Nuclear Science/Nuclear Chemistry

Nuclear Physics

The 10-MV tandem accelerator at CAMS provides a platform for conducting nuclear physics experiment both for basic science and lab mission-related programs.

Figure 1. The dependence of reaction rate on stellar temperature for the 40Ca(a,g)44Ti reaction calculated using several nucleosynthesis models and compared to other evaluations in the literature.

For example, we performed a new cross section measurement of the astrophysically important reaction ⁴⁰Ca(a,g)⁴⁴Ti in which high purity CaO targets were irradiated with helium ions at several different discrete energies. The reaction rate was measured on-line via prompt gamma ray spectroscopy and validated by post-irradiation measurement of ⁴⁴Ti at a low-level counting facility.

The relatively high beam currents available at CAMS (He²+ ~3 uA) enabled measurement of this cross section to better than 1% statistical uncertainty in a matter of several hours. The results of this experiment play an important role in constraining the nucleosynthesis models used to predict the distribution of elements produced in stars [Figure 1].

The offline counting portion of the ⁴⁰Ca(a,g)⁴⁴Ti experiment described above is an example of a direct measurement of a nuclear cross section. Direct measurements provide particularly valuable experimental data because they do not rely on theoretical model calculations that introduce intrinsic uncertainty into an evaluated cross section. For example, existing data for the ²³⁹Pu(n,2n)²³⁸Pu cross section at energies <14 MeV are largely inferred from measurements of a handful of partial g-ray cross sections coupled with Hauser-Feshbach model calculations. The unique capabilities of CAMS offer the potential to validate existing model-dependent data by directly measuring the ²³⁸Pu/²³⁹Pu atom ratio in irradiated samples.

Radiochemistry

The ion irradiation capabilities at CAMS constitute an important tool for experimental radiochemistry at LLNL. By bombarding targets with intense beams of high-energy light ions (e.g., ¹H, ²H, ⁴He, etc.), we induce nuclear reactions that yield radioactive species that can be used for a wide range of applications including tracers for development of radiochemical separation methods and diagnostic monitors for assessing the efficiency of shots at the LLNL National Ignition Facility (NIF). Co-location with CAMS is an important feature to the LLNL radiochemistry program because it provides access to short-lived activation products that are not widely-available from off-site sources.

Another important radiochemical application of CAMS is the production of realistic samples for nuclear forensic exercises. As part of this effort, we recently performed an improved measurement of the energy-dependent cross sections from proton-induced fission of ²³⁸U [Fig. 2]. This data will enable us to design future irradiations to produce mixtures of fission and activation products that will simulate the debris from a nuclear explosion. By carefully selecting the projectile/target combination we will be able to produce samples that are both realistic but also completely unfamiliar to radiochemists who participate in nuclear forensics exercises, thus creating a more challenging scenario for the exercise.

Figure 2. Cross sections of major fission products from proton-induced fission of 238U measured for several different incident energies. Irradiated targets were produced at CAMS followed by radiochemical analysis at the LLNL low-level counting facility.

Selected Reference

Hoffman RD, Sheets SA, Burke JT, Scielzo ND, Rauscher T, Norman EB, Tumey S, Brown TA, Grant PG, Hurst AM, Phair L, Stoyer MA, Wooddy T, Fisker JL, and Bleuel D. (2010) Reaction Rate Sensitivity of ⁴⁴Ti Production in Massive Stars and Implications of a Thick Target Yield Measurement of ⁴⁰Ca(α, γ)⁴⁴Ti. The Astrophysical Journal 715:1383–1399

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