Exploring the onset of collectivity approaching <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>N</mml:mi><mml:mo>=</mml:mo><mml:mn>40</mml:mn></mml:mrow></mml:math> with masses of manganese isotopes

Isotopes in the region of the nuclear chart below ⁶⁸Ni have been the subject of intense experimental and theoretical effort due to the potential onset of a new "island of inversion" when crossing the harmonic oscillator subshell closure at <em>N</em> = 40. We have measured the masses of ⁶⁴⁻⁶⁸Mn using TITAN's multiple-reflection time-of-flight mass spectrometer, resulting in the first precision mass measurements of ⁶⁷Mn and ⁶⁸Mn. These results are compared to <em>ab initio</em> calculations and modern shell model calculations and show an increase in collectivity approaching <em>N</em> = 40.