Nuclear changes and radiation evidence
Đã giới thiệu| English | Ý nghĩa |
|---|---|
| half-life/hɑːf laɪf/ | nửa đời |
| background radiation/ˈbækɡraʊnd ˌreɪdɪˈeɪʃn/ | bức xạ nền |
What would explain this observation?
- A detector records counts even when the classroom source is removed. Background must be considered before attributing every count to the source.
Build the model
- Radioactive decay is spontaneous and random for an individual nucleus.
- Half-life 半衰期 describes the time for half the undecayed nuclei in a large population to decay, or for background-corrected activity to halve.
- half-life: Time for activity or undecayed population to halve.
- background radiation 本底辐射: Radiation measured apart from the investigated source.
Half-lives: a fraction of the nuclei remains
Individual decays are random; half-life describes the pattern for a large population.
Which count should be used for source activity?
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
The background count is always present, so it must be measured separately and subtracted over equal times to leave the source activity. Using the raw count overstates the source; picking the largest repeat is just cherry-picking noise.
Match each technical term to its precise meaning.
Độ khó 2/5 · 140 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Subtract background counts measured over the same time interval.
- Distinguish irradiation from contamination.
- Shielding, distance and reduced exposure time can reduce risk under a school-controlled procedure.
In the lab: Use teacher-managed sources and the school radiation rules. Record count duration and repeat background measurements. Do not extrapolate a half-life from one nucleus or from uncorrected readings.
Which two habits make the investigation or model in this case more defensible?
Độ khó 2/5 · 140 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
Work from known quantities
- Known: measured count rate falls from 100 to 55 counts per minute; background is 10.
- Source rates are 90 and 45, so one half-life has elapsed.
- Without subtraction, 55/100 would obscure the exact half in this example.
Your turn: An initial source count is 160 per minute. Find it after three half-lives.
An initial source count is 160 per minute. Find it after three half-lives.
Độ khó 2/5 · 140 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
The result is 20 counts/min. Known: initial count 160 per minute; three half-lives. Each half-life halves the activity: 160 -> 80 -> 40 -> 20 counts per minute. After three half-lives the count is 160/(2^3) = 20 per minute.
Check the conclusion and its limits
- An irradiated object is not automatically radioactive.
- A half-life does not predict the exact decay time of one nucleus.
Not true: Half-life predicts the exact decay time of an individual nucleus. Radioactive decay is random at the level of single nuclei. Half-life is a statistical quantity: it predicts the fraction of a LARGE number of nuclei decaying in a given time, never the moment one nucleus will decay.
- Radioactive decay is spontaneous and random for an individual nucleus.
- Subtract background counts measured over the same time interval.
- Watch for: an irradiated object is not automatically radioactive.
Write from 50 counts per minute
- An initial source count is 200 per minute.
- Find the count after two half-lives. Half-life does not time one nucleus.
Radioactive decay is random at the level of single nuclei.
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
True. Radioactive decay is random at the level of single nuclei. Half-life is a statistical quantity: it predicts the fraction of a LARGE number of nuclei decaying in a given time, never the moment one nucleus will decay.
Time for activity or undecayed population to halve: write the technical term.
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
half-life means Time for activity or undecayed population to halve.
An initial source count is 200 per minute. What is the expected count after two half-lives, and why does half-life not predict the decay time of one nucleus?
Each half-life halves the expected activity. The prediction is statistical.
What is the expected count after two half-lives?
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
Two half-lives leave a quarter, and 200 divided by 4 is 50 per minute. Other options — 20 per minute (the lesson count of 160 after three half-lives); 200 per minute (the initial count).