Feature

Ames Chief Life Scientist Recognized for Radiation Breakthrough
07.12.06
 
Dr. Tore Straume Ames Chief Life Scientist Dr. Tore Straume helped settle a lengthy, and fascinating, scientific debate and earned a prestigious federal award for his groundbreaking work.

Image left: A portrait of Dr. Tore Straume, Chief Life Scientist, at the NASA Ames Research Center.

Earlier this year, the U.S. Department of Energy conferred its Special Achievement Award upon Straume. John Spitaleri Shaw, the Assistant Secretary for Environment, Safety, and Health praised Straume's contribution to a landmark international study that reassessed radiation levels in the Japanese cities destroyed by atomic bombs at the end of World War II.

Secretary Shaw lauded Straume's work as "essential to our understanding of the health effects of radiation exposure". The health-effects information obtained from the studies of Hiroshima and Nagasaki serves as the basis for treating survivors exposed to radiation and for estimating radiation risk worldwide, including those used by NASA for human spaceflight.

The study was a joint effort of U.S., Japanese, and German experts in radiation dosimetry, a field that seeks to accurately measure radiation doses and to define what is a safe radiation dose for humans. The results of the neutron measurements made by Straume and his team were incorporated into a new system for estimating radiation doses in Hiroshima and Nagasaki, known as DS02 (for Dosimetry System 2002).

DS02 will change risk estimates only a little, but the confidence in the new system will go up enormously. With the possibility of people being exposed to greater levels of radiation in their daily lives – and the threat of nuclear-based terrorism often talked about these days – the need for high confidence levels in our ability to measure radiation accurately is growing.

The decades-old debate revolved around the actual radiation dose received by A-bomb survivors in Hiroshima and Nagasaki and the resulting effects. Scientists have long sought to quantify the nature of the radiation dose unleashed by the first nuclear weapons ever used in warfare. This was particularly difficult for the Hiroshima bomb. Unlike the type of bombs tested previously or since, the Hiroshima bomb was a unique design, which was difficult to model computationally. And both of these bombs were exploded above a real city, not in the controlled environs of a test facility where measurements could be easily made.

Initial post-war studies painstakingly tried to model and recreate conditions on the morning of August 6 and 9, 1945 – the dates of the blasts. Several of the above-ground nuclear weapon tests provided experimental data showing how the radiation dose varies with bomb and distance parameters. Experts went so far as to erect a mock Japanese village in the middle of a Nevada desert, where a nuclear reactor was hoisted high in the sky, to model the radiation shielding factors provided by local construction practices and materials.

These initial studies concluded that the Hiroshima bomb emitted primarily high-energy radiation in the form of gamma rays but with a large neutron component. They concluded the Nagasaki bomb exposed survivors to a smaller amount of neutrons. Even so, scientists viewed these early estimates with caution because flaws were evident in the methodology. For one, the hoisted nuclear reactor was bare, whereas the actual Hiroshima bomb was encased in very thick steel.

Later computer studies done in the early 1980s at the Lawrence Livermore National Laboratory questioned whether there was a significant neutron component at Hiroshima. They suggested that radiation from gamma rays was far more prevalent and the doses from neutrons were much lower than previously thought. This meant that radiation such as gammas- and X-rays, rather than the neutrons, were responsible for the vast majority of the A-bomb survivor radiation effects. Considering how often X-rays are commonly used in today's technological world, this question about what type of radiation contributed most of the health effects in A-bomb survivors had to be confirmed with finality.

The Livermore computer calculations were in good agreement with measurements for gamma rays at both cities. Gamma-ray doses were validated by comparing these calculations with TLD (thermoluminescence dosimetry) measurements in roof tiles at various distances from the bomb explosions. A large number of such measurements were completed by the mid-1980s by several Japanese and U.S. scientists. Unfortunately, few validation measurements were available at that time for neutrons, and those that were available, seemed to substantially disagree with the dosimetry calculations. This was a serious problem because if left unresolved it would cast doubt on the risk estimates inferred from the A-bomb survivor data.

To help resolve this problem, the federal government funded more studies as the protracted battle of measurements versus calculation raged on. Straume was asked to lead the U.S. effort to validate the neutrons emitted from the A-bombs and the levels that irradiated the survivors in 1945. In the early nineties, Straume was head of Dosimetry and Exposure Assessment at Livermore, and had made a name for himself devising new and better ways of accurately measuring traces of decades-prior neutron interactions in durable materials such as stone or metal, materials which stand a good chance of surviving an actual blast.

Earlier work done by Straume confirmed the presence of slow neutrons interacting with mineral samples such as concrete and granite. However, Straume recognized that slow neutrons did not contribute much to the neutron dose, and that to resolve this problem, new methods had to be developed to measure traces of fast neutron interactions in samples from Hiroshima, now more than half a century later.

"There weren't many good, direct measurements for neutrons in the (earlier) work," said Straume. "Questions remained regarding the level of neutrons. Most importantly, no fast neutron measurements had been made since 1945 because the technology and capability did not exist to make such measurements."

By analyzing high purity copper samples, such as copper roofs, gutters, even lightning rods, taken from buildings in Hiroshima after the blast – and which had been carefully preserved by the Japanese since the war – the scientists were able to quantify traces of a nickel isotope produced when fast neutrons interacted with the copper atoms in the metal.

Straume and Livermore colleagues Alfredo Marchetti and Jeff McAninch devised hyper pure methods to extract the very small quantities of this nickel isotope (63Ni) from the copper metal. Advanced accelerator mass spectrometry analysis was then developed, first at Livermore and then at Munich, Germany by a joint U.S.-German team organized by Straume, to detect the amount of the nickel isotope at the atomic level. Before Straume and colleagues began the work, these analytical capabilities did not even exist.

Japanese colleagues, principally Shoichiro Fujita of the Radiation Effects Research Foundation in Hiroshima, painstakingly mapped the precise location of each metal sample in relation to the exact spot of detonation. Modeling calculations were then meticulously done in California by Stephen Egbert and colleagues at La Jolla's Science Applications International Corporation. In the end, these globe-hopping measurements validated the level of neutrons calculated by DS02 at Hiroshima.

"We had finally resolved the neutron discrepancy," said Straume recently in his Ames office crowded with material from the work. He is quick to credit the work's success to his U.S., Japanese and German counterparts in what has truly been an international collaboration.

The team's exhaustive two-volume text on their innovative methodology and techniques – with Straume as the lead author on two neutron measurement chapters – established the new benchmark for dosimetry. The research is a "tour de force experimentally," says Warren K. Sinclair, past president of the National Council on Radiation Protection and Measurements and presently president emeritus.

"(The study) is viewed as the authoritative work in radiation measurement," says Ames Life Sciences Division head Dr. Russell Kerschmann.

The team's results will be incorporated into new guidelines for risk estimates for radiation exposure and in fields such as cancer treatment and post-9/11 emergency preparedness. They will also be used in standards for space travel, as interstellar radiation will continue to be a significant obstacle to human exploration of space.

Presently, as Ames' lead scientist for Life Sciences, Straume is concerned with radiation levels that future space explorers may be subjected to on NASA missions to other planetary bodies. His groundbreaking work on terrestrial radiation will lay the foundation for similar studies in space.

"We are extremely lucky to have Tore here at Ames as the linchpin of our new space radiation program," says Ames' Kerschmann. "This recognition speaks of his stature among his peers."

In addition to his space radiation work, Straume is presently preparing a paper on the effects of the Chernobyl nuclear power plant disaster in the Ukraine. Like the Hiroshima research, this will involve reassessing historical data, coming up with new, more accurate ways of measuring radiation, and then gleaning innovative science – and hope for survivors – from past tragedies.

 
 
Terence Pagaduan
NASA Ames Research Center