an isotropic source and is useful for production rates of 10-9 minus 10-13 neutrons per second. The telescope consists of a thin hydrogenous irradiator foil and a surface-barrier detector to count recoil protons within a defined solid angle. The telescope provides n-gamma discrimination as well as discrimination against scattered neutrons. Initial tests of the counter telescope were performed using the DD reaction employed in development stages of the gas target. A clear separation of full-energy recoil protons from background and scattered neutron events was evident in the pulse-height spectra without the use of coincidence gating techniques.
Fast Neutrons↗
Utilization of the 16O(n,p) reaction for monitoring the output of 14 MeV neutron generators.
A system based upon the 16O(n,p) 16N reaction has been developed for monitoring 14-MeV neutron production. The system has proven to be quite applicable for monitoring the output of a disk-shaped neutron source and computer analysis has shown it equally suitable for monitoring the output of cone-shaped neutron sources from gas targets presently being considered for cancer therapy.
Humans↗
X-ray beam characteristics of the Varian Clinac 6-100 linear accelerator.
Beam characteristics and dosimetry measurements of the 6 MV x-ray beam from a Varian Associates Clinac 6-100 linear accelerator are presented. Percentage depth dose tables are given as a function of field size for square fields. Tissue-maximum ratios and scatter-maximum ratios are tabulated as a function of depth and field size. Investigation of surface dose and depth of maximum dose reveal a dependence on field size. Other beam parameters measured are presented include field flatness, symmetry, SSD dependence, and penumbra. Treatment planning parameters including field size output factors and wedge factors are discussed.
Particle Accelerators↗
16O(n, p)16N: a fast neutron detector for rapid tissue inhomogeneity correction.
We have devised a transit dose technique for fast neutron therapy treatment planning based on the 16O(n, p)16N reaction in recirculating water, and have determined the effect of simulated bone and lung inhomogeneities in phantom. An effective threshold of 10.2 MeV in the 16O(n, p) reaction is exploited to detect transmitted neutrons without the need detector collimation. This system has been demonstrated with 14 MeV (d, T) neutrons and with cyclotron produced p(42) + Be neutrons. 16N decays to the 6.13 MeV excited states of 16O in 7.14 s, allowing for easy identification by NaI(T1) and for rapid recirculation. The transmission of fast neutrons can thus be related to an effective thickness of soft tissue, providing a rapid and direct measure of the effects of inhomogeneities under actual treatment conditions, with the 10 MeV threshold providing a useful degree of insensitivity to multiply scattered neutrons. Equivalent thickness of compact bone and lung relative to water were found to be 1.4 and 0.34 respectively, closely resembling the effective thicknesses for Cobalt-60 gamma rays.
Biophysical Phenomena↗
Microdosimetric measurements of radiation quality variations in homogeneous phantoms irradiated by fast neutron beams.
The Dual Radiation Action Theory of Kellerer and Rossi (DRA), along with presently available microdosimetric techniques, is applied to the determination of radiation quality variation within tissue equivalent phantoms irradiated by collimated fast neutron beams. The neutron beams investigated were produced by the bombardment of 22.5 and 16 MeV d + on beryllium and by the T(d,n)4He reaction (15-MeV neutrons). Microdosimetric spectra were obtained at points of varying depth and lateral distance from the central axis within a tissue equivalent phantom, including points within the penumbra. From the microdosimetric spectra the parameter RQ, a first approximation to RBE derived from DRA theory, is calculated for each point. All RQ values are calculated for the same level of effect. For these three different beams the results show that the RQ values for the total radiation spectrum of neutron and gamma radiation remain fairly constant with depth and with lateral distance from the beam axis at 2 and 10 cm depths. The largest central axis variation in RQ is 8% for the d(16) + Be beam. The largest variation between a penumbra and an on-axis RQ value is 4% at 2 cm depth in the d(22.5) + Be beam. The results for the d (22.5) + Be beam disagree with previously reported radiological results while the 15 McV beam results are in good agreement.
Fast Neutrons↗
Neutron insensitive depolymerization of polyacrylamide for californium-252 photon dosimetry.
A photon dosimeter based on degradation of polyacrylamide (PAA) in dilute aqueous solution has been found to be insensitive to neutrons from Cf-252, independent of photon energy from 34 keVeff to 1.33 MeV, and useful over a dose range of 1 to 100 Gy. Because this dilute polymer solution is virtually 100% water, it is nearly tissue equivalent for photon absorption as well as neutron scatter and attenuation. Radiation response was determined by measuring decreases in viscosity three days after irradiation. The measured response is dose rate independent from 10 to over 300 Gy/h but increases rapidly at lower dose rates. Long-term, radiation-induced viscosity changes were also observed. The many advantages of this tissue equivalent system for photon/neutron dosimetry suggest the need for further studies of the observed over-response at low dose rates.
Acrylic Resins↗
A comparison of air-cavity inhomogeneity effects for cobalt-60, 6-, and 10-MV x-ray beams.
The inclusion of air-filled spaces in treatment fields creates a potential dosimetric problem due to the loss of charged particle equilibrium near the air-tissue interface. We have used a simulated larynx phantom and a small buildup/extrapolation chamber to compare the magnitude and spatial extent of underdosing and overdosing at the distal surface for two linear accelerators (10- and 6-MV x-rays) and a cobalt-60 machine. Surface doses were compared to doses measured in a similar but homogeneous phantom to give observed/expected ratios (O/E), which were greater than 1.0 for large field sizes and less than 1.0 for small field sizes on all machines. The minimum field sizes which produce no surface underdosing for a simulated 2-cm-diam larynx are roughly 7 X 7 cm for 10-MV x-rays, 6 X 6 cm for 6-MV x-rays, and 5 X 5 cm for cobalt-60. In addition, the depth over which underdosing occurs is seen to increase with increasing energy.
Air↗
Air cavity perturbation of a clinical p(42)Be neutron beam.
Explore the source record for details and available documents.
Air↗
Neutron and gamma dose distribution for tandem, tandem and ovoid and plaque for intracavitary 252Cf therapy of cervix carcinoma.
252Cf was used to treat tumors of the cervix in an ongoing clinical trial. Tandem and ovoids were used as well as a combination of single and multiple applications. Dosimetric analysis of these treated cases revealed the patterns of neutron dose rate and neutron/gamma ray dose ratios which were achievable using a variety of loading arrangements. Although pelvic tumor reference points and tissue dose was in general similar whether a tandem or tandem plus ovoids were used, vaginal dose was increased over two-fold or more by the use of vaginal applicators and sources. This indicates that a variety of individualized arrangements for different clinical presentations can be used for brachytherapy of gynecological pelvic neoplasm.
Brachytherapy↗
A simple method using film monitoring for Cf-252 brachytherapy: technical note.
A method was devised for monitoring of the neutron and gamma emitting isotope, Cf-252, using film autoradiography. A wide variety of loading arrangements can be used in experimental and clinical studies and the film method provides a permanent record and a simple system to record the arrangement of all sources used.
Brachytherapy↗