Polonium-210: An Interesting Nucleus

• The s-process ‘terminator’...end point of element production the stars...

• Mme. Curie (and some of her mates...) in 1898

• There’s a lot of it about...NORM and 238U decay

• Properties (alpha, energies, gammas?)

• How does it harm you…how much do you need ?

Alexander Litvinenko, aged 43 :Before and after effects of radiation poisoning with ‘polonoium – 210’ .Reported feeling ill on 1 Nov. 2006, died 23 Nov. 2006.

Alpha-decay of 210Po

4He (Z=2) (N=2)

206Pb (Z=82) (N=124)

210Po (Z=84) (N=126)

Q = energy release = [ M(210Po) – { M(206Pb) + M(4He)} ] c2 ~ 5.5 MeV ~1 pJ.

i.e., around 0.003% of total 210Po mass released kinetic energy in decay.

(rare occasion, 206Pb can be left in an excited state….803 keV above the ground state.)

Total energy released shared as KINETIC energy between 206Pb and 4He.

Cons. of linear momentum means most of the energy (5.3 MeV) goes to 4He.

Po-210Z = 84138 day

Bi-209Z = 83stable

Bi-210Z = 835 day

Pb-206Z = 82stable

Tl-206Z = 814 min

n

α (0.0001%)

5.3 MeV α (100%)2+0+ 803 keV γ (0.01%)

β

β (100%)

lethal dose ~50 Mbqactual dose ~5 GBq (30 μg)

Production by n-capture on 209Bi

Not a bad authors list....one for the RAE of circa 1930 no doubt!

Map of Europe in 1914 (pre-First World War…note, no Poland.

T1/2=140 days

Q210Pb) = 5.41 MeVE = 5.30 MeV E(206Pb) = 0.11 MeVT1/2 = 138 days.

‘218Po =Radium A’

‘218At =Radium B’

C

D

E

210Po=Radium ‘F’ Radon

=‘Emanation’

‘Radium’

C’

C’’

The Natural Decay Chain for 238U

= 214Pb

= 214Bi

Figure Wiescher, Regan & Aprahamian, Physics WorldFeb. 2002, page 33-38

Slow-neutron capture processallows formation of elementsfrom A~56 to A=209 (Bi)...

terminates at 209Bi...why?

S-process makes 209Bi from 208Pb+n → 209Pb (T1/2=3.2hr) → 209Bi

Neutron capture on stable 209Bi → 210Bi (T1/2 =5 days) → 210Po.

210Po → + 206Pb (stable nucleus, as is 207Pb and 208Pb).

…..Now the AWE files show how small that risk really is because they

record the only known secondary contamination with the element.

The Establishment, Britain's most closely-guarded nuclear weapons site

in Aldermaston, Berks, was the only place at which polonium-210, the

substance which killed Alexander Litvinenko, was regularly handled in

Britain because it was used in the 1950s as a trigger for early atom

bombs.

Only on two occasions did the metal, potentially lethal if swallowed or

inhaled in large quantities, cause a health scare at the AWE, according

to the files, unearthed at the National Archives in Kew.

On the first occasion, in 1953, a worker in the polonium department was

found to have high levels of alpha radiation in his routine urine test.

From the Daily Telegraph, 4th Dec. 2006,By Ben Fenton

How much is ‘bad’ for you ?

The second, much more serious incident occurred in 1955, when 15 workers in the section where polonium was processed were exposed when a load of waste material was spilled over a table.The material was gathered up into a large paper bag, but then the worst happened. "While collecting the waste into the bag, it was accidentally compressed and a puff of air came up into the faces of two men," the report said."Because it was known that the waste was grossly contaminated, it seems reasonable to assume that the doses were received by inhalation."This level of direct exposure by inhalation must be many times greater than any passenger would be likely to receive from sitting on a plane, according to Dr Paddy Regan, a reader in nuclear physics at the University of Surrey."They effectively breathed this in directly from a powerful source and the dose they got would have been several orders of magnitude greater than anyone could have got from simply sitting on a plane," he said.

And the top secret files show that the peak level of radiation detected in the two men was 0.6 microcuries in one and 0.2 microcuries in the second.A lethal dose for ingestion or inhalation by a human being would be around 1,000 microcuries, according to calculations by Dr Regan and his colleagues. Although they are not named, there is no suggestion in the file that either man suffered any kind of health setback from their exposure.In 1957, the fire and explosion at the nuclear reactor at Windscale (now called Sellafield), Cumbria, released 240 curies of polonium into the atmosphere, enough to kill thousands of people if ingested, but rendered harmless by being diffusion in the atmosphere.

How you estimate the damage?• Radiation damage measured in ‘Grays’

• 1 Gy = 1 Joule imparted per kilogram

• Estimates are ‘assessed’ depending on – type of radiation (i.e., gammas, betas, alphas…) – body tissues/organs affected…different cells have different tolerances to radiation damage.

• Biological effect measured in Sieverts (Sv) = WR x Grays

FromAlpen p426

SRIM output for 5.3 MeV alphas on skin (1000 histories)..typical range ~0.04mm

No problem outside the body...alphas stop indead layers of the skin.

From Alpen p427

The total sum of weighting factors = 1.00.....Stomach, colonbadly affected by radiation...uptake in stomach wall leads to blood and then other organs in body.

From J. Harrison et al., J. Radiol. Prot. 27 (2007) 17

From J. Harrison et al., J. Radiol. Prot. 27 (2007) 17

Biological Half-time ~ 60 days.shorter than radioactive half-life ~140 days

From J. Harrison et al., J. Radiol. Prot. 27 (2007) 17

Paddy’s ‘back of the envelope’ Calculation....• Need to calculate the energy emitted per second at 1mCi for 210Po andassume 'lethal' value (assume ~10 Gray = 10 J/Kg deposited as a lethal 'whole body dose’ for human over entire body)

Total alpha energy emitted by 210Po = E ~ 8.5x10-13 J x activity x time exposed

For a 1mCi 210Po source, Power ~ (8x10-13) x (0.001 x 3.7x 1010) =3x10-5 J / secondEnergy deposited in given time from 1mCi source ~ (3.6x103 s) x (3x10-5 J/s)

• ~ 0.1 J/hour ~ (neglecting excretion and biological half-life of ~60 days)• ~ 1 Joule in 10 hours, • ~ 10 Joules in 100 hours (~4 days); 100 Joules in 1000 hours (~1.25 months)• remember….radiation weighting factor of Wr=20 for particles….• Need just 100 J of alphas to get whole body dose of 20 Sv for 100 kg mass

•Conclusion...1mCi might be enough....10mCi to be sure...but must be ingested.

• Worse if you get this in the lungs (no excretion)...but assuming just the guts etc.... Mass of intestinal tract/stomach etc. ~5 kg perhaps?

Lethal dose (~20 Sv) in 1000 hours of ingestion for 1 mCi source if 100% retained….probably need x 10 more to account for 1/10 uptake if swallowed…..

So how much is that….?

A = N.ln2/T1/2 , Activity depends on number of atoms present and decay half-time.

A = 1 mCi = 3.7x107/s

T1/2 = 138 days = 1.2x107s

Therefore

N = number of Po-210 atoms for 1 mCi = 6.4x1014

= 2x10-10g = 0.2ng

Typical 238U concentrations in crustal rock ~1-3 parts per million (see ‘Radioactivity in the Environment’ R.L. Kathren, Harwood Academic publishers.

Some 210Po Numbers..(taken from ‘Radioactivity in the Environment’ R.L. Kathren, Harwood)

• Estimated annual internal ‘whole body’ dose equivalent from natural 210Po radioactivity in the USA = 30Sv (same as 222Rn). (40K =170Sv for comparison)

• Annual dose equivalent to epithelial tissue lining the lungs from Radon and its daughters (e.g., 210Po) ~20Sv (double this for cigarette smokers).

206Pb first excited state at 803keV 1:105 possible way to tell...

1mCi = 3.7 x 107 decays per second ~ 400 gammas per second....

Enough for a signal in a whole body counter ?

Biological half-life for 210Po ~50-60 days, only if you are alive to ‘excrete it’ (collects in the Kidneys).....look for activity in the (dried) urine...

How can we tell if someone has 210Po in the future ?

Summary• 210Po has an interesting history…named after Poland (which didn’t

exist at the time).

• It occurs naturally (from the decay of 238U).

• It is responsible for ‘terminating the s-process (limiting number of chemical elements).

• You can make it in neutron-capture reactions on 209Bi.

• Can be detected by - or gamma-ray spectroscopy.

• Don’t eat or drink it (few gm are deadly): No problem really outside the body.

• Physicist are useful to the public. Numbers matter in calming peoples fears….