Radiation Detection

1. -radiation from will directly hit the photocathode of the PMT without interacting with the scintillator crystal and produce a signal:
This statement is incorrect because -radiation from does not directly hit the photocathode. It first interacts with the scintillator crystal, which absorbs the energy and re-emits it as visible light (scintillation). The scintillator light is then detected by the photocathode of the PMT, which produces the signal.
Therefore, (A) is incorrect.

2. -radiation from source interacts with the scintillator crystal, producing -radiation, which will hit the photocathode of the PMT and produce a signal:
This statement is partially incorrect. While -radiation does interact with the scintillator, it does not typically produce -radiation. Instead, the scintillator converts the kinetic energy of the -particles into visible light. This visible light is then detected by the PMT.
Therefore, (B) is incorrect.

3. A mu-metal shield is put all around the PMT to nullify the effect of external electric fields:
This statement is incorrect. Mu-metal shields are specifically used to shield against external magnetic fields, not electric fields. Electric fields do not significantly affect PMT operation.
Therefore, (C) is incorrect.

4. A mu-metal shield is put all around the PMT to nullify the effect of external magnetic fields:
This statement is correct. Mu-metal is a material with high magnetic permeability, designed to shield sensitive equipment, such as PMTs, from external magnetic fields that can interfere with the electron multiplication process inside the PMT.
Therefore, (D) is correct.