How to apply CSS class only in selected item or clean CSS class applied in ion-radio

I'm new to angular / ionic development and I'm developing an application that needs to select an item from a radio and apply a CSS class only to the selected item.

In the way it is implemented I am able to apply the classes in the items but it does not automatically clean the item that is no longer selected.

<form id="gerarSimulado-form25" class="list">
      <div class="spacer" style="width: 300px; height: 18px;"></div>

      <ion-item class="simulado_enunciado_item">
        <h3 class ="simulado_enunciado_font">{{simulado.questao}}</h3>
      </ion-item>

      <ion-radio  ng-class="{'item_selecionado':selecionado}" ng-repeat="alternativa in simulado.alternativas" id="gerarSimulado-radio5"
                 name="simulado_num_questoes" ng-click="setSiuladoResposta(simulado._id,alternativa);selecionado = !selecionado;">

        {{alternativa}}
      </ion-radio>
</form>

Any ideas how to solve it?