Identify the isotopes of hydrogen

Hydrogen has three common isotopes:

• Protium: ${}^1_1\mathrm{H}$
• Deuterium: ${}^2_1\mathrm{H}$
• Tritium: ${}^3_1\mathrm{H}$

Check which isotope is radioactive and emits $\beta^-$ particles

• Protium is stable.
• Deuterium is also stable.
• Tritium is radioactive and undergoes $\beta^-$ decay: $${}^3_1\mathrm{H} \rightarrow {}^3_2\mathrm{He} + \beta^- + \bar{\nu}$$

Check the half-life condition

Tritium has a half-life of about:

This satisfies the condition $t_{1/2} > 12$ years.

Check energy of emitted $\beta^-$ particles

Tritium emits low-energy $\beta^-$ particles, which is a well-known property of tritium decay.

Evaluate the options

• A: Protium $\to$ incorrect, stable and does not emit $\beta^-$.
• B: Tritium $\to$ correct.
• C: Deuterium $\to$ incorrect, stable.
• D: Deuterium and Tritium $\to$ incorrect, because deuterium does not emit $\beta^-$.

Final answer

The isotope of hydrogen that emits low-energy $\beta^-$ particles and has $t_{1/2} > 12$ years is:

Hence, the correct option is B.