Isotopes are used for applications that touch the lives of almost everyone --
including detecting smoke from a house fire, diagnosing and irradiating cancerous
tumors, and imaging the heart in stress tests. They are also used in calibrating
detectors used to search for nuclear contraband at our ports, powering satellites
to the outer planets, performing in a wide range of industrial tasks, and enabling U.S.
scientists to remain at the forefront of scientific advances and discoveries in many fields.
Radioactive isotopes -- radionuclides -- are used for over 20000 nuclear medicine procedures
each year in the U.S., representing a multi-billion dollar/year industry. Despite this
growing need for separated isotopes, many supplies have dwindled to a critical
point and international supplies have proven unreliable.
The ORNL Tandem and associated experimental endstations are well suited for a variety of important research projects addressing critical needs in the Isotopes field. Focus areas include ion sources for electromagnetic separation, accelerator isotope production, purification techniques, and determination of isotope properties.
We can use the Tandem accelerator and associated equipment to make important contributions
to a variety of research topics crucial to isotope production: production cross section measurements,
measurements of known contaminant production, determination of unknown contaminant production,
production target design, optimization and implementation, isotope purification,
and benchmarking of simulation codes.
The beams from the ORNL Tandem are well suited to R&D related to accelerator isotope production since the energies and species available are similar to those used by isotope production cyclotrons and to the accelerator capabilities recommended in a recent NSAC Isotopes Subcommittee report. For example, light-ion beams of variable energy are available with maximum energy of 50 MeV for protons and 75 MeV for alpha-particle beams. In addition, heavy-ion beams are available with energies up to a few MeV per nucleon. Furthermore, our new resonant laser ion source system [see image at right] has numerous applications for purifying beams of radioisotopes.
One of the most important applications of this work is to evaluate the viability of alternate radioisotope production mechanisms -- of crucial importance given the limited availability and high cost of many isotopes.
As an example of work well suited for our facility, the cross sections
of some nuclear reactions needed to predict
production yields of certain important isotopes via accelerator
bombardment have never been measured or, in some cases, discrepancies
exist in the published data. While reaction models can successfully
predict some of these cross sections, others are difficult to model with
sufficient accuracy and must be measured -- especially at the low energies
typically utilized at numerous isotope production accelerator facilities.
Measurements of a few such cross sections may be extrapolated to predict cross
sections of other unmeasured reaction channels with specific beam and target
combinations. Most importantly, this work can aid in evaluating the viability
of radioisotope production mechanisms that are not yet used for routine production.
The importance of this work is evident by the recent formation of a new Cooperative Research Project on production cross sections coordinated by the Nuclear Data Section of the International Atomic Energy Agency [IAE2011].
We anticipate utilizing the ORNL Tandem to bombard thin targets with low- to moderate-intensity beams and measure the yields of reaction products to determine differential reaction cross sections of stable and radioactive species. This can be combined with studies whereby thicker targets are bombarded to determine integral cross sections of radioactive species by measurements of target activity, or integral cross sections of stable species via subsequent chemical separation and mass spectrometry.
The ORNL Tandem is extremely well suited for these measurements for numerous reasons. The 1 part in 10000 beam energy resolution of the Tandem enables us to make cross section measurements with high precision compared to other machines. Because beam energies can be quickly changed, often in 30 minutes or less, excitation functions can be mapped out much more efficiently than with a cyclotron or linac. Our facility is equipped with sophisticated detector arrays that enable us to make high-precision, coincidence measurements of emitted gamma-rays, beta-particles, heavy charged particles [see image at right of the SuperORRUBA array] [Bardayan2012], and neutrons. Furthermore, the high fidelity of these detection systems enables rejection of products of contaminant reactions that would otherwise compromise cross section measurements.
Examples of unmeasured production cross sections for medically important radionuclides include 144Sm(alpha,n)147Gd, 40Ca(alpha,p)43Sc, 197Au(p,alpha n)193Pt, and 232Th(p,alpha 2n)227Ac [see figure to right]. The IAEA Nuclear Data Section recently (2011) convened a Technical Meeting to discuss this topic and the report lists more than 70 radionuclides for which cross section data or decay data are needed [IAE2011a].
"The New SuperORRUBA Detector for Transfer Reaction Studies of
Exotic Nuclei", D.W. Bardayan et al., XII International Symposium on Nuclei in the Cosmos,
August 5-12, 2012, Cairns, Australia, Proceedings of Science PoS (in press) 2012.
[IAE2011] "Summary Report on Consultants' Meeting on Improvements in Charged-Particle Monitor Reactions and Nuclear Data for Medical Isotope Production", 21-24 June 2011, IAEA International Nuclear Data Committee INDC(NDS)-0591.
[IAE2011a] "Summary Report on Technical Meeting on Intermediate-term Nuclear Data Needs for Medical Applications: Cross Sections and Decay Data", 22-26 August 2011, IAEA International Nuclear Data Committee INDC(NDS)-0596.
For More Information
The following links will let you learn more about this topic: Charged-particle cross section database for medical radioisotope production
David Dean, Physics Division Director
deandj at ornl.gov