How are radioactive decay series distinguished ? Name the decay series which is not natural but artifical.

Explanation

### Step-by-Step Solution:

1. **Understanding Radioactive Decay Series**:
- Radioactive decay series are classified based on the mass number of the radioactive isotopes involved in the decay process. Each series is defined by the relationship of the mass number to the number 4.

2. **Classification of Decay Series**:
- There are four main types of radioactive decay series:
- **4n Series**: The mass number of the isotopes in this series is divisible by 4 (remainder 0).
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