2/20/2016Chapter N*33 Roentgen Pronounced rent’gen with a hard “g” Limitations – only applies to photons – only applies in air – only applies to energies less than 3 MeV l Named after Wilhelm C. Roentgen (thus the abbr... is capital “R”)
31
06/14/22 3 Chapter N* 1 Radiation Exposure, Radiation Exposure, Dose and Quantity Dose and Quantity Exposure is an index of the ability of a radiation field to ionize air. Dose is a measure of the energy imparted to matter, per unit mass, when an ionizing radiation field interacts with matter. OR: (The amount of energy deposited per unit mass) Quantity of radioactive material is expressed as “activity ”, the number of nuclear disintegrations that occur in a sample per second .
Transcript
05/05/23 Chapter N*3 1
Radiation Exposure, Dose and Radiation Exposure, Dose and QuantityQuantity
Exposure is an index of the ability of a radiation field to ionize air.
Dose is a measure of the energy imparted to matter, per unit mass, when an ionizing radiation field interacts with matter.
OR: (The amount of energy deposited per unit mass) Quantity of radioactive material is expressed as
“activity”, the number of nuclear disintegrations that occur in a sample per second.
05/05/23 Chapter N*3 2
Units of Exposure and Units of Exposure and QuantityQuantity
Roentgen (R) Curie (Ci) Becquerel (Bq)
2.58 x 10-4 coulombs / kg dry air
at STP
Disintegrations per second in 1 gm
radium (3.7 x 1010 dps)
Systeme Internationale unit: one dps
05/05/23 Chapter N*3 3
RoentgenRoentgen
Pronounced rent’gen with a hard “g”Limitations
– only applies to photons– only applies in air– only applies to energies less than 3 MeV
Named after Wilhelm C. Roentgen (thus the abbr... is capital “R”)
05/05/23 Chapter N*3 4
Curie (Ci)Curie (Ci)
Radiation hazard does not solely depend on the activity. It also depends on the type of decay (alpha, beta, photon, etc.)
Named in honor of Pierre Curie
05/05/23 Chapter N*3 5
Units of Absorbed DoseUnits of Absorbed Doserad Gray (Gy)
radiation absorbed dose
(100 erg/gm)
S.I. unit: 1.0 J/kg (100 rads)
05/05/23 Chapter N*3 6
Concept of Equivalent DoseConcept of Equivalent Dose
For the same absorbed dose (deposited energy), different forms of ionizing radiation can have different biological effects.
“Equivalent Dose” attempts to normalize these differences.
05/05/23 Chapter N*3 7
Equivalent DoseEquivalent Dose
Equivalent Dose is the product of the dose and a modifying factor called the quality factor (QF), which reflects the relative biological effectiveness of the radiation:
HT = D x QF
05/05/23 Chapter N*3 8
Quality Factors (QF)Quality Factors (QF)QF are indices of the “relative biological
effectiveness” (RBE) of a radiation. RBE is a complicated function of type of radiation, energy and the biological system under consideration.
QF are not measured. They are determined by a committee.
Effective Dose Equivalent (EDE) is intended to reflect the total biological effect of a given exposure on a human. It is a weighted average of the individual doses to a number of important tissues:
DistanceMaintain maximal practical distance from radiation source
ShieldingPlace radioactive sources in a lead container
To Limit Caregiver Dose to 5 rem
Distance Rate Stay time
1 ft 12.5 R/hr 24 min
2 ft 3.1 R/hr 1.6 hr
5 ft 0.5 R/hr 10 hr8 ft 0.2 R/hr 25 hr
05/05/23 Chapter N*3 25
05/05/23 Chapter N*3 26
05/05/23 Chapter N*3 27
05/05/23 Chapter N*3 28
05/05/23 Chapter N*3 29
The radiated energy can be considered to be spread over a sphere of area 4r2 at any distance r from the source. Since the radiation intensity is equal to the energy per unit area :
If the radiation intensity is I0 at a distance r0 from the source, the intensity I at any other distance r will be given by :
Since the radiation dose is proportional to the intensity, the dose D can be calculated by the same type of relationship :
24 rEnergy
2
20
0 rrII
2
20
0 rrDD
05/05/23 Chapter N*3 30
Example 1If a nurse received a radiation dose of 100 millirems(0.1rem) by spending one hour at a distance of 2ft from a radioactive implant in a patient, how much would she have received at a distance of 4ft? At a distance of 20ft? ( 1 ft=30.48cm)
Example 2If a patient has received 100 millicuries of cobalt-60 as a therapeutic measure, the dose received by an attendant at a distance of one foot from the patient might be on the order of 200 millirems per hour. How long could the attendant spent at this distance in a week’s time? How long could he spent at 2ft? At 10 ft?
