RARAF – Charged Particle Beams

• the track-segment facility.
  
• the molecular beam facility.
  

• the microbeam.

Track-segment

Because the beam is vertical, the track-segment facility can irradiate attached cells growing in dishes filled with culture medium. The dishes are 3.5 cm i.d. stainless steel rings with 6 micron thick Mylar epoxied onto them. The vertical beam passes through a thin metal foil into the atmosphere, through the Mylar dish bottom, and irradiates the sample attached to the Mylar. A slot-shaped aperture approximately 6 mm wide defines the beam irradiating the samples. A stepping motor rotates a wheel containing up to 20 dishes at a rate defined by the desired dose and the instantaneous beam current striking the beam-defining aperture. Each point on a dish passes through the beam in about 25 steps. A heated enclosed wheel to which humidified gas can be supplied is available for oxygen enhancement ratio (OER) or fractionation experiments. In a track-segment irradiation, an initially monoenergetic beam of charged particles passes through the thin sample (~10mm) so that the same segments of the particle tracks are deposited in all the material of interest. Particles with linear energy transfer (LET) between 10 and 200 keV/mm are available utilizing beams of protons, deuterons, helium-3, and helium-4 ions as listed below.

Charged-Particle Beams for Radiobiology

For these low-velocity beams, the use of LET to describe radiation quality is fairly valid because the delta-ray tracks are short and the energy deposition events are closely spaced along the track. The energy and energy loss for helium-4 ions as a function of the particle range are shown below.

For a fixed sample thickness the LET spread through the sample is considerably less with high-energy, low-LET particles than with lower-energy, higher-LET particles.

Energy loss for helium-4 as a function of residual range. In charged-particle irradiations of thin samples, the same segment of all the tracks passes through the sample so the dose is deposited at a desired LET with a minimal spread on that LET.

To provide a reference radiation and an on-line test for hypoxia in OER experiments, a 50 kVp, 50 mA x-ray machine is part of the track-segment facility.

Track-segment irradiations and dosimetry are performed under computer control. The desired dose for each dish is entered into the program. During an irradiation, the computer inserts and withdraws the main beam stop and controls the rotation rate of the sample wheel so the dose across each dish is uniform. A typical irradiation takes less than 20 minutes. Beam tuning and dosimetry may take several hours.

For dosimetry, the sample wheel is replaced with a wheel holding three radiation detectors (right). A solid-state detector which can be moved radially under computer control is used to measure beam uniformity and particle fluence and check beam purity by taking energy spectra. Dose is measured with an ionization chamber having an equivalent thickness of about 1 micron. The chamber is placed at an appropriate equivalent distance along the particle track. The LET spectrum is measured with a self-calibrating LET counter simulating the appropriate tissue thickness using TE gas. The sample thickness and density must be specified by the experimenter for the LET, LET spread, and dose to be determined accurately. Dosimetry is also provided for the 50 kVp x-ray generator attached to the track-segment fixture. Caution should be exercised in using this radiation source for determinations of RBE – the average x-ray energy is less than 30 keV, so the biological effectiveness of these x-rays is significantly greater than that of orthovoltage x-rays.

Molecular Beam

In another of RARAF's charged-particle irradiation facilities, the thin Mylar bottom of a cell culture dish acts as a vacuum window and is the only material the beam encounters before striking the cells. This facility was designed to expose cells to spatially correlated ions generated by molecular beams. In the molecular-beam experiment, cells attached to Mylar are irradiated with equal doses of particles with the same LET, either uncorrelated ions or correlated ions from a molecule. The molecules break apart in the Mylar substrate into a pair of spatially-correlated ions which diverge because of multiple scattering in the foil and the cellular material. A distribution of separations between the pairs of ions results and can be used to probe the distribution of radiosensitive material in the cell nucleus. In the molecular-beam facility, the beam is horizontal and the plane of the dishes vertical, so culture medium must be removed from the dishes before irradiation. In the original fixture used at BNL and briefly at Nevis, the dishes were mounted in a partially evacuated wheel within a high vacuum chamber. The cells were typically exposed to this adverse condition for an hour. The new fixture allows the cells to remain at atmospheric pressure. To accomplish this, the inside diameter of the cell dishes has been reduced to less than 1.3 cm. Operation of the new molecular-ion fixture for irradiation and dosimetry is similar to operation of the track-segment fixture except that the dishes are inserted into the sample wheel one by one. The molecular-ion fixture is mounted on the 50º beam line in the U area, where access is not restricted when the accelerator is running.

Site developed by CE, page last modified by JL on November 29, 2007 .

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