Radioactive waste used to explore exploding stars

A map of the titanium-44 and iron-60 in the Cassiopeia-A supernova remnant. Image: NASA/NuSTAR satellite

An experiment led by Alex Murphy of the School’s Nuclear Physics Group has used a highly novel technique to explore a key nuclear reaction needed to interpret satellite-based gamma-ray observations of core collapse supernovae.

Core collapse supernovae, which are some of the most violent and extreme phenomena in the Universe, are thought to produce about half the heavy elements found in Nature. Refining this understanding is a major theme of nuclear astrophysics research, and both uses data from, and motivates, major space-based gamma-ray observatories such as ESA’s INTEGRAL and NASA’s NuSTAR satellites.

One of the most important observations these instruments make is that of gamma rays emitted by the isotope titanium-44, since the abundance of this isotope produced in a supernova is thought to be closely related to the details of the underlying explosion mechanism. Several observations of titanium-44 in supernovae have been reported, but intriguingly, they all report significantly more activity than present computer models of the explosions can explain.

Titanium-44 nuclear ion beam

Titanium-44 has a half-life of 60 years, so does not naturally occur on Earth. In this work, reported recently in Physics Letters B, the Edinburgh team worked with scientists from the Paul Scherrer Institute in Switzerland to obtain about a microgram of the isotope from highly irradiated parts of a synchrotron. This was then transported to the REX-ISOLDE accelerator facility at CERN and used to produce a titanium-44 nuclear ion beam, with kinetic energy similar to that which the ions would have in the heart of the exploding star.

In the supernova explosion, the final amount of titanium-44 produced is the net results of reactions that create and reactions that destroy the isotope. By studying the interactions of this beam with helium in a gas cell, the researchers were able to place an upper limit on the rate of the most important destruction reaction. Including this new information in the computer models brings the satellite observed amounts of titanium-44 into much better agreement with expectation.

Find out more

The work is reported in Physics Letters B 731 (2014) 358-361 and is highlighted in New Scientist April 5 2014 (page 12).