Where is Bob?
Thursday 9th May 2013 – Today is our third rest day in a row at Everest Base Camp in Tibet. We heard today that preparations are ongoing higher up the mountain for our summit push. Our Sherpa support were at 8,300m earlier today dropping off some equipment for us (oxygen, tents, etc). The ropes are not yet fixed to the summit on the North side so we will continue resting until the ropes are in position and a suitable weather window comes along for our summit bid.
Today I thought that I would tell you a little more about a topic that is rarely, if ever, discussed when people talk about Everest ascents and that topic is cosmic radiation doses.
When you consider all of the risks involved in an ascent of a high altitude mountain such as Everest then cosmic radiation is significantly less immediately life-threatening or dangerous than the lack of oxygen, potential altitude sickness, risk of rockfall, risk of serac fall, risk of avalanche, etc. However, exposure to this type of radiation could be detrimental many years down the line (e.g. increased risk of cancers, increased risk of eye cataracts, etc).
We are all exposed to cosmic radiation all of the time. There are two principal sources of it – our own sun and from elsewhere in the universe. The relative contribution of the two sources varies with our sun’s 11 year solar cycles – when there is a lot of sun spot activity our sun is spewing more cosmic radiation in our direction and the solar wind increases which has the effect of deflecting cosmic radiation from other origins. Occassionally our sun will eject a solar flare and this sends a large amount of charged particles etc. towards the Earth and this cosmic radiation can affect our satellites, installations on the ground, give us a higher radiation dose and produce beautiful aurora in the northern and southern polar regions.
At sea level the amount of radiation dose received from cosmic radiation is low due to the amount of shielding provided by the Earth’s atmosphere.
As you go higher above sea level then there is less and less atmosphere to shield you from cosmic radiation therefore the amount of radiation dose that you receive increases with increasing altitude.
This is a well-known effect and significant amounts of work have been done to measure the radiation doses received by astronauts and aircraft crew. With these occupations, their total time spent at altitude is generally quite short.
The cruising height of a plane flying from say Aberdeen to Wick in Scotland may vary somewhere between 17,000ft and 23,000ft and the total flight time is 25 minutes of which maybe only 10 minutes is spent at the maximum altitude. This will result in a small dose to the flight crew and the passengers. This pilot and crew might make 8 or so similar flights in a day giving say 90 minutes of elevated dose rate.
Everest Base Camp on the North side of Everest is at about 5,200m altitude or 17,050ft. We are either relaxing at, and living at, the cruising height of small aircraft or climbing above that and we are being exposed to these higher levels of cosmic radiation 24 hours per day.
The occupational radiation doses received by aircrew are assessed and monitored routinely.
With an ascent of Everest we are going to be going up to just over 29,000ft and hence receiving even higher radiation dose rates than experienced by some aircrews.
There is limited data available on the likely doses being received by UK-based high altitude mountain guides or their clients (whom are members of the public) so I performed a hypothetical assessment in a paper that I produced for the IRPA conference held in Glasgow last year. This paper estimated that during a typical Everest expedition just over 1 milliSievert of dose would be received (this is a ‘significant dose’ under the regulations). It is one thing doing a hypothetical assessment – but it is always better to gather some real data to support such assessments so that appropriate recommendations can be made if any are appropriate.
Throughout my expedition to Everest I have been making a series of measurements using a range of units to gather information to support a future scientific paper – this is my “Everest for Science” part of the expedition. Some of the units I can download and interrogate here, but other units are retrospective and I have to send them off for assessment at the end of the expedition so it will be a few months after the expedition before I can fully pull together all if the data and report the findings.
One of the reasons for using various different units is that by taking the units up Everest I am taking the units outside of their standard design altitudes – most electronics are only designed to go up to about 4000m unpressurised so I am pushing the limits of available technologies.
There are three different unit types that I am using to measure radiation doses received whilst on Everest. These are the Mirion Technologies Instadose v2.1, the Mirion Technologies Genesis Ultra Thermoluminescent Dosemeter (TLD) and the Tracerco T404 Personal Electronic Dosemeter (PED).
Each unit has slightly different radiation detection characteristics and with this range of equipment not all types of radiations that we are being exposed to will be detected. However, due to the studies perforned on airline pilots, with some careful interreptation/assessment, it will be possible to produce a defensible assessment of the typical dose received during an Everest expedition.
The Instadose v2.1 dosemeters are small, lightweight devices that measure the photon radiation received. These units have been pre-programmed for me such that every two hours they will record the total dose that has been accrued. When I get the results back I will be able to calculate the average dose rate for each 2 hour period and tie this in with the associated altitudes. One of these units is at home in the UK to allow for comparisons against sea level. These units will provide a good overall picture of radiation doses across the entire expedition as I have been carrying two of them with me at all times.
The Instadose v2.1 is electronic and in case the extreme altitude is too much for this device, which I hope it isn’t, I have got a Genesis Ultra TLD with me. This unit is completely passive with no electronics. The lithium fluoride chips inside the badge will allow an assessment of the photon and lower energy neutron total dose received to be made when the badge is sent off to California for processing (along with it’s associated background control badge which is in the UK at sea level).
The third unit being used for radiation dose and dose rate measurements is the Tracerco T404 PED. This electronic unit displays real time information on it’s LCD screen and can also be downloaded onto a netbook as the expedition progresses. The unit contains a miniature geiger-muller tube which is used to detect the radiation. I have already had this unit up at 7,200m altitude and it has been operating well.
The preliminary data downloaded from the Tracerco T404 PED ties in neatly with the variations in altitude that have occurred during this expedition. Specifically, the data clearly shows the flights from the UK, the initial acclimatisation in the Khumbu valley, cycle 1, rest at Base Camp and the second cycle. Doses are creeping up on this expedition and there has been in excess of a third of a milliSievert of photon dose recorded so far. Full analysis and assessment of doses will occur after the expedition but the initial hypothetical predictions look like they will be of the right order of magnitude – it will be interesting to see the end results.
A further tool that has been taken out to Everest is Kromek’s miniature gamma spectrometry instrument. This 60g detector plugs into the USB port of my netbook. The main limitation with use of this detector on Everest is the operational altitude of my netbook – at Base Camp with it’s 5,200m altitude I am already using it outside its design capacity and I don’t want to wreck it as it is acting as my photo backup device too. However, the Kromek detector has been successfully used at sea level, at various altitudes up to 5,200m and inside a pressurised aircraft at 30,000ft above Kathmandu to gain some spectral information on the energy ranges and magnitudes of the photon radiations that we are being exposed to.
All in all, there is quite a bit of measurement and science ongoing on a daily basis and I look forward to having a completed dataset for final analysis and interpretation.
I am a member of the Society for Radiological Protection and as part of my expedition I am also trying to fundraise for their Benevolent Fund – please click here to make a donation to this worthy cause.