Scientists directly measure a key reaction in neutron star binaries by StuffsEarth

An X-ray burst (XRB) is a violent explosion that occurs on the surface of a neutron star as it absorbs material from a companion star. During this absorption, increasing temperatures and densities on the surface of the neutron star ignite a cascade of thermonuclear reactions.

These reactions create atoms of heavy chemical elements. A study, published in Physical Review Letters, presents an investigation of one of these reactions, ²²Mg(α,p)²⁵Al (magnesium-22 and helium-4, producing a proton and aluminum-25). The rate of this reaction plays a major role in informing models of XRBs and determining the reaction mechanisms that power these explosions. The researchers found that the reaction rate is four times higher than the previous direct measurement.

XRBs are driven by a sequence of reactions involving unstable nuclei that rapidly capture protons before the nuclei have a chance to decay. During this sequence, the rate of particular proton capture reactions decreases at multiple “waiting point” nuclei (such as magnesium-22), causing the nuclear flow to slow down.

Research has found that the capture of alpha particles (helium-4) by these nuclei instead of protons could bypass these waiting points and continue the synthesis of heavier elements. Precisely determining the rates of possible reactions at the waiting points—including the ²²Mg(α,p)²⁵Al reaction at the magnesium-22 waiting point—can help scientists improve their understanding of XRBs.

The ²²Mg(α,p)²⁵Al reaction involves unstable nuclei with lifetimes too short for the nuclei to be made into targets. To measure this reaction, scientists performed the measurement in inverse kinematics using the Argonne Tandem Linac Accelerator System (ATLAS), a Department of Energy user facility at Argonne National Laboratory.

The researchers developed an in-flight radioactive beam with the ATLAS in-flight system. The beam was delivered to the MUlti-Sampling Ionization Chamber (MUSIC) detector filled with pure helium gas, recreating conditions relevant for XRBs.

The experiment yielded a new direct measurement of the angle and energy-integrated cross-section of the ²²Mg(α,p)²⁵Al reaction. The cross-section is a measure of the probability that the reaction will occur.

The experiment found that this probability is four times higher than the previous direct measurement. This higher rate indicates a higher likelihood that the ²²Mg waiting point is bypassed by the ²²Mg(α,p)²⁵Al reaction. In addition, the scientists found that the reaction begins to occur at lower temperatures than previously thought.

The new result provides insight into the underlying physics of the nucleosynthesis reaction flow through the ²²Mg waiting point in XRBs.