Acid fumigation - Preparing C-13 solid samples for organic analysis

 

TIPS FOR PREPARING LARGE BULK DIFFICULT COMBUSTION ¹³C & ¹⁵N SOLID SAMPLES – SOILS, SEDIMENTS, FILTERS, WOOD, AND CARBONATES

Removing carbonates from calcareous soils and sediments before organic ¹³C analysis

Inorganic C in the form of carbonates can interfere with the measurement of organic ¹³C in soils. Remove inorganic C by acid fumigation. Weigh soil samples into silver capsules (tin decomposes when exposed to acid) and arrange samples in a 96-well tray. Add a small amount of water to each open capsule to wet the soil. Place the whole 96-well tray in a desiccator containing a beaker of concentrated (12M) HCl. Carbonates are released as CO₂ in 6 to 8 hours. Dry the samples at 60°C and carefully crimp-seal the capsules. The capsules become brittle after drying, resulting in leaks; be careful not to lose material when crimping. We recommend placing the whole capsule into a new tin (Sn) capsule and crimp it closed. The additional tin capsule is an important combustion catalyst, so it is advantageous to use tin capsules for re-encapsulating leaking samples.

Reference:

Harris, D., Horwath, W. R., and van Kessel, C., 2001. Acid fumigation of soils to remove carbonates prior to total organic carbon or carbon-13 isotopic analysis. Soil Science Society of America Journal 65: 1853-1856.

Source : http://stableisotopefacility.ucdavis.edu/sample-weight-calculator.html

(Accessed 26 August 2015)

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apllo 15 gamma experiment

Science Experiments - Gamma-Ray Spectrometer

The Gamma-ray Spectrometer Experiment measured the composition of the lunar surface.

The Apollo Moon landings returned samples for study on Earth from only six locations on the Moon. In order to better understand the Moon's overall chemical composition, the Gamma-ray Spectrometer and the X-ray Fluorescence Spectrometer studied the composition of the Moon's surface from lunar orbit. These experiments were flown on both Apollo 15 and Apollo 16. The Gamma-ray Spectrometer was deployed on a 7.6-meter-long boom, which is visible in some Metric Camera photographs.

Some elements, such as uranium and thorium, are naturally radioactive and emit gamma-rays as part of their radioactivity. The bombardment of the lunar surface by galactic cosmic rays causes some other elements to emit gamma-rays. The Gamma-ray Spectrometer measured this radiation from lunar orbit to produce maps of the abundances of thorium, iron, and titanium on the lunar surface. The different elements were identified based on the energy of the gamma-rays that they emitted. The abundances of these elements were determined from the intensity of the gamma-rays (the greater the intensity of the gamma-rays, the greater the abundance of the element). Unlike the X-ray Fluorescence Spectrometer, which worked only in sunlight, the Gamma-ray Spectrometer functioned both in sunlight and in darkness. As a result, the Gamma-ray Spectrometer mapped the entire region of the Moon flown over by Apollo 15 and 16, or about 20% of the lunar surface in all. Features as small as 100 kilometers across can be distinguished in the resulting maps. Maps of the distribution of iron and titanium for most of the Moon's surface were later derived by analyzing visible and near-infrared images obtained by the Clementine spacecraft. In addition to studying the lunar surface, the Gamma-ray Spectrometer was also used on the return voyage to Earth to survey the sky for astronomical gamma-ray sources.

This experiment found high iron abundances over all mare regions and lower abundances elsewhere. Thorium and titanium abundances were also highest over mare regions, but these two elements varied considerably in abundance in different parts of the maria. More details about these measurements and their relationship to lunar rock compositions are presented in the following documents.

Gamma-Ray Spectrometer Iron Abundance

The figure above shows the iron abundance measured by the Apollo Gamma-Ray Spectrometer. Red is a high abundance, yellow and green are intermediate abundances, and blue and purple are low abundances. High iron abundances are found in all mare regions and lower abundances are found elsewhere. In combination with the low aluminum abundance found in the maria by the X-ray Fluorescence Spectrometer, this indicates that the maria are covered by basalt. This was known from laboratory analysis of samples obtained by Apollo missions that landed in mare regions, but the orbital geochemistry experiments allowed this knowledge to be extended to a larger segment of the Moon. Data are only available for portions of the Moon overflown by Apollo 15 and Apollo 16. Another map of iron abundance was produced using observations made by the Clementine spacecraft. The Clementine results cover almost the entire Moon and are based on the effects of iron on the Moon's near-infrared spectrum rather than on its gamma-ray emissions. (From Plate 10.2 of Lunar Sourcebook, Cambridge University Press, 1991.)

Gamma-ray Spectrometer Thorium Abundance

The figure above shows the thorium abundance measured by the Apollo Gamma-ray Spectrometer. Data are only available for portions of the Moon overflown by Apollo 15 and 16. Red is a high abundance, yellow and green are intermediate abundances, and blue and purple are low abundances. Mare regions on the western part of the lunar nearside (Mare Imbrium and Oceanus Procellarum) show relatively high thorium abundances. Mare regions on the eastern part of the lunar nearside have lower thorium abundances. The lowest thorium abundances are on the lunar farside. Samples from the western part of the Moon's nearside obtained by Apollo 12, 14, and 15 include a type of basalt known as KREEP. KREEP is a chemical acronym, denoting rocks that are high in potassium (denoted chemically as K), rare earth elements (REE) and phosphorus (P). Thorium behaves chemically as one of the rare earth elements, and the high thorium regions in the above map indicate regions where KREEP is likely to be present on the Moon's surface. In contrast, samples from mare regions on the eastern part of the Moon's nearside obtained by Apollo 11and 17 do not include significant amounts of KREEP. The Gamma-ray Spectrometer results indicate that KREEP is not important in this region of the Moon. (From Plate 10.1 of Lunar Sourcebook, Cambridge University Press, 1991.)

Gamma-ray Spectrometer Titanium Abundance

The figure shows the titanium abundance measured by the Apollo Gamma-Ray Spectrometer. Red is a high abundance, yellow and green are intermediate abundances, and blue and purple are low abundances. Titanium on the Moon's surface occurs primarily in the mineral ilmenite. Titanium is very rare in the lunar highlands and of variable abundance in the lunar maria. High titanium abundances are found in Mare Serenitatis and Mare Tranquillitatis. Among the lunar samples returned to Earth, the highest titanium abundances were in fact observed in Apollo 11 and Apollo 17 samples from these maria, although some very low titanium basalts were also sampled by Apollo 17. Another high titanium area is in western Oceanus Procellarum, a region that was not sampled by Apollo. The Gamma-ray Spectrometer data indicates that other mare regions have lower Ti abundances. Samples returned by Apollo 12, 14, and 15 and by Luna 16 and 24 are consistent with this observation. Data is only available for portions of the Moon overflown by Apollo 15 and Apollo 16. Another map of titanium abundance was produced using observations made by the Clementine spacecraft. The Clementine results cover almost the entire Moon and are based on the effects of titanium on the Moon's visible and near-infrared spectrum rather than on its gamma-ray emissions. (From Plate 10.3 of Lunar Sourcebook,Cambridge University Press, 1991.)

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gamma spectroscopy, lsc report

 

Md. Mahbubul Hassan sohag

Six months Scientific Attachment at Material Technology Group (MTEG), Nuclear Malaysia (NM).

Liquid Scintillation Counter (LSC) Report:

Liquid scintillation counting is an analytical technique that measures activity of radionuclides from the rate of light photons emitted by a sample. This measurement technique is applicable to all types of emissions.

Sample Preparation:

Environment sample Preparation for H-3 Analysis:

According to procedure took 100ml of water (environment sample).Weighted 0.05gm of Potassium permanganate (KMnO4) and 0.02 gm of Potassium hydroxide (KOH). Mixed sample water with Potassium permanganate (KMnO4) and Potassium hydroxide (KOH).

Put 5 or 6 pieces of boiling chip, refluxed it for 30 minutes and cool down for 15 minutes. Now it was distilled through the column that contained activated charcoal and fiber glass. Now this distilled water was collected into a plastic bottle and labeled it.

Same procedure was followed for distilled water and collected in plastic bottle. This sample is considered as background.

For background, pipette 10ml of distill water into first vial and add 12ml scintillator, then sake it.

In second vial pipette 10ml of sample solution add 12ml scintillator, then sake it.

For third vial pipette 10ml of sample solution with 0.1 ml of standard solution (H-3) add 12ml scintillator, then sake it.

Counting Efficiency:

Counting efficiency =

Quenching Effect:

Quenching is the loss of counts due to sample or cocktail. Quenchers are divided into two category one chemical quenchers and other color quenchers. chemical quenchers absorb radioactive energy before it is converted into photon. Color quenchers absorb light in the range of wavelength emitted by scintillator. In this case the number of photon is not changed but the number reaching photomultiplier tube is reduced.

Gamma Spectroscopy Report:

Sample Preparation:

For gamma spectroscopy, To obtain get good number of counts samples will concentrate (reduce its volume but not reduce its mass). Several types of sample was prepared like as soil sample, Water sample, Plant sample, Beverage Sample that all are given below.

Soil Sample:

Collect soil samples and dry sample. Crash samples, put in specific geometry sample bottle. Now seal sample bottle using tape. Put it in incubator at least 21 days to produce homogeneous condition.

Water Sample:

Took collected environment water sample 2 liter. Then using filtration pump, which is consist of suction pump with filter, clean the water sample. Pour it into a beaker. Put the beaker on hot plate fix the temperature 60 degree for three days until two liter convert into 350ml. Seal the sample for 21 days in incubator.

Beverage Sample:

Cash the sample and put into specific geometry bottle sample is ready for count. Incase sample is liquid then follow same thing.

Plant sample:

For plant sample, weight sample then dry it in oven for two days at 105 ˚c. Again weight this sample. Now grind this sample, put into furnish about 500˚c for one day. Now make palette using this sample.

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 Principles of Gamma-ray Spectroscopy and Applications in Nuclear Forensics

Introduction

Gamma-ray (γ-ray) spectroscopy is a quick and nondestructive analytical technique that can be used to identify various radioactive isotopes in a sample. In gamma-ray spectroscopy, the energy of incident gamma-rays is measured by a detector. By comparing the measured energy to the known energy of gamma-rays produced by radioisotopes, the identity of the emitter can be determined. This technique has many applications, particularly in situations where rapid nondestructive analysis is required.

Background principles

Radioactive decay

The field of chemistry typically concerns itself with the behavior and interactions of stable isotopes of the elements. However, elements can exist in numerous states which are not stable. For example, a nucleus can have too many neutrons for the number of protons it has or contrarily, it can have too few neutrons for the number of protons it has. Alternatively, the nuclei can exist in an excited state, wherein a nucleon is present in an energy state that is higher than the ground state. In all of these cases, the unstable state is at a higher energy state and the nucleus must undergo some kind of decay process to reduce that energy.

There are many types of radioactive decay, but type most relevant to gamma-ray spectroscopy is gamma decay. When a nucleus undergoes radioactive decay by α or β decay, the resultant nucleus produced by this process, often called the daughter nucleus, is frequently in an excited state. Similar to how electrons are found in discrete energy levels around a nucleus, nucleons are found in discrete energy levels within the nucleus. In γ decay, the excited nucleon decays to a lower energy state and the energy difference is emitted as a quantized photon. Because nuclear energy levels are discrete, the transitions between energy levels are fixed for a given transition. The photon emitted from a nuclear transition is known as a γ-ray.

Radioactive decay kinetics and equilibrium

Radioactive decay, with few exceptions, is independent of the physical conditions surrounding the radioisotope. As a result, the probability of decay at any given instant is constant for any given nucleus of that particular radioisotope. We can use calculus to see how the number of parent nuclei present varies with time. The time constant, λ, is a representation of the rate of decay for a given nuclei,

If the symbol N₀ is used to represent the number of radioactive nuclei present at t = 0, then the following equation describes the number of nuclei present at some given time.

The same equation can be applied to the measurement of radiation with some sort of detector. The count rate will decrease from some initial count rate in the same manner that the number of nuclei will decrease from some initial number of nuclei.

The decay rate can also be represented in a way that is more easily understood. The equation describing half-life (t_1/2) is shown in 

The half-life has units of time and is a measure of how long it takes for the number of radioactive nuclei in a given sample to decrease to half of the initial quantity. It provides a conceptually easy way to compare the decay rates of two radioisotopes. If one has a the same number of starting nuclei for two radioisotopes, one with a short half-life and one with a long half-life, then the count rate will be higher for the radioisotope with the short half-life, as many more decay events must happen per unit time in order for the half-life to be shorter.

When a radioisotope decays, the daughter product can also be radioactive. Depending upon the relative half-lives of the parent and daughter, several situations can arise: no equilibrium, a transient equilibrium, or a secular equilibrium. This module will not discuss the former two possibilities, as they are off less relevance to this particular discussion.

Secular equilibrium takes place when the half-life of the parent is much longer than the half-life of the daughter. In any arbitrary equilibrium, the ratio of atoms of each can be described 

Because the half-life of the parent is much, much greater than the daughter, as the parent decays, the observed amount of activity changes very little.

This can be rearranged to show that the activity of the daughter should equal the activity of the parent.

Once this point is reached, the parent and the daughter are now in secular equilibrium with one another and the ratio of their activities should be fixed. One particularly useful application of this concept, to be discussed in more detail later, is in the analysis of the refinement level of long-lived radioisotopes that are relevant to trafficking.

Detectors

Scintillation detector

A scintillation detector is one of several possible methods for detecting ionizing radiation. Scintillation is the process by which some material, be it a solid, liquid, or gas, emits light in response to incident ionizing radiation. In practice, this is used in the form of a single crystal of sodium iodide that is doped with a small amount of thallium, referred to as NaI(Tl). This crystal is coupled to a photomultiplier tube which converts the small flash of light into an electrical signal through the photoelectric effect. This electrical signal can then be detected by a computer.

Semiconductor detector

A semiconductor accomplishes the same effect as a scintillation detector, conversion of gamma radiation into electrical pulses, except through a different route. In a semiconductor, there is a small energy gap between the valence band of electrons and the conduction band. When a semiconductor is hit with gamma-rays, the energy imparted by the gamma-ray is enough to promote electrons to the conduction band. This change in conductivity can be detected and a signal can be generated correspondingly. Germanium crystals doped with lithium, Ge(Li), and high-purity germanium (HPGe) detectors are among the most common types.

Advantages and disadvantages

Each detector type has its own advantages and disadvantages. The NaI(Tl) detectors are generally inferior to Ge(Li) or HPGe detectors in many respects, but are superior to Ge(Li) or HPGe detectors in cost, ease of use, and durability. Germanium-based detectors generally have much higher resolution than NaI(Tl) detectors. Many small photopeaks are completely undetectable on NaI(Tl) detectors that are plainly visible on germanium detectors. However, Ge(Li) detectors must be kept at cryogenic temperatures for the entirety of their lifetime or else they rapidly because incapable of functioning as a gamma-ray detector. Sodium iodide detectors are much more portable and can even potentially be used in the field because they do not require cryogenic temperatures so long as the photopeak that is being investigated can be resolved from the surrounding peaks.

Gamma spectrum features

There are several dominant features that can be observed in a gamma spectrum. The dominant feature that will be seen is the photopeak. The photopeak is the peak that is generated when a gamma-ray is totally absorbed by the detector. Higher density detectors and larger detector sizes increase the probability of the gamma-ray being absorbed.

The second major feature that will be observed is that of the Compton edge and distribution. The Compton edge arises due to Compton Effect, wherein a portion of the energy of the gamma-ray is transferred to the semiconductor detector or the scintillator. This occurs when the relatively high energy gamma ray strikes a relatively low energy electron. There is a relatively sharp edge to the Compton edge that corresponds to the maximum amount of energy that can be transferred to the electron via this type of scattering. The broad peak lower in energy than the Compton edge is the Compton distribution and corresponds to the energies that result from a variety of scattering angles. A feature in Compton distribution is the backscatter peak. This peak is a result of the same effect but corresponds to the minimum energy amount of energy transferred. The sum of the energies of the Compton edge and the backscatter peak should yield the energy of the photopeak.

Another group of features in a gamma spectrum are the peaks that are associated with pair production. Pair production is the process by which a gamma ray of sufficiently high energy (>1.022 MeV) can produce an electron-positron pair. The electron and positron can annihilate and produce two 0.511 MeV gamma photons. If all three gamma rays, the original with its energy reduced by 1.022 MeV and the two annihilation gamma rays, are detected simultaneously, then a full energy peak is observed. If one of the annihilation gamma rays is not absorbed by the detector, then a peak that is equal to the full energy less 0.511 MeV is observed. This is known as an escape peak. If both annihilation gamma rays escape, then a full energy peak less 1.022 MeV is observed. This is known as a double escape peak.

Example of experiments

Determination of depleted uranium

Natural uranium is composed mostly of ²³⁸U with low levels of ²³⁵U and ²³⁴U. In the process of making enriched uranium, uranium with a higher level of ²³⁵U, depleted uranium is produced. Depleted uranium is used in many applications particularly for its high density. Unfortunately, uranium is toxic and is a potential health hazard and is sometimes found in trafficked radioactive materials, so it is important to have a methodology for detection and analysis of it.

One easy method for this determination is achieved by examining the spectrum of the sample and comparing it qualitatively to the spectrum of a sample that is known to be natural uranium. This type of qualitative approach is not suitable for issues that are of concern to national security. Fortunately, the same approach can be used in a quantitative fashion by examining the ratios of various gamma-ray photopeaks.

The concept of a radioactive decay chain is important in this determination. In the case of ²³⁸U, it decays over many steps to ²⁰⁶Pb. In the process, it goes through ^234mPa, ²³⁴Pa, and ²³⁴Th. These three isotopes have detectable gamma emissions that are capable of being used quantitatively. As can be seen in , the half-life of these three emitters is much less than the half-life of ²³⁸U. As a result, these should exist in secular equilibrium with ²³⁸U. Given this, the ratio of activity of ²³⁸U to each daughter products should be 1:1. They can thus be used as a surrogate for measuring ²³⁸U decay directly via gamma spectroscopy. The total activity of the ²³⁸U can be determined by , where A is the total activity of ²³⁸U, R is the count rate of the given daughter isotope, and B is the probability of decay via that mode. The count rate may need to be corrected for self-absorption of the sample is particularly thick. It may also need to be corrected for detector efficiency if the instrument does not have some sort of internal calibration.

Half-lives of pertinent radioisotopes in the 238U decay chain
Isotope	Half-life
238U	4.5 x 109 years
234Th	24.1 days
234mPa	1.17 minutes

A gamma spectrum of a sample is obtained. The 63.29 keV photopeak associated with ²³⁴Th was found to have a count rate of 5.980 kBq. What is the total activity of ²³⁸U present in the sample?

²³⁴Th exists in secular equilibrium with ²³⁸U. The total activity of ²³⁴Th must be equal to the activity of the ²³⁸U. First, the observed activity must be converted to the total activity using Equation A=R/B. It is known that the emission probability for the 63.29 kEv gamma-ray for ²³⁴Th is 4.84%. Therefore, the total activity of ²³⁸U in the sample is 123.6 kBq.

The count rate of ²³⁵U can be observed directly with gamma spectroscopy. This can be converted, as was done in the case of ²³⁸U above, to the total activity of ²³⁵U present in the sample. Given that the natural abundances of ²³⁸U and ²³⁵U are known, the ratio of the expected activity of ²³⁸U to ²³⁵U can be calculated to be 21.72 : 1. If the calculated ratio of disintegration rates varies significantly from this expected value, then the sample can be determined to be depleted or enriched.

As shown above, the activity of ²³⁸U in a sample was calculated to be 123.6 kBq. If the gamma spectrum of this sample shows a count rate 23.73 kBq at the 185.72 keV photopeak for ²³⁵U, can this sample be considered enriched uranium? The emission probability for this photopeak is 57.2%.

As shown in the example above, the count rate can be converted to a total activity for ²³⁵U. This yields a total activity of 41.49 kBq for ²³⁵U. The ratio of activities of ²³⁸U and ²³⁵U can be calculated to be 2.979. This is lower than the expected ratio of 21.72, indicating that the ²³⁵U content of the sample greater than the natural abundance of ²³⁵U.

This type of calculation is not unique to ²³⁸U. It can be used in any circumstance where the ratio of two isotopes needs to be compared so long as the isotope itself or a daughter product it is in secular equilibrium with has a usable gamma-ray photopeak.

Determination of the age of highly-enriched uranium

Particularly in the investigation of trafficked radioactive materials, particularly fissile materials, it is of interest to determine how long it has been since the sample was enriched. This can help provide an idea of the source of the fissile material—if it was enriched for the purpose of trade or if it was from cold war era enrichment, etc.

When uranium is enriched, ²³⁵U is concentrated in the enriched sample by removing it from natural uranium. This process will separate the uranium from its daughter products that it was in secular equilibrium with. In addition, when ²³⁵U is concentrated in the sample, ²³⁴U is also concentrated due to the particulars of the enrichment process. The ²³⁴U that ends up in the enriched sample will decay through several intermediates to ²¹⁴Bi. By comparing the activities of ²³⁴U and ²¹⁴Bi or ²²⁶Ra, the age of the sample can be determined.

In , A_Bi is the activity of ²¹⁴Bi, A_Rais the activity of ²²⁶Ra, A_U is the activity of ²³⁴U, λ_Th is the decay constant for ²³⁰Th, λ_Ra is the decay constant for ²²⁶Ra, and T is the age of the sample. This is a simplified form of a more complicated equation that holds true over all practical sample ages (on the order of years) due to the very long half-lives of the isotopes in question. The results of this can be graphically plotted as they are in .

Ratio of ²²⁶Ra/²³⁴U (= ²¹⁴Bi/²³⁴U) plotted versus age based on . This can be used to determine how long ago a sample was enriched based on the activities of ²³⁴U and ²²⁶Ra or ²¹⁴Bi in the sample.

Exercise: The gamma spectrum for a sample is obtained. The count rate of the 121 keV ²³⁴U photopeak is 4500 counts per second and the associated emission probability is 0.0342%. The count rate of the 609.3 keV ²¹⁴Bi photopeak is 5.83 counts per second and the emission probability is 46.1%. How old is the sample?

Solution: The observed count rates can be converted to the total activities for each radionuclide. Doing so yields a total activity for ²³⁴U of 4386 kBq and a total activity for ²¹⁴Bi of 12.65 Bq. This gives a ratio of 9.614 x 10^-7. Using , as graphed this indicates that the sample must have been enriched 22.0 years prior to analysis.

References

• G. Choppin, J.-O. Liljenzin, and J. Rydberg. Radiochemistry and Nuclear Chemistry, Elsevier Press, Oxford (2006).

• W. Loveland, D. J. Morrissey, and G. T. Seaborg. Modern Nuclear Chemistry, Wiley, New Jersey (2006).

• K. Mayer, M. Wallenius, and I. Ray. Analyst, 2005, 130, 433.

• J. T. Mihalczo, J. A. Mullens, J. K. Mattingly, and T. E. Valentine. Nucl. Instrum. Meth. A, 2000, 450, 531.

• J. T. Mihalczo, J. K. Mattingly, J. S. Neal, and J. A. Mullens, Nucl. Instrum. Meth. B, 2004, 213, 378.

• K. J. Moody, I. A. Hutcheon, and P. M. Grant. Nuclear Forensic Analysis, CRC Press, Boca Raton (2005).

• C. T. Nguyen. Nucl. Instrum. Meth. B, 2005, 229, 103.

• C. T. Nguyen, and J. Zsigrai. Nucl. Instrum. Meth. B,2006, 246, 417.

• D. Reilly, N. Ensslin, and H. Smith, Jr. Passive Nondestructive Assay of Nuclear Materials, National Technical Information Service, Springfield, VA (1991).

• M. Wallenius, A. Morgenstern, C. Apostolidis, and K. Mayer. Anal. Bioanal. Chem., 2002, 374, 379.

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