RARAF – Slow Neutrons

Low-energy Neutron Facility

In the final design below, the target surface is normal to the beam direction, and the cell samples are positioned above the beam, making placement much easier and permitting the use of multiple samples. To enable the use of high beam currents without melting the lithium, the target is rotated at high speed. A beam spot 1 mm wide by 5 mm high is used to minimize the time that the lithium is heated by the beam. The lithium is evaporated in place in an annular ring using the port located directly below the axis of rotation while the target is being rotated.

The samples are positioned 2.5 cm from the center of the beam in order to maximize the dose rates, which are as low as 0.1 Gy/hr for the lowest-energy spectrum, but considerably higher for the higher-energy spectra. A filter up to 1 cm long consisting of depleted uranium, tin, copper and aluminum is inserted between the target and the sample to reduce the dose rate from the 477 keV gamma rays arising from the ⁷Li(p,p’g )⁷Li reaction and the characteristic X rays from the uranium.

Dose contributions as a function of neutron energy are presented below for the three low-energy neutron spectra. Note that the bin widths double above 121 keV.

Mechanistic Rationale

In the past two decades, it has become clear that energy deposition at the nanometer level is the prime determinant of biological effectiveness. This conclusion has been largely based on studies using ultrasoft x-rays as a mechanistic probe, whose secondary photoelectrons have nanometer dimensions, However, the interpretation of these soft x-ray studies has been strongly hampered by the non-uniform dose distribution produced by these x rays over the range of a cellular nucleus, due to the high attenuation coefficient of the x rays. Unlike soft x-rays, however, low-energy neutrons produce a uniform dose distribution across cell nuclei, thus avoiding the major problems of dosimetry and interpretation inherent in the soft x ray approach.

Occupational Exposure to low-energy neutrons

 

A significant number of people are potentially exposed occupationally over a protracted period to low doses of neutrons. In DOE facilities (1988 figures, Merwin et al 1990), about 92,000 individuals were monitored as potentially receiving neutron dose, and about 7,000 individuals absorbed measurable neutron doses. In addition, of the approximately 600,000 monitored workers under NRC regulation, about 6,000 per year (primarily research workers, well loggers and reactor workers) receive measurable neutron doses (NRC, 1988 and private communication from C. Raddatz, NRC, 1991). There is also increasing concern about the neutron dose to which airline crew members (300,000 in U.S. airlines) are exposed. Calculations (e.g. Friedberg, 1989, Wilson and Townsend, 1988) indicate that in some cases crew members will receive more than the maximum permissible dose for non-radiation workers, about half the dose equivalent coming from neutrons.

For reactor workers the neutron energy spectrum to which occupationally exposed individuals will be subject varies widely, even within a given reactor facility. The neutron spectrum depends on the neutron source and on the degree of shielding, and thus moderation, to which the neutrons are exposed. In addition, of course, the neutrons are moderated by the body of the exposed individual. Whether this is important in terms of the biological effectiveness depends on whether the neutron biological effectiveness varies significantly in the neutron energy range of interest for occupational exposure.

The significant neutron energy range, in terms of dose deposited, varies according to the fluence spectrum to which the individual is exposed. The neutron energy range from 10 to 200 keV is, however, the energy range where there is evidence that there may be significant variations in biological response. In the neutron energy range below ~100 keV, there are two major groups of data sets available, both based on filtered reactor beams; these are from Sevankaev et al (1979) in the Soviet Union (nominal 40, 90 keV) and Lloyd and colleagues (nominal 24 keV), in the U.K. (e.g. Lloyd et al, 1988, Morgan et al, 1988). The yield (per unit dose at low doses) of chromosomal aberrations in human lymphocytes, as measured by Sevankaev et al (1979), is considerably decreased compared with the yield at a neutron energy of a few hundred keV. This is in accord with earlier results for cellular survival (Hall et al, 1973) and is also in accord with biophysical expectations (e.g. ICRU 1986, Blue et al 1995), as well as recent ICRP recommendations (ICRP, 1991). On the other hand, the results of the Harwell group, both for chromosomal aberration yields in human lymphocytes, and for other end points in rodent cells (e.g. Morgan et al 1988), suggest comparable yields to those at a few hundred keV.

This disagreement is significant on two levels. First, in terms of the radiation protection issues addressed in the current proposal, a significant decrease in the biological effectiveness of neutrons from the hundreds of keV to the tens of keV range would result in a decrease in the quality factor appropriate for most occupational exposure situations. Second, in terms of biological mechanisms, radiobiological models based on energy deposition in cellular or nucleus-sized targets unequivocally predict a decrease in biological effect as the neutron energy decreases; if this decrease were not to be confirmed, then such models would be substantially falsified.

 

Boron Neutron Capture Therapy

 

Slow neutrons are aimed at a tumor containing a borated drug, and neutron capture by boron causes the emission of a highly-damaging alpha-particle in the tumor. The limiting normal tissue damage will be produced by the soft neutrons themselves, the biological effectiveness of which is poorly understood.