I agree this should be a property but I can't foresee a case where it would ever be initialized (or set) to some value at least not with .spe files. I think just a basic property would be better a la:
```python

def __init__(self, ..., ...):
    """define gamma spectra specific variables """    
    ...
    self._dead_time = 0
    ...

@property
def dead_time(self):
    self._dead_time = self.real_time - self.live_time
    return self._dead_time
```
Of course if you're going to make dead time a property then most of the rest of the initialized values may be better as properties too to keep them from being overwritten on accident.