Authors of this article are Dr. Carola A. Laue and Professor Darleane C. Hoffman (left). Professor Hoffman is a member of the NMT Division Review Committee. Dr. Laue is an Alexander von Humboldt Fellow, a joint postdoctoral position between the Alexander von Humboldt Foundation and The Lawrence Berkeley National Laboratory. Dr. Laue works in the laboratory of Professor Hoffman.
Nearly 60 years after its discovery, plutonium still has the magic associated with being one of the first synthetic elements produced by humans. After so many years, chemists are still thrilled by the complexity of its chemical properties and behavior. Mastering the chemical complexity of plutonium has always been a challenge, but the key challenge in the identification of the long-sought 231Pu isotope was its expected brief half-life of 3 to 30 minutes.
Soon after plutonium's initial discovery in 1941, researchers associated with Glenn T. Seaborg's group at the University of California Radiation Laboratory began looking intensively for the lighter (neutron-deficient) plutonium isotopes. In 1951, D. A. Orth found, 232Pu, the lightest plutonium isotope known at that time and remaining so for nearly 40 years. In the early 1990s, a Russian team at the Joint Institute of Nuclear Research at Dubna identified three lighter isotopes, 228,229,230Pu, leaving a vacancy in the chart of nuclides for 231Pu, as Figure 1 shows.
In our successful identification experiment, an array of eleven very thin (~52 mg/cm2) 233U targets was irradiated with 48-MeV 3He ions from the 88-Inch Cyclotron at the Lawrence Berkeley National Laboratory. In this experimental setup, nuclear reaction products recoil out of the thin 233U targets, stop in the helium atmosphere, and attach to potassium chloride aerosols, which are continuously introduced into the target chamber by flowing helium gas. The activity-laden aerosols were then transported by the flowing helium through a capillary to a collection site away from the target chamber.
Figure 1. Region of the Chart of the Nuclides indicating the missing 231Pu.
The entire setup is shown schematically in Figure 2. Experiments consisting of 10-minute collections of the aerosols followed by chemical separation and measurement of the a-particle energies of the isotopes were repeated every 10 minutes. The decay data from all of the single experiments were summed to provide sufficient data for determination of the half-life and decay modes of 231Pu.
Figure 2. Schematic of the experimental setup. Helium is saturated with KCl aerosols, produced in an oven at 400°C. This helium/aerosol stream flows through the light-ion multiple (LIM) target system, picking up the recoiling reaction products and transporting them via a ~10-m-long capillary (2-mm ID) to a four-position rotatable wheel at the collection site outside the irradiation area. The aerosol deposit is then removed manually and chemically processed.
231Pu was expected to decay by orbital electron capture (EC) to 231Np, as well as by emission of an a-particle to 227U. Both decay branches were found and are illustrated in Figure 3. The 231Pu identification could only be performed by using the a-a-correlation technique. This technique can be used when a-decay of a parent nucleus leads to a known daughter nucleus, which decays by a-emission. The individual times and energies of each detected a-particle were stored sequentially using a coumputer-controlled data-aquisition system. The recorded data were analyzed off-line. Time-correlated pairs of a-particles were sorted according to the time of occurrence of the parent, the time interval between parent and daughter decay, and the characteristic a-particle energies of parent and daughter. This information provided the powerful tool needed for positive identification of the atomic and mass numbers of the isotopes.
Figure 3. Decay chains of the long-sought isotope, 231Pu, and of 232Pu, the most abundant isotope in the purified plutonium fraction. All a-decay energies are given in MeV.
Using this a-a-correlation technique, we measured a half-life of 8.6 ± 0.5 minutes for 231Pu. The energy of its emitted a-particle was found to be 6.72 ± 0.03 MeV. The 231Pu a-decay branch is approximately 10%, the remainder decaying by electron capture.
Often, we are asked why our experiment was successful in identifying 231Pu when so many previous attempts had failed. Primarily, because we devised the following improvements, which increased the sensitivity of our experiment and were crucial in identifying this elusive isotope.
In the recoil technique, the momentum of nuclei formed in the production reaction causes the nuclei to recoil out of the target, provided the recoil range is long compared to the target thickness. The disadvantage in light- ion-induced reactions is that the recoil ranges of the reaction products are rather small. Consequently, the light-ion multiple (LIM) target system (Figure 2) was developed in our group some years ago to overcome this disadvantage. The light-ion beam passes successively through closely spaced thin targets, losing only a small fraction of its energy in each target. The beam still has enough energy to induce the desired nuclear reaction and, as a result, the production rate increases proportionally to the number of targets. In the much earlier attempts at Berkeley, very heavy uranium oxide targets were irradiated, but because of the target thickness, the recoil technique was not applicable. Although large amounts of plutonium were produced, the entire target had to be dissolved, making the separation slow and difficult, thus limiting the possibility of finding shorter-lived isotopes.
Figure 4. Flow chart of the plutonium separation method using solid-phase extraction.
Previous researchers tried to save time by skipping the separation of neptunium from plutonium, but the interferences resulting from the decay of neptunium isotopes (see Figure 3) completely obscured the 231Pu decay.
In summary, the increases in sensitivity, including larger production rates, and advances in chemical separation techniques and in data acquisition and analysis enabled us to successfully identify the long-sought 231Pu isotope nearly 50 years after the search began.
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