Isospin symmetry in the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>s</mml:mi><mml:mi>d</mml:mi></mml:mrow></mml:math>shell: Transition strengths in the neutron-deficient<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>s</mml:mi><mml:mi>d</mml:mi></mml:mrow></mml:math>shell nucleus<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi mathvariant="normal">Ar</mml:mi><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>33</mml:mn></mml:mrow></mml:mmultiscripts></mml:math>

Reduced transition strengths of the deexciting transitions from the first two excited states in $^{33}\mathrm{Ar}$ were measured in a relativistic Coulomb excitation experiment at the GSI Helmholtz center. The radioactive ion beam was produced by fragmentation of a primary $^{36}\mathrm{Ar}$ beam on a $^{9}\mathrm{Be}$ target followed by the selection of the reaction product of interest via the GSI Fragment Separator. The $^{33}\mathrm{Ar}$ beam hit a secondary $^{197}\mathrm{Au}$ target with an energy of approximately 145 MeV/nucleon. An array of high-purity germanium cluster detectors and large-volume ${\mathrm{BaF}}_{2}$ scintillator detectors were employed for $\ensuremath{\gamma}$-ray spectroscopy at the secondary target position. The Lund-York-Cologne Calorimeter was used to track the outgoing ions and to identify the nuclear reaction channels. For the two lowest energy excited states of $^{33}\mathrm{Ar}$ the reduced transition strengths have been determined. With these first results the ${T}_{z}=\ensuremath{-}3/2$ nucleus $^{33}\mathrm{Ar}$ is now, together with $^{21}\mathrm{Na} ({T}_{z}=\ensuremath{-}1/2)$, the only neutron-deficient odd-$A sd$ shell nucleus in which experimental transition strengths are available. The experimental values are compared to results of shell-model calculations which describe simultaneously mirror-energy differences and transition-strength values of mirror pairs in the $sd$ shell in a consistent way.

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