cosmic radiation in australia carpenter
DESCRIPTION
TRANSCRIPT
Cosmic Radiation in Australia
Julia Carpenter, Brendan Tate, Rick Tinker, Stephen SolomonARPANSA
Contact: [email protected]
Background Radiation:
• Terrestrial Sources
• Man‐Made Sources
• Cosmogenic Sources
• Galactic (108
to 1020+
eV)
• Solar (100 MeV or less)
2
Background Radiation
Webb, Solomon and Thomson (1999) – Sources and Effects of Ionizing Radiation Background Radiation Levels and Medical Exposure Levels in Australia.
Aims:• To produce a detailed map of cosmic radiation variability in Australia• To find a population‐weighted annual dose from cosmic radiation in Australia
This is part of a larger project within ARPANSA to reassess background radiation pathways in Australia
• Altitude‐
As altitude
decreases, particle energy decreases
‐
Decay and interaction with
particles in the earth’s atmosphere.
3
Cosmic radiation
• Solar Cycle
• Latitude‐
Charged particles
interact with the earth’s magnetic field
• Use CARI‐6 to generate a data set of average annual cosmic radiation dose as a function of longitude, latitude
and altitude.• Digital elevation grid (Geoscience Australia)
4
Calculating Australian Exposure to Cosmic Radiation
• Repeat for each year from 1996 to 2008
• Use MapInfo to create thematic maps
5
Annual outdoor doses (uSv) from cosmic radiation
• US Federal Aviation Administration (FFA) Civil Aerospace Medical
Institute
• To calculate the dose an adult would receive from cosmic radiation on a designated flight path or at a user‐specified altitude and geographic
location.
• Uses a heliocentric potential model of solar modulation which has been shown to accurately calculate effective doses to aviation
workers in a range of solar conditions (O’Brien et al., 2005)
• Effects of geomagnetic field also taken into account, however radiation from solar particle events not included.
• Dose rates from LUIN99, a database that accounts
for fluence when calculating effective dose
coefficients.
• So how well does it work at sea level?
6
CARI‐6
• Grasty and LaMarre (2004) –
Canada and USA
• Shielding factor of 0.8• Indoor time fraction 0.8
• Results averaged of 4 solar cycles• Annual effective dose (population weighted) = 318µSv
• NCRP‐160 (2009) – USA
• Weighting factor of 0.83 applied to correct for shielding and time spent indoors
• Annual effective dose of 330µSv for adults in the 99 most populated US cities
7
CARI‐6
• Ground‐based measurements of cosmic radiation in Melbourne
• Yarra River near Fairfield (freshwater region)
• Depth greater than 3m
• At least 20m to banks in all directions
• Low banks
• Access to wooden rowboats
8
Verification
70 m
60 m
9
10
11
• Reuter‐Stokes High Pressure Ionisation Chamber (HPIC) (80mins)
• Sodium Iodide (NaI(Tl)) scintillation detector. (Background spectrum)
• Distinct peaks corresponding to K‐40 and Th‐228 from terrestrial background radiation were observed in the spectrum. These were
assessed by stripping off their contributions to the background spectrum and calculating the fraction of the air kerma contribution (approx
4nSv/h)
12
Results
The net cosmic ray contribution measured at this site was 45.2 +/‐
2.1 nGyh‐1
CARI‐6 calculated the cosmic ray dose to be 43.9 nSvh‐1
at the corresponding geographic
coordinates, based on the heliocentric potential model of
solar modulation data for that day.
13
Population‐weighted Dose (Outdoors)
Average annual cosmic radiation dose for 208 regions (ABS Census Data)
14
Population‐weighted dose –
Capital Cities (Outdoors)
• Population weighted outdoor dose rate for Australia is 360µSva‐1 averaged over the 11 year solar cycle
• Range of 262µSva‐1
to 565µSva‐1 (over 208 regions)
• Population‐weighted indoor dose rate for Australia is estimated to be 302µSva‐1
• Indoor time fraction = 0.8 (UNSCEAR, 2008)
• Shielding factor = 0.8 (UNSCEAR 2008)
15
Population‐weighted dose (Indoors)
• We now have a detailed map of cosmic radiation variability in Australia.
• The average population‐weighted dose for Australia is estimated to be 302µSva‐1.
• This dose is below the world‐average dose
• In Australia, the factors that have the largest influence on cosmic radiation dose are altitude and latitude.
16
Summary/Conclusion