If $N_0$ is the original mass of the substance of half life period $T_{1/2}$= 5 years, then the amount of substance left after 15 years is

1. $N_0$/8 

2. $N_0$/16

3. $N_0$/2

4. $N_0$/4

The nuclei of which one of the following pairs of nuclei are isotones? 

1. ${}_{34}S{e}^{74},<mspace\; width="1em"></mspace>{}_{31}G{a}^{71}$ 

2. ${}_{42}M{o}^{92},<mspace\; width="1em"></mspace>{}_{40}Z{r}^{92}$

3. ${}_{38}S{r}^{84},<mspace\; width="1em"></mspace>{}_{38}S{r}^{86}$ 

4. ${}_{20}C{a}^{40},<mspace\; width="1em"></mspace>{}_{16}S_{32}$

A radioactive substance disintegrates 1/64 of initial value in 60 s. The half-life of this substance is

1. 5 s 

2. 10 s

3. 30 s 

4. 20 s

Half-life of a radioactive substance is 12.5 h and its mass is 256 g. After what time, the amount of remaining substance is 1 g? 

1. 75 h

2. 100 h 

3. 125 h

4. 150 h

If in a nuclear fusion  process, the masses of the fusion nuclei be $m_1<mspace\; width="1em"></mspace>and<mspace\; width="1em"></mspace>m_2$ and the mass of the resultant nucleus be $m_3$, then 

1. $m_3=m_1+m_2$

2. $m_3=\left|m_1-m_2\right|$

3. $m_3<\left(m_1+m_2\right)$ 

4. $m_3>\left(m_1+m_2\right)$

The constituents of atomic nuclei are believed to be 

1. neutrons and protons

2. protons only

3. electrons and protons

4. electrons, protons and neutrons

The half-life of radium is about 1600 yr. Of 100 g of radium existing now, 25 g will remain unchanged after 

1. 4800 yr

2. 6400 yr 

3. 2400 yr

4. 3200 yr

The nucleus ${}_6{}^{12}C$ absorbs an energetic neutron and emits a beta particle $\left({\beta }^{-}\right)$. The resulting nucleus is 

1. ${}_7{}^{14}C$

2. ${}_7{}^{13}N$ 

3. ${}_5{}^{13}B$ 

4. ${}_6{}^{13}C$

The counting rate observed from a radioactive source at t = 0 second was 1600 counts per second and at t = 8 seconds it was 100 counts per second. The counting rate observed, as counts per second, at t = 6 seconds will be:

1. 400

2. 300

3. 200

4. 150