GetQuanta at zero drift field nuclear recoil

[Alex-X-Li]

I am just getting start to use NEST, so this might be a dumb question to ask.

I was trying to calculate the quanta (photon and electrons) produce in nuclear recoil with zero drift electric field in your nestpy_tutorial.ipynb. With the code
`

N = 50000
E_mono = 2 # keVnr
E_nr = np.ones(N)*E_mono
field = 0.

photons, electrons = GetQuanta(nestpy.nr, E_nr, field)
print(photons, electrons)
kwargs = {'energy': E_mono, 'drift_field': field}
nest_ly = nestpy.PhotonYield(interaction=nestpy.nr, **kwargs)
nest_qy = nestpy.ElectronYield(interaction=nestpy.nr, **kwargs)

`
And the photon and electrons produce is always 0.

However, for the electron recoil in zero field given by your given example
`

N = 50000
E_mono = 2 # keVee
E_er = np.ones(N)*E_mono
field = 0.

photons, electrons = GetQuanta(nestpy.beta, E_er, field)
kwargs = {'energy': E_mono, 'drift_field': field}
nest_ly = nestpy.PhotonYield(interaction=nestpy.beta, **kwargs)

nest_qy = nestpy.ElectronYield(interaction=nestpy.beta, **kwargs)

`
Shows non-zero photons and electrons production.

Physically speaking the nuclear recoil should produce photons and electrons as well. What's the reason for getting this result?