Sunday, February 20, 2011

Making Radioactive 63Ni to Detect Explosives at Airports

When Transportation Security Administration (TSA) inspectors swipe a cloth over your luggage and then place it in an analyzer to check for explosives residue, they are using a device containing 63Ni, a radioactive isotope of nickel, made at Oak Ridge National Laboratory (ORNL).

Targets of enriched stable 62Ni such as these are bombarded with neutrons in HFIR to make 63Ni. Each target contains 25 grams of pressed 62Ni metal pellets stacked in a 35-inch-long aluminum target capsule, 12.5 grams at each end.
Targets of enriched stable <sup>62</sup>Ni such as these are bombarded with neutrons in HFIR to make <sup>63</sup>Ni. Each target contains 25 grams of pressed <sup>62</sup>Ni metal pellets stacked in a 35-inch-long aluminum target capsule, 12.5 grams at each end.
Credit: Oak Ridge National Laboratory

ORNL is the exclusive producer of 63Ni in North America and perhaps worldwide. "Our only competition would probably be Russia. They have high-flux research reactors and may well be supplying the material also," explained Mitch Ferren in the ORNL Isotope Business Office. The office coordinates production of the 63Ni and other isotopes.

To detect explosives, or hazardous chemicals and vapors, an area of public safety increasingly important since the September 11 attacks, the 63Ni's beta emitter acts as an annihilation source, stripping the molecules that are given off by a material and analyzing these in the device.

To make 63Ni, technicians at the Radiochemical Engineering Development Center at the high flux isotope reactor (HFIR) prepare targets of enriched stable 62Ni and then bombard them with neutrons in the HFIR. Each target contains 25 grams of pressed 62Ni metal pellets stacked in a 35-inch-long aluminum target capsule, 12.5 grams at each end. Under bombardment with neutrons, 62Ni becomes activated, and the result is a new, radioactive isotope useful for airport and transportation security applications.

The 62Ni comes from an inventory of stable isotopes maintained by ORNL's Isotope Development Group.

The HFIR is uniquely suited to the activation work, Ferren said, because the reactor has probably the highest flux available—average number of neutrons bombarding the target—and ample volume for large target irradiations. Consequently, it can produce large curie quantities of 63Ni more quickly than other reactors. The result is more 63Ni, a quicker production process, lower irradiation costs, and therefore, a less expensive end product.

62Ni is not naturally abundant, occurring at only 3.6% in nature. "The 62Ni material we use for the target is 96% enriched," Ferren explained. "We increase the percentage of 62Ni, compared to the other isotopes, to get more atoms of 62Ni per unit volume to hit with the neutrons." This stable isotope enrichment process was achieved with electromagnetic separators located at the Y-12 site. The irradiation of the highly enriched 62Ni results in a high "specific activity" product containing fewer impurities at the end of bombardment.

Industry specifications require the specific activity of the final product to be at least 15 curies (a measure of radioactivity) of 63Ni per gram of enriched Ni target material. To achieve this, the 63Ni is placed in the reactor for approximately 15 reactor cycles, where it becomes progressively more radioactive. Each target produces approximately 375 curies of 63Ni.

The target irradiation takes a long time. "Each cycle lasts about 23 days, and we need to do 15 of those," Ferren said. The HFIR burns through its fuel in 23 days and must shut down for refueling. The reactor is typically off for 19 days between cycles and has semi-annual maintenance shutdowns. The result is 7 cycles a year—so each target stays in the HFIR for more than two years. This is not a problem for sample integrity.

An attractive feature of 63Ni is that it has a half life of 101 years—so nobody needs to be in a hurry for fear the radioactive isotope will decay. "This allows us to let the targets cool prior to processing. Some isotopes have a 10-day half-life and we have to get them moving. The 63Ni is a good one; we don't have to rush," said Ferren.

After the cycles are complete, the targets are cooled for six months to a year to allow the aluminum part of the target and other trace elements,which become activated during bombardment, to decay out. Then the radiochemists at the Radiochemical Engineering Development Center (REDC) cut the nickel pellets out of the aluminum and chemically process them to remove impurities.

63Ni is a beta emitter, so it does not need much shielding. The prepared bulk material is packaged in a solution of hydrochloric acid, and the final product is sent to Brookhaven National Laboratory, where it is distributed to commercial source fabricators.

Ferren said the bulk is routinely shipped to commercial fabricators in increments of 20 curies. The commercial fabricators then refine it further, even to the bare metal, so they can electroplate it or adhere it to a backing such as foil in detection devices.

63Ni is made at the HFIR under the Department of Energy's Isotope Production and Applications Program. Through the program, radioactive and stable isotopes and byproducts are produced and sold worldwide. The program is funded through the Office of Science's Office of Nuclear Physics.

Contacts and sources:
Agatha Bardoel
Federal Labs Consortium

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