大多数人都知道电子可以改变能量水平，并且可以在兴奋状态下找到。当质子或中子（核子）被激发时，在原子核中发生类似的过程。激发的核子占据了更高能量的核轨道。大多数时候，激发的核子立即返回基态，但如果激发态的半衰期长于正常激发态的100到1000倍，则认为它是亚稳态。换句话说，激发态的半衰期通常为10-12秒，而亚稳态的半衰期为10-9秒或更长。一些来源将亚稳态定义为具有大于5×10-9秒的半衰期，以避免与伽马发射的半衰期混淆。虽然大多数亚稳态很快衰减，但有些持续数分钟，数小时，数年或更长时间。形成亚稳态的原因是因为需要更大的核自旋变化以使它们返回基态。高自旋变化使衰变“禁止过渡”并延迟它们。衰变半衰期也受到可用衰减能量的影响。大多数核异构体通过γ衰变返回基态。有时来自亚稳态的γ衰变被称为异构跃迁，但它与正常的短寿命伽马衰变基本相同。相反，大多数激发的原子态（电子）通过荧光返回基态。亚稳态异构体可以衰变的另一种方式是通过内部转化。在内部转换中，衰变释放的能量加速内部电子，使其以相当大的能量和速度离开原子。对于高度不稳定的核异构体存在其他衰变模式。
英国拉夫堡大学Assignment代写：核异构体定义和实例
Most people are aware electrons can change energy levels and be found in excited states. An analogous process occurs in the atomic nucleus when protons or neutrons (the nucleons) become excited. The excited nucleon occupies a higher energy nuclear orbital. Most of the time, the excited nucleons return immediately to the ground state, but if the excited state has a half-life longer than 100 to 1000 times that of normal excited states, it is considered a metastable state. In other words, the half-life of an excited state is usually on the order of 10-12 seconds, while a metastable state has a half-life of 10-9 seconds or longer. Some sources define a metastable state as having a half-life greater than 5 x 10-9 seconds to avoid confusion with the half-life of gamma emission. While most metastable states decay quickly, some last for minutes, hours, years, or much longer. The reason metastable states form is because a larger nuclear spin change is needed in order for them to return to the ground state. High spin change makes the decays “forbidden transitions” and delays them. Decay half-life is also affected by how much decay energy is available. Most nuclear isomers return to the ground state via gamma decay. Sometimes gamma decay from a metastable state is named isomeric transition, but it’s essentially the same as normal short-lived gamma decay. In contrast, most excited atomic states (electrons) return to the ground state via fluorescence. Another way metastable isomers can decay is by internal conversion. In internal conversion, the energy that is released by the decay accelerates an inner electron, causing it to exit the atom with considerable energy and speed. Other decay modes exist for highly unstable nuclear isomers.