Radiation: Effects and Dose

Radiation effects shows two types of Poisoning according to the exposure obtained:

Acute Radiation Poisoning

Chronic Radiation Poisoning

*Acute radiation syndrome (ARS) also known as radiation poisoning,radiation sickness or radiation toxicity, is a constellation of health effectswhich occur within several months of exposure to high amounts of ionizingradiation.

Relatively smaller doses results: *gastrointestinal effects:

-nausea and vomiting *symptoms related to falling blood count:

-infection and bleeding.

Relatively larger doses can result: *neurological effects *rapid death.

Treatment :*blood transfusions *antibiotics

Chronic radiation syndrome is a constellation of health effects that occurafter months or years of chronic exposure to high amounts of ionizingradiation.

It may manifest with lowblood cell counts andneurological problems.

Radiation exposure can alsoincrease the probability ofdeveloping some other diseases,mainly different types of cancers.

Radiation Doses SI unit Traditional unit Equivalency

Absorbed doseis the concentration of energy deposited in tissue as a result of an exposure to ionizing radiation. It tells us the energy deposit in a small volume of tissue.

gray rad 1 gray = 100 rad

Equivalent doseis an amount that takes the damaging properties of different types of radiation into account. It addresses the impact that the type of radiation has on that tissue. (Different types of radiation have different damaging properties.)

sievert rem 1 Sievert = 100 rem

Effective doseis the sum of the weighted equivalent doses in all the tissues and organs of the body.

sievert rem 1 Sievert = 100 rem

“RADIATION PROTECTION”

*EXTERNAL RADIATION PROTECTION

A.TimeB. DistanceC. Shielding

The amount of radiation an individualaccumulates will depend on how long theindividual stays in the radiation field.

Dose (mrem) = Dose Rate (mrem/hr) x Time (hr)

stay time- how long a person can stay in an areawithout exceeding a prescribed limit.

Stay Time = DoseRate (mrem/hr) Limit (mrem)

Example:How long can a radiation worker stay in a 1.5 rem/hr radiation field if we wish to limit his dose to 100 mrem?

Stay Time = 1500 mrem/hr 100 mrem= 0.067 hr= 4 minutes

The amount of radiation an individual receiveswill also depend on how close the person is tothe source.

1. The Inverse Square Law- Point sources of x- and gamma radiation

follow the inverse square law, which states thatthe intensity of the radiation (I) decreases inproportion to the inverse of the distance from thesource (d) squared:

21

2

2

2

1

d

d

I

I

Example: Radiographer A receives 5 mR per second of scatterradiation standing 3 feet from the patient. What rate of radiationdoes radiographer B standing 7 feet away?Given

Reqd:

Answer: 0.92 mR/sec

ftd

ftd

mRI

7

3

sec5

2

1

1

?2 I

Example: The exposure rate one foot from a source is 500 mR/hr. What would be the exposure rate three feet from the source?Given:

Reqd:

Answer: 55.56 mR/hr

ftd

ftd

hr

mRI

3

1

500

2

1

1

?2 I

2. Gamma Exposure Rate Formula

The exposure rate from a gamma point source can be approximated from the following

expression:

Where:

C is the activity of the gamma emitter, in Curies

E is the gamma ray energy in MeV

f is the fraction of disintegrations yielding the gamma of energy E

d is the distance from the source in feet or meter

When d is in feet When d is in meter

I= 6CEf /d2 I= 0.5CEf /d2

An individual walks into a room containing a 500-Curie 60Co beam type irradiator. All

indications are that the beam exit port is closed. The individual stands in the path of

the beam and performs work on a piece of medical equipment located approximately

one meter from the source. The individual works for about 5 minutes and then exits

the room. Upon exiting, the individual discovers that the beam port was open the

entire time. You are the resident expert. You are called at home and asked to estimate

the dose received. What is the individual’s estimated dose?

Given:

C= 500 Curie

d= 1 m

t = 5mins

E= 1.33 MeV (see table for Co 60)

f = 0.9998 (see table for Co 60)

Required:

Dose=?Answer: 28 R

When reducing the time or increasing the distance may not be possible,one can choose shielding material to reduce the external radiation hazard.The proper material to use depends on the type of radiation and itsenergy.

*Alpha and Beta Radiation

*X and Gamma Radiation

*Half Value Layer

*Alpha and Beta Radiation

Maximum Range of Beta Particles vs. Energy.

*X and Gamma Radiation

Monoenergetic x- or gamma rays collimated into a narrow beam are

attenuated exponentially through a shield according to the following

equation:

I = Ioe-μx

Where:

I is the intensity outside of a shield of thickness x

Io is the unshielded intensity

μ is the linear attenuation coefficient of the shielding material

x is the thickness of shielding material.

*X and Gamma Radiation

The linear attenuation coefficient, μ, is the sum of the probabilities of

interaction per unit path length by each of the three scattering and

absorption processes - photoelectric effect, Compton effect, and pairproduction. Note that μ has dimensions of inverse length (1/cm).

Mass attenuation coefficient : μm = μ𝜌

Where μm = cm2/g

𝜌 = density (g/cm3)

Example: What is the dose rate after shielding a source that emits only 1 MeV photons if the unshielded dose rate is 100 mrem/h and the source is shielded by 1/2 inch lead?

Given:Io = 100 mrem/hrX= 0.5 in Pb

From the table @ 1 MeV:𝜌 = 11.35 g/cm3

μm=0.068 cm2/g Required:

I = ?Answer: 37mrem/hr

*Half Value LayerThe half value layer (HVL) is the thickness of a shielding material requiredto reduce the intensity of radiation at a point to one half of its originalintensity. It can be calculated by setting I=0.5Io and solving theattenuation equation for x:

From the equation:

I = Ioe-μx

Let I=0.5Io Thus,

0.5Io= Ioe-μx

0.5=e-μx

HVLX

5.0ln5.0

Determine the lead HVL for Cs-137 photons (0.662 MeV) using the shielding

equation.

Given:

E=0.662 MeV

From the Table:μm=0.107 cm2/g 𝜌 = 11.35 g/cm3

Required:HVL=?

Answer: 0.57 cm Pb