The A = 32, T = 2 quintet is the most precisely measured quintet so far. We remeasured the mass of the T_z = 0 member of this quintet (³²S) with unprecedented accuracy and precision using the ³¹P(p,γ) reaction. This is a stringent test of the isobaric multiplet mass equation.

The lowest T = 2 state in ³²S was produced using the ³¹P(p,γ) reaction. A proton beam at ≈ 3285 keV was bombarded on an implanted ³¹P target. The target was seated on a target ladder that allowed for direct water cooling on the Tantalum backing. The corresponding de-excitation γ rays registered using HPGe detectors.  The energies of these γ rays give us the excitation energy of the T = 2 state and thus the mass of the state. The figure on the left shows the gamma decays of the lowest T = 2 state of  ³²S.

We performed two independent measurements at different times. The first set of data were obtained using two HPGe detectors at +90° and -90^° to the beam. These set of gamma-rays had minimal Doppler shift. The energy calibration was done using a ⁵⁶Co source, and capture radiation from ²⁷Al(p,γ) and ³⁵Cl(n,γ) reactions. Neutrons were produced using the ⁷Li(p,n) reaction. A proton beam at E_p≈1912 keV impinged a thick Lithium Oxide target (≈ 500 μg/cm²) to produce neutrons that were moderated by a 4 cm thick paraffin slab before capturing on a block of NaCl.

The second set of data were obtained using a HPGe detector at 0° to the beam. The gamma-rays were detected with maximum Doppler shift, but had least sensitivity to target ladder and detector misalignments.  The energy calibration was done using the ⁵⁶Co source and ²⁷Al(p,γ) lines. The experimental setups are shown below (top view).

        
Results and Conclusions

Systematic effects such as gain-shifts, Doppler effects and ADC non-linearities were taken into consideration and corrected for to obtain peak centroids. The Doppler effects on γ energies were determined using precise Monte Carlo simulations. Our results agree well with previous determinations of excitation energies of the three T = 1 levels fed by the T = 2 state, but disagree with the previously cited value of the excitation energy of the T = 2 state itself by ≈ 7σ. The figure on the right shows the γ-ray spectrum from the 0° detector.
On using the best available masses for the other members of the T = 2, A = 32 quintet we obtain significant disagreement with the IMME prediction with Q(χ²,ν) = 0.0001. Our result provides the best demonstration of the validity of the approximations inherent in the IMME and its utility for predicting masses away from the valley of stability. The figure below shows the residuals between the IMME fit and measured mass excesses for the quintet.