Nuclear radiation emitted from a Co60 radioactive source is detected by a
photomultiplier tube (PMT) coupled to a scintillator crystal. Which of the following
option(s) is/are correct?
Options:
(A) \gamma radiation from Co60 will directly hit the photocathode of the PMT without
interacting with the scintillator crystal and produce a signal
(B) \beta radiation from Co60 source interacts with the scintillator crystal, producing \gamma
radiation, which will hit the photocathode of the PMT and produce a signal
(C) A mu-metal shield is put all around the PMT to nullify the effect of external electric
fields
(D) A mu -metal shield is put all around the PMT to nullify the effect of external
magnetic fields
Official Answer Key:
B;D
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE PH 2025 Q.29)**
**1. Radiation Detection & Photomultiplier Tubes (PMTs)**:
- Detection of ionizing radiation from a $^{60}\text{Co}$ radioisotope source using a scintillation detector coupled to a PMT.
**2. Detailed Analysis of Statements (MSQ)**:
- **(A) False**: Photons from $^{60}\text{Co}$ interact predominantly via photoelectric effect / Compton scattering inside the dense scintillator crystal, producing visible scintillation light rather than direct photocathode detection.
- **(B) Beta radiation interaction producing optical photons [TRUE]**:
- $^{60}\text{Co}$ decays via $\beta^-$ emission ($E_{\max} = 0.318\text{ MeV}$) followed by cascade $\gamma$ emissions ($1.17\text{ MeV}$ and $1.33\text{ MeV}$).
- Charged $\beta$ particles deposit kinetic energy inside the scintillator crystal through electronic ionization and excitation, which de-excite by emitting scintillation photons that strike the PMT photocathode.
- **(C) False**: Mu-metal shields against magnetic fields, not electrostatic fields.
- **(D) Mu-metal shielding against magnetic fields [TRUE]**:
- PMTs operate by accelerating secondary electrons across a series of dynodes at high voltage.
- Stray external magnetic fields (even Earth's magnetic field $\sim 0.5\text{ G}$) bend low-energy electron trajectories away from dynode collection apertures, drastically degrading gain.
- A cylindrical **mu-metal shield** (high magnetic permeability $\mu_r \sim 10^5$) channels magnetic flux lines around the tube, shielding the dynode cascade.
**3. Official Verification**:
The verified official IIT answer is **Options (B) and (D)** (`['B', 'D']`).
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