Recap after Easter

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Across
  1. 3. an alpha particle can be described as being the same as a _ _ _ _ _ _ nucleus
  2. 7. this type of radiation is stopped by a few mm of aluminium
  3. 9. this particle has a negative charge and orbits the nucleus
  4. 10. this is the time taken for the amount of radiation to decrease to half of the original amount
  5. 12. this particle has no charge and is found in the nucleus
  6. 14. exposure to large amounts of radiation can cause...
  7. 15. this type of radiation is stopped by paper or skin
  8. 17. the people that killed Litvinenko were from this country
  9. 18. Li, Na and K are all elements found in this group
Down
  1. 1. you can calculate this by multiplying the length, width and depth of an object together - it is measured in cm3
  2. 2. this is the 'official' unit for mass, usually shortened to 'kg'
  3. 4. this element has the symbol 'Ca'
  4. 5. this type of radiation is only reduced by thick lead or concrete
  5. 6. this is calculated by doing mass divided by volume
  6. 8. this is the maximum number of electrons allowed in the first shell
  7. 11. the polonium in Litvinenko's tea was so dangerous because it gave off this type of radiation
  8. 13. this element is radioactive and has the symbol 'U'
  9. 16. this particle has a positive charge and is found in the nucleus