Proton-emission branches of the <math altimg="si1.svg"><msup><mrow><mn>10</mn></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">+</mo></mrow></msup></math> isomer in the <math altimg="si2.svg"><msub><mrow><mi>T</mi></mrow><mrow><mi>z</mi></mrow></msub><mo linebreak="goodbreak" linebreakstyle="after">=</mo><mo linebreak="badbreak" linebreakstyle="after">−</mo><mn>1</mn></math> nucleus <sup loc="post">54</sup>Ni have been imaged with the active target and time projection chamber (ACTAR TPC) in an experiment conducted at the Grand Accélérateur National d'Ions Lourds (GANIL). The completed decay scheme allows derivation of the reduced transition strengths, <math altimg="si3.svg"><mi>B</mi><mo stretchy="false">(</mo><mi>E</mi><mn>2</mn><mo>;</mo><msup><mrow><mn>10</mn></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">+</mo></mrow></msup><mo stretchy="false">→</mo><mspace width="0.2em"/><msup><mrow><mn>8</mn></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">+</mo></mrow></msup><mo stretchy="false">)</mo></math> and <math altimg="si4.svg"><mi>B</mi><mo stretchy="false">(</mo><mi>E</mi><mn>4</mn><mo>;</mo><msup><mrow><mn>10</mn></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">+</mo></mrow></msup><mo stretchy="false">→</mo><mspace width="0.2em"/><msup><mrow><mn>6</mn></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">+</mo></mrow></msup><mo stretchy="false">)</mo></math>, for the two competing γ-ray transitions. By means of a comparison with their well-known ‘mirror transitions’ in <math altimg="si5.svg"><msub><mrow><mi>T</mi></mrow><mrow><mi>z</mi></mrow></msub><mo linebreak="goodbreak" linebreakstyle="after">=</mo><mo linebreak="badbreak" linebreakstyle="after">+</mo><mn>1</mn></math><sup loc="post">54</sup>Fe, and aided by a variety of shell-model calculations in the fp model space, effective charges for E4 transitions near <math altimg="si6.svg"><mi>N</mi><mo linebreak="goodbreak" linebreakstyle="after">=</mo><mi>Z</mi></math><sup loc="post">56</sup>Ni can be deduced: <math altimg="si7.svg"><msub><mrow><mi>ε</mi></mrow><mrow><mi>π</mi></mrow></msub><mo>≈</mo><mn>1.40</mn></math> and <math altimg="si8.svg"><msub><mrow><mi>ε</mi></mrow><mrow><mi>ν</mi></mrow></msub><mo>≈</mo><mn>0.30</mn></math>. Mirror-energy differences are explored with various shell-model interactions and isospin-symmetry breaking terms.Read less <