30 Nov 2018

Preparation for first experiments: Photodisintegration of Li-7

By Catalin Matei

One of the long-standing questions in nuclear astrophysics is the so-called “cosmological Li problem”. The light elements 4He, 2H, 3He and Li isotopes are produced during the first 15 minutes after the Big Bang, the initial event that created the Universe. Big-Bang Nucleosynthesis (BBN) is a theory that predicts the abundances of these light elements based on the cross sections of nuclear reactions. There is very good agreement between calculated and observed abundances for these light elements except for 7Li. The discrepancy is a factor of 3-4, i.e. the calculated abundance of 7Li is 3-4 times larger than what we observe in older stars.

Scientists from both sides, nuclear physicists and astrophysicists, spent the last 30 years trying to solve the mystery of Li in the early Universe. One of the nuclear reactions that creates 7Li after the Big Bang is 3H(α,γ)7Li, but experimental measurements of the cross section are more than 30 years old and in conflict with recent theoretical calculations. Stricter regulations on using 3H in nuclear physics experiments mean that the 3H(α,γ)7Li reaction cross section can’t be re-measured directly. Measuring the inverse reaction, the photodisintegration of 7Li through the 7Li(γ,3H)4He reaction, would bring new light into the Li production mechanism in the early Universe.

The Charged Particle Detection (CPD) group at ELI-NP proposed the cross section measurement of the photodisintegration of 7Li through the 7Li(γ,3H)4He reaction as one of the first experiments to be performed at ELI-NP using the gamma-ray beam and the ELISSA detector array. However, before the ELI-NP facilities and detectors are implemented, the CPD group initiated in 2016 a proof-of-principle proposal for 7Li(γ,3H)4He at the High Intensity Gamma-ray Source (HIGS) in the USA. 

Silicon detectors from Oak ridge National Laboratory in USA, arranged in lampshade configuration for detecting the 4He and 3H particles from the photodisintegration of 7Li

The team assembled at HIGS in March 2017 and carried out measurements of the cross section at gamma-ray beam energies above 4.5 MeV during 5 days of beam time. The proposal and the experimental campaign at HIGS were the effort of more than 20 scientists from across Europe, the USA and Asia: ELI-NP from Romania, Oak Ridge National Laboratory, Rutgers University, University of Michigan, Ohio University, University of North Carolina from USA, University of York from the UK, Aarhus University from Denmark, INFN - Laboratori Nazionali del Sud from Italy, and Sungkyunkwan University from Korea.

Waiting for the first data. Team members from the USA, Italy, and Romania in the User Room at HIGS in March 2017.

The analysis of the data and preliminary results show that the cross section of 7Li(γ,3H)4He reaction can be measured with good accuracy given a well characterized detector array and gamma-ray beam parameters. A manuscript on a new method to measure the properties of photon beams has been submitted to an instrumentation journal while the physics results are in the final stages of manuscript drafting.