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- Half-life T 1/2 The amount of time required for a substance to be reduced to one-half of its previous level by degradation and/or decay–radioactive half-life, by catabolism–biological half-life, or by elimination from a system–eg, half-life in serum Hematology The time that cells stay in the circulation–eg, RBCs 120 days–which ↑ after splenectomy, platelets–4-6 days, eosinophils.
- Half-life, in radioactivity, the interval of time required for one-half of the atomic nuclei of a radioactive sample to decay, or, equivalently, the time interval required for the number of disintegrations per second of a radioactive material to decrease by one-half.
- The Half-Life: Day One model is identical to the Half-Life one, except that it has 'gender' body parts. The 'male' version is Gordon, and the 'female' is an early ' gina.mdl ' / ' holo.mdl '. Furthermore, it has an animation removed from the more recent models, named 'ambush player', consisting of Freeman being hit twice, then falling on the.
The Half-Life calculator can be used to understand the radioactive decay principles. It can be used to calculate the half-life of a radioactive element, the time elapsed, initial quantity, and remaining quantity of an element. Half-life is a concept widely used in chemistry, physics, biology, and pharmacology.
What is half life?
Half-life
There are stable and unstable nuclei in each radioactive element. Unstable nuclei are radioactive decay and emit alpha, beta, or gamma-rays that eventually decay to stable nuclei while stable nuclei of a radioactive don’t change. Half-life is defined as the time needed to undergo its decay process for half of the unstable nuclei.
Each radioactive element has a different half life decay time. The half-life of carbon-10, for example, is only 19 seconds, so it is impossible to find this isotope in nature. Uranium-233 has a half-life of about 160000 years, on the other hand. This shows the variation in the half-life of different elements.
The concept if half-life can also be used to characterize some exponential decay. For instance, the biological half-life of metabolites.
Half-life is more like a probability measure. It doesn't mean that half of the radioactive element would have decayed after the half-life is over. However, it is a highly accurate estimate when enough nuclei are available in a radioactive element.
Half life formula
By using the following decay formula, the number of unstable nuclei in a radioactive element left after t can be calculated:
(N(t) = N_0 times 0.5^{(t/T)})
In this equation:
N(t) refers to the quantity of a radioactive element that exists after time t has elapsed.
N(0) refers to the initial amount of the element.
T refers to the half-life of an element.
The remaining amount of a material can also be calculated using a variety of other parameters:
(N (t) = N_0 times e^{-dfrac{t}{tau}})
(N (t) = N_0 times e^{(-lambda t)}) Desktop tree crack.
λ refers to the decay constant, which is the rate of decay of an element.
Half Life Equation
τ refers to the mean lifetime of an element. The average time a nucleus has remained unchanged.
The foregoing are all three equations that characterize the radioactivity of material and are linked to each other, which can be expressed as follow:
How to calculate half life?
As of now, you have been through the formula for half-life, and you may be wondering how to find half-life by using that half-life equation. Calculating half-life is somewhat complicated, but we will simplify the process for your understanding. Let’s calculate the half-life of an element by assuming a few things for the sake of calculations.
- Suppose the original amount of a radioactive element is:
N (0) = 200 g
- Now let’s assume the final quantity of that element is:
N (t) = 50 g
- If it took 120 seconds to decay from 3 kg to 1 kg, the time elapsed would be:
t = 120 seconds
- Write the half-life equation and place the above values in that equation:
T = 60 seconds
So, if an element with the initial value of 200 grams decayed to 50 grams in 120 seconds, its half-life will 60 seconds.
Similarly, you can also calculate other parameters such as initial quantity, remaining quantity, and time by using the above equations. If you don’t want to get yourself into these complex calculations, just put the values in the above calculator. Our calculator will simplify the whole process for you.
Check out a few more calculators by us, designed specifically for you.
Example
How many grams of an isotope will remain in 30 years if the half-life of 500 grams of a radioactive isotope is 6 years?
Solution:
In this half-life problem, we already have a half-life, time, and initial quantity of a radioactive isotope. We need to calculate the remaining quantity of that isotope. Let’s find the remaining quantity step by step:
- Identify the values from the above problem description.
![Half-Life Half-Life](/uploads/1/2/6/6/126680190/904818039.jpg)
N (t) =?
N (0) = 500 g
T = 6 years
t = 30 years
- Write the equation of half-life and substitute the values.
- Solve the equation for the remaining quantity N (t). After simplifying these values, we will get:
N (t) = 15.625 g
A radioactive isotope will remain 15.625 grams after 30 years if its half-life is 6 years, and initial values are 500 grams. Similarly, the elapsed time t and the initial quantity N (0) of a radioactive isotope can also be calculated by following the same process.
How to use our half life calculator?
Calculating half-life using the above calculator is very simple because you just have to input values to get half-life of any element. This calculator not only calculates the half-life, but it can also be used to calculate the other parameters of the half-life equation such as time elapsed, initial and remaining value. You can find the different tabs for calculating each parameter.
To calculate the half-life of an element, go to the half-life tab:
- Enter the initial and remaining quantity of the element in the corresponding input boxes.
- Enter the total time it took to decay. You can select the unit of time from seconds, minutes, hours, months, year, etc.
- Press the Calculate It will instantly show you the half-life of the element.
Similarly, you can calculate initial and remaining values as well as the time elapsed by clicking on the respective tabs and entering values in the input boxes. You don’t need to put any effort into calculating half life because this calculator does all the complex calculations by only taking values and give the results in a blink of an eye. Moreover, you can use this calculator to solve any type of half-life problems in school or college.
What is a pesticide half-life?
A half-life is the time it takes for a certain amount of a pesticide to be reduced by half. This occurs as it dissipates or breaks down in the environment. In general, a pesticide will break down to 50% of the original amount after a single half-life. After two half-lives, 25% will remain. About 12% will remain after three half-lives. This continues until the amount remaining is nearly zero. See Figure 1.
Figure 1. Approximate amount of pesticide (shaded area) remaining at the application site over time.
Each pesticide can have many half-lives depending on conditions in the environment. For example, permethrin breaks down at different speeds in soil, in water, on plants, and in homes.
- In soil, the half-life of permethrin is about 40 days, ranging from 11-113 days.
- In the water column, the half-life of permethrin is 19-27 hours. If it sticks to sediment, it can last over a year.
- On plant surfaces, the half-life of permethrin ranges from 1-3 weeks, depending on the plant species.
- Indoors, the half-life of permethrin can be highly variable. It is expected to be over, or well over, 20 days.
Why is a pesticide's environmental half-life important?
The half-life can help estimate whether or not a pesticide tends to build up in the environment. Pesticide half-lives can be lumped into three groups in order to estimate persistence. These are low (less than 16 day half-life), moderate (16 to 59 days), and high (over 60 days). Pesticides with shorter half-lives tend to build up less because they are much less likely to persist in the environment. In contrast, pesticides with longer half-lives are more likely to build up after repeated applications. This may increase the risk of contaminating nearby surface water, ground water, plants, and animals.
However, pesticides with very short half-lives can have their drawbacks. For example, imagine that a pesticide is needed to control aphids in the garden for several weeks. One application of a pesticide with a half-life of a few hours will probably not be very effective several weeks out. This is because the product would have broken down to near-zero amounts after only a few days. This type of product would likely have to be applied multiple times over those several weeks. This could increase the risk of exposure to people, non-target animals, and plants.
What can influence a pesticide's environmental half-life?
Many things play a role in how long a pesticide remains in the environment. These include things like sunlight, temperature, the presence of oxygen, soil type (sand, clay, etc.), how acidic the soil or water is, and microbe activity. See Table 1. Pesticide half-lives are commonly reported as time ranges. This is because environmental conditions can change over time. This makes it impossible to describe a single, consistent half-life for a pesticide.
A pesticide product's formulation can also change how the active ingredient behaves in the environment. In fact, the properties of the formulation may dominate initially, until enough time has passed to allow the ingredients to separate This is because small amounts of an active ingredient are 'formulated' with larger amounts of 'other' ingredients to make a whole pesticide product.
Table 1. Environmental factors that affect pesticide persistence.4
Environmental Factors | Role in Chemical Degradation |
---|---|
Sunlight | Radiation from the sun breaks certain chemical bonds, creating break down products. |
Microbes | Bacteria and fungi can break down chemicals, creating biodegradation products. |
Plant / Animal Metabolism | Plants and animals can change chemicals into forms that dissolve better in water (metabolites). This makes removal from the body easier. |
Water | Water breaks chemicals apart to make pieces that dissolve better in water (hydrolysis). This is typically a very slow process. |
Dissociation | Chemicals can break apart into smaller pieces (dissociation products). |
Sorption | Chemicals that stick tightly to particles can become inaccessible and/or move away with those particles. |
Bioaccumulation | Some chemicals can be absorbed by plants/animals from the soil, water, food, and air. When the plant/animal is exposed again before it can remove the chemical(s), accumulation can occur. |
How is a pesticide's half-life determined?
Pesticide half-lives are often determined in a laboratory. There, conditions like temperature can be controlled and closely monitored. Soil, water, or plant material is mixed with a known amount of a pesticide. The material is then sampled and tested over time to determine how long it takes for half of the chemical to break down.
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Field studies are also performed for some chemicals. A known amount of the pesticide is mixed with soil, water, or plant material. It is then placed in an outdoor environment where it is exposed to various environmental conditions and tested over time. Field studies provide researchers with a more realistic idea of how the pesticide will act in the environment. However, half-life values from such studies can vary greatly depending on the exact conditions. See Figure 2.
Before a pesticide product is registered, manufacturers measure their half-lives. You can find their research results in a variety of databases, books, and peer-reviewed articles. If you need help, call the National Pesticide Information Center.
What happens to pesticides after they 'go away'?
When a pesticide breaks down it doesn't disappear. Instead, it forms new chemicals that may be more or less toxic than the original chemical. Generally, they are broken into smaller and smaller pieces until only carbon dioxide, water, and minerals are left. Microbes often play a large role in this process. In addition, some chemicals may not break down initially. Instead, they might move away from their original location. It all depends on the chemical and the environmental conditions.
Inorganic pesticides like iron phosphate and copper sulfate don't break down in the same way as organic pesticides.10,12 The 'half-life' concept only applies to organic pesticides, those that contain carbon components.
Figure 2. The soil half-life of five pesticides.8,9,11,13,15
Where can I get more information?
For more detailed information about pesticide half-lives please visit the list of referenced resources below or call the National Pesticide Information Center, Monday - Friday, between 8:00am - 12:00pm Pacific Time (11:00am - 3:00pm Eastern Time) at 1-800-858-7378 or visit us on the web at http://npic.orst.edu. NPIC provides objective, science-based answers to questions about pesticides.
Please cite as: Hanson, B.; Bond, C.; Buhl, K.; Stone, D. 2015. Pesticide Half-life Fact Sheet; National Pesticide Information Center, Oregon State University Extension Services. http://npic.orst.edu/factsheets/half-life.html.