It seems like `dead_time` should be a property

```python
def __init__(self, ..., dead_time=None, ...):
    """define gamma spectra specific variables """    
    ...
    self._dead_time = dead_time
    ...

@property
def dead_time(self):
    if self._dead_time is None:
        self._dead_time = self.real_time - self.live_time
    return self._dead_time

@dead_time.setter
def dead_time(self, val):
    self._dead_time = val
```