F. D. Becchetti

On‐line monitoring of radiotherapy beams: Experimental results with proton beams

Proton radiotherapy is a powerful tool in the local control of cancer. The advantages of proton radiotherapy over gamma-ray therapy arise from the phenomenon known as the Bragg peak. This phenomenon enables large doses to be delivered to well-defined volumes while sparing surrounding healthy tissue. To fully realize the potential of this technique the location of the high-dose volume must be controlled very accurately. An imaging system was designed and tested to monitor the positron-emitting activity created by the beam as a means of verifying the beams range, monitoring dose, and determining tissue composition. The prototype imaging system consists of 12 pairs of cylindrical BGO detectors shielded in lead. Each crystal was 1.9 cm in diameter, 5.0 cm long, and separated by 0.5 cm from other detectors in the row. These are arranged in two rows, 60 cm apart, with the proton beam and tissue phantoms half-way between and parallel to the detector rows. Experiments were conducted with 150 MeV continuous and macro-pulsed proton beams which had beam currents ranging from 0.14 nA to 1.75 nA. The production and decay of short-lived isotopes, 15O and 14O, was studied using 1 min irradiations with a continuous beam. These isotopes provide a significant signal on short time scales, making on-line imaging possible. Macro-pulsed beams, having a period of 10 s, were used to study on-line imaging and the production and decay of long-lived isotopes, 13N, 11C, and 18F. Decay data were acquired and on-line images were obtained between beam pulses and indicate that range verification is possible, for a 150 MeV beam, after one beam pulse, to within the 1.2 cm resolution limit of the imaging system. The dose delivered to the patient may also be monitored by observing the increase in the number of coincidence events detected between successive beam pulses. Over 80% of the initial positron-emitting activity is from 15O while the remainder is primarily 11C, 13N, 14O with traces of 18F, and 10C. Radioisotopic imaging may also be performed along the beam path by fitting decay data collected after the treatment is complete. Using this technique, it is shown that variations in elemental composition in inhomogenous treatment volumes may be identified and used to locate anatomic landmarks. Radioisotopic imaging also reveals that 14O is created well beyond the Bragg peak, apparently by secondary neutrons.

Transfer and/or Breakup Modes in The He-6+Bi-209 Reaction Near the Coulomb Barrier

Reaction products from the interaction of 6He with 209Bi have been measured at energies near the Coulomb barrier. A 4He group of remarkable intensity, which dominates the total reaction cross section, has been observed. The angular distribution of the group suggests that it results primarily from a direct nuclear process. It is likely that this transfer and/or breakup channel is the doorway state that accounts for the previously observed large sub-barrier fusion enhancement in this system.

6 Li elastic scattering ON 12 C, 16 O, 40 Ca, 58 Ni, 74 Ge, 124 Sn, 166 Er and 208 Pb at E( 6 Li ) = 50.6 MeV

Abstract The elastic scattering of 6 Li ions from a variety of targets, A = 12 to 208, has been measured at a bombarding energy of 50.6 MeV. The angular distributions are characteristic of strongly absorbed particles, such as 3 He and heavy ions, and less diffractive than for 4 He. A simple optical model with Woods-Saxon real and imaginary volume potentials is adequate to fit the data. Spin-orbit effects are not apparent in the data.

6Li elastic scattering ON 12C, 16O, 40Ca, 58Ni, 74Ge, 124Sn, 166Er and 208Pb at E(6Li) = 50.6 MeV

Abstract The elastic scattering of 6 Li ions from a variety of targets, A = 12 to 208, has been measured at a bombarding energy of 50.6 MeV. The angular distributions are characteristic of strongly absorbed particles, such as 3 He and heavy ions, and less diffractive than for 4 He. A simple optical model with Woods-Saxon real and imaginary volume potentials is adequate to fit the data. Spin-orbit effects are not apparent in the data.

An apparatus for applying strong longitudinal magnetic fields to clinical photon and electron beams

Monte Carlo studies have recently renewed interest in the use of the effect of strong transverse and longitudinal magnetic fields to manipulate the dose characteristics of clinical photon and electron beams. A 3.5 T superconducting solenoidal magnet was used to evaluate the effect of a longitudinal field on both photon and electron beams. This note describes the apparatus and demonstrates some of the effects on the beam trajectory and dose distributions for measurements in a homogeneous phantom. The effects were studied using film in air and in phantoms which fit in the magnet bore. The magnetic field focused and collimated the electron beams. The converging, non-uniform field confined the beam and caused it to converge with increasing depth in the phantom. Due to the fields collecting and focusing effect, the beam flux density increased, leading to increased dose deposition near the magnetic axis, especially near the surface of the phantom. This study illustrates some benefits and challenges associated with the use of non-uniform longitudinal magnetic fields in conjunction with clinical electron and photon beams.

Nuclear and Coulomb Interaction in 8B Breakup at Sub-Coulomb Energies

The angular distribution for the breakup of 8B-->7Be+p on a 58Ni target has been measured at an incident energy of 25.75 MeV. The data are inconsistent with first-order theories but are remarkably well described by calculations including higher-order effects. The comparison with theory illustrates the importance of the inclusion of the exotic proton halo structure of 8B in accounting for the data.

A radioactive beam facility using a large superconducting solenoid

Abstract A facility for producing well-focussed (~ 5 mm diameter) secondary beams of ≤ 20 MeV 6 He, 7 Be, 8 Li, and similar ions has been implemented. The heart of the apparatus is a superconducting solenoid operating in an asymmetric lens configuration, with large solid angle (dΩ ≥ 100 msr) for the collection of reaction products. A beam of 15-MeV 8 Li ions ( ΔE ≤ 0.6 MeV) of intensity 5 × 10 4 ions/s, produced via the 9 Be( 7 Li, 8 Li) 8 Be reaction, was scattered from targets of CH 2 , CD 2 , 9 Be, 12 C, 27 l, and 197 Au. Elastic scattering and ( 8 Li, 7 Li) reaction products are unambiguously identified. Preliminary results of measurements on the 2 H( 8 Li, 9 Be)n reaction, as well as the status of a project to upgrade the 8 Li beam intensity to ≥ 1 ×10 6 ions/s, are reported. ft]∗|Permanent address: Physics Department, Oberlin College, Oberlin, OH 44074, USA.

Nuclear and Coulomb Interaction in the 8B to 7Be + p Breakup Reaction at sub-Coulomb Energies

The angular distribution for the breakup of 8B-->7Be+p on a 58Ni target has been measured at an incident energy of 25.75 MeV. The data are inconsistent with first-order theories but are remarkably well described by calculations including higher-order effects. The comparison with theory illustrates the importance of the inclusion of the exotic proton halo structure of 8B in accounting for the data.

Isobaric analog states of the tin and tellurium isotopes studied with the (3He, t) reaction at θL = 0°

Abstract The Sn( 3 He, t)Sb and Te( 3 He, t)I reactions to the g.s. isobaric analog states (IAS) of 112, 114, 116, 117, 118, 119, 120, 122, 124 Sn and 124, 125, 126, 128, 130 Te have been studied at θ = 0° and E( 3 He ) = 45.9 MeV . The Coulomb displacement energies decrease more slowly than A − 1 3 indicating that the rms radius of the nuclear matter distribution increases more slowly than A 1 3 . The total widths of the IAS are 10 to 60 keV and exhibit a pronounced odd- A even- A variation correlated with the excitation and decay energies of the IAS. Estimates of the decay and spreading widths have been obtained, with the latter yielding an isospin-mixing matrix element of about 1 keV. The ( 3 He, t) cross sections at 0° appear to be directly related to (p, n) total cross sections for the 0 + IAS. The cross sections for the Sn-Sb and Te-I isotopes, in contrast to accepted expressions, depend in a complicated manner on the neutron excess. A microscopic analysis gives an effective interaction ( μ = 1 fm −1 ) of V t · t = 11 ± 4 MeV. This is smaller than values obtained from analyses of ( 3 He, t) reactions on nuclei with A ⪅ 90, if one assumes a simple direct charge-exchange reaction.

Alpha-particle pickup on rare-earth nuclei with the (d, 6Li) reaction at 35 MeV

Abstract Spectra and angular distributions have been measured for the (d, 6Li) reaction on 138Ba, 140, 142Ce, 142, 144, 146Nd, 144, 148, 150, 154Sm and 166Er using 35 MeV deuterons and magnetic analysis. Good agreement between measured and calculated angular distributions was obtained employing one-step zero-range distorted wave Born approximation (DWBA) theory with α-cluster wave functions. Alpha-particle spectroscopic factors Sα and reduced widths γα2 have been extracted for the transitions to ground and excited states. Included in the analysis are earlier results for the (d, 6Li) reaction on 160Dy, 208Pb and 238U. The spectroscopic factors range from approximately 0.002 to 0.1. The transitions to the ground states of nuclei with a few neutrons beyond N = 82 are enhanced. A close correspondence with the (p, t) two-neutron pickup reaction was observed. Several of the target nuclei are known α-particle emitters, and the direct comparison between reaction and decay data leads to a consistent description of these processes making it possible to deduce α-decay lifetimes and branching ratios from the (d, 6Li) data.