Mass measurements demonstrate a strong N=28 shell gap in argon.

Area of Science:

• Nuclear Physics
• Atomic Physics

Background:

• The N=28 shell closure is a key feature in nuclear structure, influencing atomic masses and stability.
• Investigating this phenomenon in lighter even-Z elements provides crucial data for nuclear models.

Purpose of the Study:

• To perform precise mass measurements of argon isotopes, specifically ^{48}Ar and ^{49}Ar.
• To provide experimental evidence for the N=28 shell closure in argon.

Main Methods:

• Time-of-flight nuclear mass measurements were conducted at the National Superconducting Cyclotron Laboratory.
• Analysis of experimental data to determine atomic mass excesses.

Main Results:

• The first atomic mass excesses for ^{48}Ar and ^{49}Ar were determined to be -22.28(31) MeV and -17.8(1.1) MeV, respectively.
• These measurements strongly support the closed shell nature of neutron number N=28 in argon.
• Argon is identified as the lowest even-Z element exhibiting the N=28 closed shell.

Conclusions:

• The experimental masses of ^{48}Ar and ^{49}Ar confirm the N=28 shell closure in argon.
• The observed trend in binding-energy differences aligns with shell-model calculations (SDPF-U, SDPF-MU) in the sd-pf shell, validating theoretical predictions.