Neutron Activation Analysis of Absolutely-Dated Tree Rings
| Participants: | Kenan Ünlü, Professor of Mechanical and Nuclear Engineering |
| D. K. Hauck, Ph.D. student at Mechanical and Nuclear Engineering | |
| T. H. Daubenspeck, Activation & Irradiation Specialist | |
| Services Provided: | Radionuclear Applications Laboratory |
| Sponsor: | Cornell University |
| NSF and RSEC |
Introduction
The Dendrochronology project taking place at the Breazeale Nuclear Reactor is a search for heightened gold concentration in tree rings with the use of Instrumental Neutron Activation Analysis (INAA). The usefulness of this study lies in the environmental and climactic history that can be deduced from the data. Large amounts of stratospheric sulfur from volcanic eruptions, forest fires or industrial pollution can stress trees and result in larger heavy ion uptake. We are focusing on gold uptake because of its suitability to INAA. In addition, climate cycles over 10s of years can be identified and correlated with such well-known cycles as the El Nino Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). The current climatological record recording these cycles only exists for the past 100-150 years. This relatively short record makes Earth’s climate cycles and effects like Global Warming difficult to understand. A correlation with ion uptake in tree rings would allow us to extend the climatological record back thousands of years.
PSU NAA Facility
In the past year, the INAA facility was completed at the Breazeale Nuclear Reactor. This includes installing two Dry Irradiation Tubes (DT) in the reactor core, getting the Automatic Sample Handling System (ASHS) up and running, setting up the analysis system with a High-Purity Germanium (HpGe) Detector, a Digital Spectrum Analyzer and Genie-2000 software, and doing the flux measurements necessary to identify flux peak area for sample irradiation positions. Nearly 400 tree ring samples have been analyzed from a tree which grew in Turkey between the years 1473 and 1980 A.D. The tree rings that have been analyzed so far are from the years 1600 – 1980 A.D. We will first complete the analysis of this tree (1473-1980) and continue with a tree from same region and covering similar time period.
The Dry Tubes
Two Dry Irradiation Tubes (DT) were installed in the reactor core for irradiation of NAA samples. The DT’s occupy approximately symmetric positions within the core, as shown in Figure 1. The bottom of the DTs was designed to be identical to the bottom fuel rod tips and fit into the bottom grid plate. The height of the DTs are approximately 21 feet and bend to 6 inches off the centerline approximately one third of the way up. This allows for samples to be pulled out of the core while they decay but maintain a depth of 14 feet of water as a radiological shield.
Figure 1: The Dry Tubes occupy positions E4 and E13 in the reactor core.
Flux measurements were performed in the DTs at 10 W, 800 kW and 1 MW show that the height of the flux peak is strongly dependent on the position of the control rods but fairly independent of core position within the reactor pool. Measurements were done both in the center of the reactor pool position and coupled core-D 2O tank position. The peak flux at 1 MW was approximately 1.5 x 10 13 n/cm 2/s with DT1 being consistently slightly higher than DT2. The flux measured inside the polyethylene vials used for wood irradiations is approximately 1.2 x 10 13 n/cm 2/s with the difference presumably the position of polyethylene vials and possibly from moderation by the polyethylene vial.
The Automatic Sample Handling System (ASHS) brought from Cornell University, Ward Center for Nuclear Sciences by Dr. Ünlü was installed and tested at Penn State, RSEC. Batch programming files have been written specifically for the Dendrochronology project and for general usage. This allows the user to define the number of samples to be run in the ASHS in sequence and the amount of time that they are to be counted. Special presets can be used, for example real time vs. live time and files can be automatically named and analyzed according to user specified values. The ASHS holds up to 90 samples at one time, allowing for a consistent through put of sample especially useful for such an extensive study as the Dendrochronology project. A close-up picture of ASHS is shown in Figure 2.
Figure 2: A close up picture of the Automatic Sample Handling System (ASHS) showing sample tray and robot arm.
HpGe Detector
A new HpGe detector was purchased, allowing the former HpGe detector associated with the Dendrochronology project to be dedicated to teaching and demonstrations. The new HpGe detector has a 36% relative efficiency and 1.8 keV resolution at the higher energy cobalt peak (1332 keV). This has allowed for more accurate isotope identification and area calculations. The detector is surrounded by 3000 lbs of pre-WWII lead bricks to block out environmental background radiation.
Data Analysis
Before irradiating wood samples, the natural radioactivity of the wood should be considered. In order to find the typical isotopes found in un-irradiated wood, and how their abundance differed in different tree rings, approximately 30 tree ring samples were analyzed. The samples that analyzed are from the years 1940-1968 and they are from a tree which grew in Turkey between 1628 and 1998. This measurement period corresponds to the heaviest years of nuclear weapons testing. Consistently the following isotopes were identified in tree samples: Ac 228, Ra 223, U 235,Bi 214 and Pa 234.
Recent samples that have been analyzed since the inclusion of the new irradiation facilities in the Breazeale Nuclear Reactor are from a Turkish pine tree and range in date from 1600 to 1980. The average isotope concentrations differ slightly from another Turkish tree which was analyzed previously. A comparison of average concentrations of some isotopes for these trees is given in Table 1. The smaller quantity of Gold in the tree samples requires long irradiations or higher neutron fluences. At RSEC, the tree samples are irradiated at 4MW-hour for each batch of runs. In comparison, typical irradiations of tree samples were 1.2 MW-hour at Cornell University. This more powerful irradiation has allowed for the identification of a new isotope, La-139, which has promising qualities as an indicator of biological stress.
Table 1: Average concentrations of isotopes in two Turkish trees spanning the years 1650 to 1980. The tree CTUCAT14C was analyzed recently at Penn State. CTUKLK10B was analyzed previously at Cornell University. Differences may be due to different uptake properties of the two trees or different regional environmental stresses. Concentrations are in parts per million.
The chronology of data from the samples recently analyzed is also promising. The data shows a clearly defined base continuum in the isotopes that will behave chemically as salts in the tree, especially including Sodium, Potassium and Bromine. This continuum indicates a steep rise around the late eighteenth century that may be due to increased industrialization, metabolism with in the tree or large time-scale climate cycles. Any of the three possible causes for this increase in background would be an important indicator for future studies. All identified isotopes show very strong peaks (as much as an order of magnitude higher) on top of the background continuum (see Figure 3). The peaks are very consistent among the salt-like isotopes and slightly different for the heavier, non-reactive elements like gold. This may be a result of active transport with in the tree of chemicals that take part in the biological processes of the tree. Gold, on the other hand, is inactive and expected to have no active transport and very little passive transport across tree rings, making it ideal for this study. Three example isotopes are shown in Figure 3. Potassium, Zinc and Bromine behave similarly to Sodium. The data we are acquiring now from another tree that grew in the same region of Turkey and covers the similar time period. We will be looking same trends for this tree in order to verify our previous observations.
Conclusions
The tree samples that are currently being analyzed have great promise as an indicator of major environmental events in addition to climatic change and plant uptake properties. In particular, the comparison of Gold uptake properties with that of the salts should reveal which effects are due to tree biology and which are due to external stresses. The next immediate goal is to complete the analysis of several trees and make a correlation with environmental effects and the ion uptake behavior of trees.
Figure 3. Gold, Sodium and Lanthanum concentrations in the tree sample CTUCAT14C that grew in Turkey between 1600 to 1980. The concentrations are in parts per million.
