Environmental Physics Chapter 15: Effects and Uses of Radiation Copyright © 2008 by DBS.

Environmental Physics

Chapter 15:Effects and Uses of Radiation

Copyright © 2008 by DBS

Introduction

• Radiation is present everywhere

• Can have both positive and negative effects on living things

• Earth has been subject to cosmic rays since it was formed

Question

Calculate the energy (in eV) of visible light and Gamma rays

E = hν = hc/λ= (6.626 x 10-34 Js x 3.00 x 108 m/s)/ 10-6 m = 2 x 10-19 J x (1 eV/1.6 x 10-19J) = 1 eV

E = hν = hc/λ= (6.626 x 10-34 Js x 3.00 x 108 m/s)/ 10-12 m = 2 x 10-13 J x (1 eV/1.6 x 10-19J) = 1 MeV

Introduction

• We will study radiation that produces harmful biological effects - ionizing radiation

• Radiowaves, microwaves, visible light etc are of too low energy to cause damage

• Energy around 12 eV is required to ionize water

Nuclear Physics

Type Description Penetration Shielding Charge

α He nucleus Low Skin +ve

β Electrons Medium 1-5 mm metal

-ve

γ-ray, x-ray EM radiation High 10 mm + lead, 1 m concrete

0

Heavy particles less penetrating

Radiation Dose

• Ionizing Radiation:

– Gamma rays

– X-Rays

– Beta particles

– Alpha particles

• Gamma and X-Rays are not charged, do not ionize through electrical interactionProduce e- indirectly via the Compton effect

Radiation Dose

• Activity (A) – measures the no. of decays per second (Ci or Bq)

• Absorbed dose (D) – energy absorbed per gram of tissue (rads or Grays, 1 Gy = 100 rads)

• Dose equivalent (H) – absorbed dose multiplied by a biological effectiveness factor (rems or Sieverts, 1 Sv = 100 rems

• Quality factor (QF) (see table)

H = D x QF

Radiation Dose

• Absorbed Dose, D. is the energy deposited in an organ or a mass of tissue per unit mass of irradiated tissue. A common unit for absorbed dose is the rad, which is 100 ergs (100 x 10 -7 J) per gram of material.

• To calculate D you usually calculate the total absorbed energy first

• You use the activity (Bq), the energy (eV) of the radiation and exposure time to calculate the total amount of energy that arrives at the surface of your body

• Tissue absorption coefficients are then used to calculate the total energy absorbed by the body

• The last step is to calculate the absorbed energy per unit mass, which requires a decision on what mass of tissue to use—the mass of the whole body or just the mass of the irradiated tissue

Radiation Dose

• Dose-equivalent, H. A key factor that the absorbed dose doesn’t take into account is the density of the ionization created by the radiation

e.g. α leaves an ion track that’s several hundred times more dense than that of β

• Generally, one rad of α ~20 x as effective at causing cellular damage—and thus cancer—than one rad of γ or β radiation.

• We account for these differences using a radiation-weighting factor, QF, that represents the effectiveness of each type of radiation to cause biological damage. The factors are determined by measuring the occurrence of various biological effects for equal absorbed doses of different radiations

H (rem, Sv) = QF x D (rad, Gy)

• Why does gamma radiation have the same weighting factor as beta particles?

– γ-radiation is more diffuse but does the same amount of damage

Radiation Dose

0.1 μg Pu-239 in your hand (5 cm diameter area), what dose do you receive from 5-MeV α-particles in one hour?

Assume 50% penetrates the body (50% attenuated by the air), energy of α-particle = 5 MeV, and 1 g Pu-239 emits 2.2 x 109 α –particles/s

Abs. Dose (rads) = (Activity) x (Ave. Energy per particle) x (Fraction Absorbed) x (Time)

Skin absorbs energy of 0.1 x 10-6 g x (2.2 x 109 α –particles/s / 1 g) x 0.5 = 110 α-particles/s

110 α-particles/s x 5 MeV/particle = 550 MeV/s x (1.6 x 10-19J/ 1 eV) x erg/10-7 J = 0.0009 erg/s

Assume α penetrates 30 μm, energy is deposited in a disc of tissue 0.06 cm3, using density of 1 g/cm3, we have 0.0009 erg/s / 0.06 g tissue = 0.015 erg/s per g

Abs. Dose (rads) = 0.015 erg/s per g x (1 rad / 100 ergs/g) = 1.5 x 10 -4 rads/s

Dose equivalent = 1.5 x 10-4 rads/s x 20 = 0.003 rem/s x 3600 s/hr = 11 rem

Radiation Dose

Biological Effects

• Non-ionizing radiation is thought to be essentially harmless below the levels that cause heating.

• Ionizing radiation is dangerous in direct exposure, although the degree of danger is a subject of debate

• Humans and animals can also be exposed to ionizing radiation internally: if radioactive isotopes are present in the environment, they may be taken into the body

Biological Effects

• Average adult has ~ 50 trillion cells

• Nucleus and chromosomes carries DNA with instructions for replication

– Ionizing radiation may cause direct DNA damage, inhibiting repair

• Cell are made of 70 % water

– may be ionized forming highly reactive free radicals

Figure 15.1: Diagram of a cell.

alpha alpha particleparticle

Highly energetic α, β particles rip through tissue causing cellular and genetic damage

Biological Effects

• Damage may be:

– Somatic – affecting the individual

– Genetic – affecting offspring

• Damaged cells may grow in an uncontrolled manner and invade and destroy surrounding host cells becoming malignant cancer

• This is an over simplification… cancer is probably caused by several factors

Biological Effects

• Effects may be acute or chronic

• For small doses cancers may not be seen for many years – hard to establish cause and effect

• For high doses under short time spans we have data from human and animal experiments

Figure 15.2: Dose-effect distribution curve for mice. The point at which 50% of the population dies is called the lethal dose-50, or LD-50. It is about 1000 rads in this example.

Biological Effects

• Biological effects depend on what part of the body is irradiated

– Hands and feet can take higher doses

– Internal organs are more susceptible

• Standards governing exposure are determined based on past exposures

• Health effects of high doses of radiation are well known, information on low dose effects is difficult to obtain

• Two very different sources of data…

Biological Effects

• Hiroshima and Nagasaki survivors received average exposure of 130 rem (this is still 100 x higher than occupational low-dose exposure)

• Created 120 additional cancers over 27 years

• Both somatic and genetic studies reported

• One example of a radiation effect curve is that for leukemia

• 2 cases per million people per year per rem

Figure 15.3: Leukemia mortality dose-response curves for Hiroshima and Nagasaki.

Biological Effects

• Uranium miners

– High incidence of lung cancer

• X-ray technicians

• Radium dial painters

• Studies generally assume a linear dose-response relationship

• Curve can be extrapolated back from high-dose data to the low dose region

• Assumes no threshold, any radiation exposure produces a harmful effect

• Considerable debate as to the validity of this argument

Figure 15.4: (a) No threshold dose-response curve—that is, that the data from high doses (solid curve) can be extrapolated back to low doses, (b) threshold curve

Dose (rem)

Ad

vers

e H

ea

lth E

ffect

s

Atomic Bomb Survivors

Medical Patients

Observed EffectsObserved EffectsObserved EffectsObserved Effects

? ?

?

Underground MinersUnderground Miners

Linear

No threshold

End

• Review

Background Radiation

• Natural sources

• Man-made sources

Eckhardt, R. (1995)

Radon and its decay products make up 55% of background

What is Radon?Natural Radioactivity

22286Rn

Invisible, odorless, colorless, tasteless

Only gas in 238U decay chain

Question

Radon is said to be a daughter of radium-226, polonium-218 is a daughter of radon. Why are these not called sons?

Radon decay products (RDP’s) continue to decay giving birth to new daughters (progeny).

Indeed these RDP are the real culprits in the radon story!

What is Radon?Radon Gas

50 minutes

26.8m

Health EffectsRadon Gas

Progenies (‘daughters’) build up in confined space –are breathed in, stick to surface of airways and emit α-particles

Source: Turco (2002)

Background Radiation

• Medical studies of uranium miners has shown that long-term exposure causes cancer

• Generally agreed that Rn is a major cause of cancer

• 10-15 % of all lung cancers

Figure 15.6: Comparable risks from exposure to radon gas.

?

Health Effects How Radon Compares To Other Causes Of Death

0

5,000

10,000

15,000

20,000

25,000

Dea

ths

per y

ear

Low

er e

stim

ate

Drunk Driving

Drownings Fires/Burns Air Transportation

Radon

Upp

er

estim

ate

Source: U.S. EPA’s Home Buyer’s and Seller’s Guide (Radon: National Academy of Sciences, Non-radon: National Safety Council)

Question

Convert pCi L-1 to Bq m-3

1 pCi L-1 x 1000 L m-3 = 1000 pCi m-3

1000 pCi m-3 x 3.7 x 1010 Bq / Ci

= 1 x 10-9 Ci m-3 x 3.7 x 1010 Bq / Ci

= 37 Bq m-3

Background Radiation

• How did we discover radon was a problem?

Background RadiationStanley Watras

• Limerick Nuclear Power Plant, Christmas 1984

• Set off radiation alarm bells

• Home basement Rn ~ 100 000 Bq m-3

• Risk equivalent smoking 135 packs of cigarettes per day

Designed to detect radiation on workers leaving…Watras was entering!

What is Radon? 1988 EPA orders every home tested

What is Radon?

Background Radiation

Pressure driven air flow

• Warm air rises and escapes home

• Lower pressure inside than out

• Replacement air drawn in from below

• Increases with wind speed

http://www.cornwallradon.co.uk/page21.html

Stack effect enhances Rn movement

Radon-222Zone 1: > 4 pCi/L (red)

Zone 2: 2-4 pCi/L (orange)

Zone 3: < 2 pCi/L (yellow)

Based on indoor measurements, geology, aerial radioactivity, soil parameters and foundation type

Basement level

Background Radiation

• At 4 pCi/L people receive a dose of ~ 7700 mrem to the lung or 1000 mrem whole body equivalent each year (based on 75 % of time spent inside)

• National Academy of Sciences BEIR VI Report (1998) stated an average effective dose equivalent for US from radon to be 200 mrem/y (2 mSv/y)

• Basis for working out our average background radiation exposure

Background Radiation

• Remediation

– Seal cracks

– Pump air to roof from underneath basement

Figure 15.7: Key to major radon entry routes.

Background Radiation

• Cosmic rays (30 mrem/y)

– High energy particles (p+) and gamma rays from outer space

– Dose depends on altitude and latitude

– Atmosphere attenuates radiation – lower doses at sea-levele.g. 30 mrem/y Al, CA, MA vs. 120 mrem/y CO

– Earth’s magnetic field deflects best at equator, least at the poles

– Increased exposure on airplane flights ~ 2 mrem for a transcontinental flight

• Terrestrial (30 mrem/y)

Background Radiation

• Internal Exposure (40 mrem/y)

– K-40 in the body

– Food and liquid ingested

– U-238 and K-40 from phosphate fertilizers

– Ra-226 from cereals and Brazil nuts

– Cosmogenically produced C-14

Background Radiation

Artificial sources

• Medical and Dental – diagnostic/therapeutic (53 mrem/y)

– Typical chest X-ray about 10 mrem

– Thyroid scan using radioactive Iodine 200-300 mrem

– Have to do a risk assessment analysis for yourself

e.g. for a thyroid scan I-131 is a beta emitter with a 8 day half-life vs. I-123 is a gamma emitter with a 13 hr half-life

e.g. breast cancer screening for women – when to start

Background Radiation

Varies – depends on the personAverage is 4 dental X-rays and 1 medical X-ray every 10 years

Background Radiation

Consumer Products (9 mrem/y)

• Luminous paint on dials

• Smoke detectors (Am-241)

• Fiestaware

• Porcelain teeth and crowns

• Rose tinted glasses

• Tobacco (leaves absorb Rn) – Po daughter suspected to be major cause of lung cancer

Background Radiation

End

• Review

Radiation Standards

• Biological Effects of Ionizing Radiation (BEIR) VI report of 1998

• Obtained risk estimates from animal studies

• Genetic doubling dose (dose to achieve number of mutations equal to natural occurrence) = 100 rem

• Somatic (cellular) effects determined from atom bomb survivors and miners

• 560 excess cancer deaths per million people per extra (above BG) rem of annual whole body dose (all cancers)

• Compare with 18,000 cancer deaths per year per million people (all causes)

• NRC has public limit of 100 mrem above background (1994)

• Radiation workers allowed up to 5 rem/y whole body

Radiation Standards

• f

Medical and Industrial Uses

• Treatment of cancer

– Cobalt-60 treatment

– Cancer cells divide rapidly, more sensitive to radiation than normal cells

– Patient receives treatment from all sides

– 10,000 Ci source, dose of ~100 rem

– Small doses given periodicaly

Figure 15.8: A cobalt-60 unit for radiation therapy. The machine allows the radioactive source to rotate around the stationary patient. The tumor being treated is placed exactly at the center of rotation, so that damage to good tissue is minimized.

Medical and Industrial Uses

Medical and Industrial Uses

• Technitium-99

Fig. 15-9, p. 518

Figure 15.9: Radioisotope use in industry. An increase or reduction in the intensity of gamma rays transmitted through the sheet metal indicates a thinner or thicker sheet, respectively. This is used as a thickness gauge.

p. 519

Fig. 15-10, p. 520

Figure 15.10: Computed Tomography (CT- or CAT-scan) equipment.

Fig. 15-11, p. 521

Figure 15.11: MRI image of a head. A tumor appears as a white spot.

Radiation Protection

• Best dose is zero!

• Distance – intensity of radiation is inversely proportional to square of the distance from source

• Shielding – concrete, lead etc. Intensity reduces exponentionally

• Exposure Time – dose is cumulative

Figure 15.12: Radiation detection devices. The film badge in the center consists of photographic film, which darkens as a function of the amount of ionizing radiation received. The pocket dosimeter on the left can be read at any time to see how much radiation has been accumulated. The Geiger tube on the right is used with a counter to measure radiation intensity, while the large device in the background is a detector to monitor neutron activity.

p. 521

Being checked for contamination by whole-body scan (using NaI detectors), upon exiting nuclear power plant.

Table 15-7a, p. 526

Radiation Standards

Eck

hard

t, R

. (1

995)

Radiation Detection

• Instruments for monitoring radiation exposure or determining type and energy of radiation

• Detect ions produced by radiation

– Gas-Filled Detectors

– Scintillators

– Solid-state detectors

Fig. 15-13, p. 528

Figure 15.13: A gas-filled radiation detector.

A common radiation detector is the Geiger-Müller counter, which has a long, narrow tube containing a gas that’s easily ionized. The tube has a wire down its center, and a voltage drop is created between the wire and the sides of the tube. Whenever radiation penetrates the tube and ionizes some of the gas, the voltage causes positive ions to be pulled toward the walls and negative electrons to accelerate toward the wire, creating an electrical discharge—a miniature lightning bolt. The resulting current pulse in the circuit is registered by the counter.

Sometimes radiation may not be counted by the detector because it’s blocked by the wall of the tube or it passes through the tube without ionizing any of the gas. So to understand your measurements fully, you need to know the type of radiation you’re trying to measure and the efficiency of the counter for detecting that radiation.

Still, a Geiger-Müller counter with a thin mica window on one end of the tube (to let some of the weakly penetrating radiations into the gas) is a good all-around tool for detecting most types of ionizing radiation. You can learn a lot about your environment and your own exposures taking measurements with a simple hand-held Geiger-Müller counter. There are several such counters available today in the $250 to $350 price range.

Radiation Detection

• Scintillation Counters– Sodium iodide crystal– Emits light when exposed to gamma radiation– Photomultiplier uses photoelectric effect to produce a current

Figure 15.14: Analysis of γ rays from a radioactive source, using a NaI scintillation detector. The NaI crystal is coupled to a photomultiplier tube, whose output is amplified and directed to an analyzer. The analyzer determines the number of γ rays as a function of their energy, and the results are displayed on the monitor.

Radiation Detection

• Semiconductor Detectors

– Similar in principle to a GM tube

– Gas is replaced by Si or Ge

– Current is proportional to the energy of the radiation

Figure 15.15: Gamma-ray spectrum of Eu-152, obtained with a germanium detector. The spectrum shows the number of counts (on the y-axis) as a function of γ-ray energy.

End

• Review

Summary

• As you can see, we live in a sea of ionizing radiation, most of which has been here from the birth of the planet. Man’s ability to manipulate radioactive materials and to create new sources of radiation is adding to the amount of ionizing radiation we receive each year.