1 Curie is equal to 

(1) $3\times {10}^{10}$ disintegrations/sec

(2) $3.7\times {10}^7$ disintegrations/sec

(3) $5\times {10}^7$ disintegrations/sec

(4) $3.7\times {10}^{10}$ disintegrations/sec

The electron emitted in beta radiation originates from

(1) Inner orbits of atoms

(2) Free electrons existing in nuclei

(3) Decay of a neutron in a nucleus

(4) Photon escaping from the nucleus

When a radioactive substance emits an $\mathrm{\alpha }$-particle, its position in the periodic table is lowered by

(1) One place             

(2) Two places

(3) Three places           

(4) Four places

A radioactive nucleus emits a beta particle. The parent and daughter nuclei are 

(1) Isotopes             

(2) Isotones

(3) Isomers               

(4) Isobars

If a radioactive substance reduces to $\frac{1}{16}$ of its original mass in 40 days, what is its half-life 

(1) 10 days           

(2) 20 days

(3) 40 days           

(4) None of these

A radioactive nucleus with Z protons and N neutrons emits an $\mathrm{\alpha }$-particle, 2$\mathrm{\beta }$-particles and 2 gamma rays. The number of protons and neutrons in the nucleus left after the decay respectively, are 

(1) Z – 3, N – 1                 

(2) Z – 2, N – 2

(3) Z – 1, N – 3                 

(4) Z, N – 4

99% of a radioactive element will decay between

(1) 6 and 7 half lives             

(2) 7 and 8 half lives

(3) 8 and 9 half lives             

(4) 9 half lives

The half-life of a radioactive substance against $\mathrm{\alpha }$-decay is $1.2\times {10}^7$ s. What is the decay rate for $4\times {10}^{15}$ atoms of the substance

(1) $4.6\times {10}^{12} \mathrm{atoms}/\mathrm{s}$         

(2) $2.3\times {10}^{11} \mathrm{atoms}/\mathrm{s}$

(3) $4.6\times {10}^{10} \mathrm{atoms}/\mathrm{s}$          

(4) $2.3\times {10}^8 \mathrm{atoms}/\mathrm{s}$

In a sample of radioactive material, what percentage of the initial number of active nuclei will decay during one mean life 

(1) 69.3%           

(2) 63%

(3) 50%               

(4) 37%

The S.I. unit of radioactivity is 

(1) Roentgen         

(2) Rutherford

(3) Curie               

(4) Becqueral