CONCEPT OF TIME,DISTANCE AND SHILDING

 

INTRODUCTION

        Any work involving ionising radiations may entail some potential hazard. Based on ICRP's system of Dose Limitation, it is essential that the doses are kept as low as reasonably achievable (ALARA). 

•       A knowledge of the nature and the degree of the potential hazard will help to take appropriate control measures.

•       In tele gamma unit In the ON position, the source is adequately shielded except for the primary beam.

•       In the case of X-ray machines, radiation is emitted only during the time when the unit is energized.

Radiation hazard may be broadly classified under

•       internal hazard

•       external hazard.

INTERNAL HAZARD

•       Internal hazard arises due to actual entry of radioactive material into the living organism.

The main routes of intake are:

1. Inhalation of contaminated air.
2. Ingestion of contaminated food and water.
3. Entry of radionuclide directly into the blood stream,                                                                            e.g., through open wound.

      

EXTERNAL HAZARD

•       External hazard is caused when the source of radiation is outside the body,

•        e.g. X-rays and sealed radioactive sources.

•        From external point of view, X-rays and gamma rays are most hazardous on account of their higher penetrating power, beta rays next and lastly the alpha particles.

•       Hazard evaluation is necessary in order to take control measures in radiation areas.

•        The evaluation of radiation hazard starts at the design stage of the radiation equipment.

•        Under the present regulations, it is the duty of the manufacturer to ensure that the equipment meets the stipulated radiation safety standards.

•       Similarly, the user of radiation sources is required to ensure safety for patients, personnel and public.

•       It is essential to evaluate the hazard likely to arise, under normal working conditions, as also under potential accident conditions (e.g., fire, explosion).

TIME, DISTANCE AND SHIELDING

•       Three basic fundamental factors by which external exposure to radiation can be controlled are time, distance and shielding.

A) TIME

•       For a given shielding and distance of source, the exposure from a source at a point will be directly proportional to the time during which the exposure is `ON’.

LESSER THE TIME OF OPERATION, LESSER WILL BE THE RADIATION DOSE

•       The exposure rate from a telecobalt source is 200 R per minute at 1 meter, what are the exposure in (a) 15 sec., (b) 3 min, and (c) 1 hour?

 For 15 Sec.

   Convert seconds to minutes  15 sec=60/15​=0.25 min.

          formula- Exposure=Exposure Rate×Time

                                     =200×0.25 =50 R(ans.

    b) for 3 min.

    formula- Exposure=Exposure Rate×Time

                                      =200×3=600 R(ans.)

 (c) Exposure for 1 hour

Convert hour to minutes- 1 hr=60 min

  formula- Exposure=Exposure Rate×Time

                                  =200×60=12000 R(ans.)

The exposure increases linearly with time.

   Double the time → double the dose.

2. The exposure rate from a source at a point where the engineers are doing servicing is 120 mR/hr. On a average for how much time can the engineer work at this point without exceeding the equivalent dose limits?

Exposure rate = 2 mR/min.

Exposure allowed/day  =  8 mR.

•        time/day = 8/2 = 4 min.

 B) DISTANCE

•       The exposure rate at any point from a source of X or gamma rays varies inversely as the square of the distance of the point from the source (inverse square law).

•       LARGER THE DISTANCE, LESSER WILL BE THE RADIATION DOSE

C) SHIELDING