Radioactivity is very dangerous to human health. What are these rays and how we can take measures to prevent this radiation?

these rays are X rays and gamma rays.

Actually we can't prevent exposure to these rays but we can protect ourselves using "Alara (As Low As Reasonably Achievable) principle" to minimize radiation exposure.

this safety principle consists of thee keys:

1. TIME—Minimize the time of exposure.

2. DISTANCE—Double the distance between your body and the radiation source; this reduces the radiation exposure by a factor of 4.

3. SHIELDING—Use absorber materials such as Plexiglas® for beta particles and lead for X-rays and gamma rays.