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THE NUCLEUS: RADIOACTIVITY & HALF-LIFE

THE NUCLEUS: RADIOACTIVITY & HALF-LIFE. THE NUCLEUS OF AN ATOM. Zoom In to Nucleus. Enlarged view of the Nucleus. Nucleus contains NUCLEONS: Protons: Positive Neutrons: Neutral (almost identical to the Proton, but for the charge). Atom: Electrons + Nucleus. Nuclear Forces.

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THE NUCLEUS: RADIOACTIVITY & HALF-LIFE

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  1. THE NUCLEUS: RADIOACTIVITY & HALF-LIFE

  2. THE NUCLEUS OF AN ATOM Zoom In to Nucleus Enlarged view of the Nucleus • Nucleus contains NUCLEONS: • Protons: Positive • Neutrons: Neutral (almost identical to the Proton, but for the charge) Atom: Electrons + Nucleus

  3. Nuclear Forces There are two kinds of forces in the nucleus: • Electrical Forces (Repulsive) • Between positively charged protons. • Strong Forces (Attractive) • Between protons & neutrons. • Always attractive. Does NOT depend upon charge. Strong forces are responsible for holding the nucleus together.

  4. Why do some nuclei disintegrate? Strong attraction dominate when two protons (A and B) are close together. Electrical repulsion dominates when two protons (A and C) are far apart. Close together A Far apart B C So, in larger nuclei(e.g. Uranium) when the protons are more likely to be far apart, the electricalrepulsion tends to dominate thestrong attraction, causing the nucleus to disintegrate.

  5. What is Radioactivity? The disintegration of a nucleus and release of several different kinds of rays and particles is called radioactivity. More energetic than X-Rays

  6. How long does disintegration take? The rate at which an atom disintegrates is measured by its half-life. Half-Life is the time in which one-half of any number of nuclei will have disintegrated. • E.g. Half-Life of Radium is 1,620 Years. If we start out with a certain amount of say 1kg, then … • after 1,620 years we only have ½ kg. • after another 1,620 years we have ¼ kg. • after yet another 1,620 years we have 1/8 kg. After each half-life, we have half of what we had previously.

  7. HALF - LIFE 1half life 1kg (1/2) x 1kg = (1/2) kg 2half lives (1/2) x (1/2) x 1kg = (1/4) kg Weight of Radon 3half lives 1/2 kg (1/2) x(1/2) x (1/2) x 1kg = (1/8) kg 4half lives 1/4 kg (1/2) x (1/2) x (1/2) x (1/2) x 1kg = (1/16) kg 1/8 kg 1/16 kg 1 x 1,620 Years 2 x 1,620 Years 3 x 1,620 Years 4 x 1,620 Years Time

  8. Figuring Amount Left Over 15 years is 3 half-lives. So amount left over is: (1/2) x (1/2) x (1/2) x 1kg = 1/8 kg. 3half-lives The half-life of Cobalt-60 is about 5 years. If you have 1kg of Cobalt-60 today, about how much of it would you have 15 years later? • 1/16 kg. • 1/8 kg. • 1/6 kg. • 1/4 kg. • 1/3 kg.

  9. 16→2 in ~ 9 days

  10. Figuring Half - Life You start out with a 1,000 of bacterial cells and 20 days later, you have only 250 of them still alive. What is the half-life of the bacterial cells? • 4 days. • 5 days. • 10 days. • 20 days. • Not enough info. 250 out of 1,000 is 1/4. (1/4) is the same as (1/2) x (1/2) So, it means 2 half-lives have passed. So: 20 days is 2half lives. So, a half-life is 10 days.

  11. Applications of Half – LifeCarbon-14 Dating Every living organism contains a very small percentage of radioactive Carbon-14, which decays but is replenished from the Carbon-14 in the food. When the organism dies it no longer consumes Carbon-14. And Carbon-14 decays into Carbon-12, with a half-life of about 5,730 years. So, by measuring the percentage of Carbon-14 to Carbon-12 in a fossil of the organism it is possible to work backward and figure how long ago it lived.

  12. Zone 1 counties have a predicted average indoor radon screening level greater than 4 pCi/L (pico curies per liter) (red zones) Zone 2 counties have a predicted average indoor radon screening level between 2 and 4 pCi/L (orange zones) Zone 3 counties have a predicted average indoor radon screening level less than 2 pCi/L (yellow zones) http://www.epa.gov/radon/zonemap.html

  13. http://www.epa.gov/radon/zonemap/kansas.htm

  14. Today’s lab – Zorkium data Plot Corrected Activity vs. Time.  Perform an Automatic Curve Fit using the Natural Exponent fity=A exp(-Cx) + B Calculate the effective half life of the daughter products of Ra-222 collected from the air on the balloon.  Use the formulat1/2 = ln(2)/C i.e. t1/2 = 0.693/C Obtained value for the half life is in the same time units as units used to plot time on the graph. The constant C from the graph is known as the decay constant and is usually referred to as "lambda" (λ).

  15. Today’s lab – Balloon data Plot Corrected Activity vs. Time.  Perform an Automatic Curve Fit using the Natural Exponent fity=A exp(-Cx) + B Calculate the effective half life of the daughter products of Ra-222 collected from the air on the balloon.  Use the formulat1/2 = ln(2)/C i.e. t1/2 = 0.693/C Obtained value for the half life is in the same time units as units used to plot time on the graph. The constant C from the graph is known as the decay constant and is usually referred to as "lambda" (λ).

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