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Revision as of 17:21, 16 September 2014
Introduction
Methods of determining atomic concentration in Material
Neutron Activation Analysis (NAA)
Neutron activation analysis (NAA) is a high-sensitivity and non-destructive multi-elemental analytical technique used for analysis of major, minor, and trace elements in samples. NAA is significantly different from other spectroscopic analytical techniques in that it is based on nuclear transitions rather than electronic transitions. The basic essentials to carry out an analysis of samples by NAA are a source of neutrons to bombard the sample, instruments suitable for detecting gamma rays, and knowledge of the reactions that occur when neutrons interact with target nuclei.
The sequence of events taking place during the most common type of nuclear reaction for NAA, namely neutron capture, is illustrated in the figure. (make figure) Upon irradiation, a neuron interacts with the target nucleus via a non-elastic collision. A compound nucleus forms in an excited and usually unstable state. This unfavourable state will almost instantaneously de-excite into a more stable configuration by emitting one or more characteristic prompt gamma rays. In most cases, this new and stable configuration yields a radioactive nucleus. The newly formed radioactive nucleus now decays by the emission of one of more characteristic delayed gamma rays. This decay process is at a much slower rate according to unique half-life of the radioactive nucleus. Measurement, in principle, falls into two categories: (1) prompt gamma-ray neutron activation analysis (PGNAA), where measurements are taken during the irradiation of a sample, or (2) the more common delayed gamma-ray neutron activation analysis (DGNAA), where measurements follow radioactive decay.
A range of different neutron sources can be used for NAA. These include reactors, accelerators, fusors, and radioisotopic neutron emitters. Since they have high fluxes of neutrons from uranium fission, nuclear reactors offer the highest available sensitivities for most elements.
There are several types of detectors and configurations employed in NAA. Most are designed to detect the emitted gamma radiation. The instrumentation most commonly used consists of scintillation type or semiconductor type detector(s), associated electronics, and a computer-based, multi-channel analyser. Scintillation type detectors use a radiation-sensitive crystal, usually thallium-doped sodium iodide (NaI(Tl)). Hyperpure or intrinsic germanium (HpGe) detectors that operate at liquid nitrogen temperatures (77 degrees K) are the semiconductor type most commonly operated for NAA.
There are a few drawbacks to the use of NAA. Even though the technique is non-destructive, the irradiated sample will remain radioactive for great lengths of time depending on the half-lives, requiring handling and disposal protocols. Also, not all labs have convenient access to a suitable reactor for a neutron source to irradiate a sample. As with any other analytical method, NAA is also not universal. For instance, the determination of low-Z elements, such as C, N, O, F, or several other elements such as Mg, Si, Ca, Ti, Ni, Sr, Y, Zr, Nb, Sn, and Tl, is not sufficiently sensitive or impossible.
Inductively coupled plasma mass spectrometry(ICP-MS)
Inductively coupled plasma atomic emission spectroscopy(ICP-OES)
Atomic Absorption Spectrometry (multi-element AAS)
Particle-induced X-ray Emission (PIXE)
Another method, uses X-ray from a synchrotron light source and look at the de-excitation of atomic electrons to measure the atomic number. Reports are that they can measure pico-gram quantities.
table of detection limits -vs- Method
Coincidence Counting Setup
Y-88 CAA
Using our Y-88 source, our set-up allows us to perform runs with the detectors in coincidence (AND mode) or singles (OR mode). The figure on the left shows the HpGe detector's coincidence events (red) that occurred within a 200 ns timing window. This graph has been overlaid with the same detector’s single events (blue). We can see that by having the detectors in coincidence, the noise is reduced and several peaks, which do not have multiple photons in coincidence, are removed.
The coincidence run does not show the two energy peaks associated with Y-88 decay alone. Even though we require coincidence for the system to trigger, there is still noise. These peaks could be Compton events contaminating the coincidence and accidentals. Photons can loss a portion of their energy traveling to the detector. They do not necessarily have to deposit all of their energy into the detector either. The photon can Compton scatter out. It is not possible to tell the difference between a photon that deposited all of its energy from a scattered photon, since the speed of light is so fast and the resolution of the detector so poor. The photons can, however, be distinguished if an energy cut is applied. In order to remove the Compton events and the accidentals, we can require the photon energy to be completely deposited into the detector. This can be achieved by placing an energy cut on the NaI detector around the high energy Y-88 peak. This will remove other unwanted photon energies. The graph on the right is an overlay of the coincidence signal before (blue) and after (red) an energy cut is applied to the companion detector (NaI detector). The noise seen at low energies is greatly reduced after including the cut and improved the signal.
Background subtraction
898 keV analysis
The raw Histograms and fits may be found here A_W_thesis_old#898_keV_signal_table
The Measured half life is
Days | Trig | Integral | Statistical Error | Systematic Error | Systematic Error without Coincidence Energy Cut | |
105 | 0.98 | sing | 1.364 | N/A | ||
coin | 0.0052 | |||||
156 | 1.46 | sing | 0.8697 | N/A | ||
coin | 0.0042 | |||||
226 | 2.12 | sing | 0.4733 | N/A | ||
coin | 0.0034 | |||||
242 | 2.27 | sing | 0.5807 | N/A | ||
coin | 0.0015 |
The signal to noise ratio is
Days | Trig | SNR | |
105 | 0.98 | sing | 4.143 |
coin | 52.050 | ||
156 | 1.46 | sing | 10.165 |
coin | 42.263 | ||
226 | 2.12 | sing | 3.529 |
coin | 36.865 | ||
242 | 2.27 | sing | 5.305 |
coin | 13.295 |
N_0 = xxxx
When SNR = 0 then t= 301.1 days (s) and 343.3 days (c)
N_{sing} = ZZZZ = lowest measurable activity using singles
N_{conic} = AAAA = lowest measurable activity using conic
1836.1 Analysis
The raw Histograms and fits may be found here A_W_thesis_old#1836_keV_signal_table
The Measured half life is
Days | Trig | Integral | Statistical Error | Systematic Error | Systematic Error without Coincidence Energy Cut | |
105 | 0.98 | sing | 0.8414 | N/A | ||
coin | 0.0062 | |||||
156 | 1.46 | sing | 0.5015 | N/A | ||
coin | 0.0046 | |||||
226 | 2.12 | sing | 0.2822 | N/A | ||
coin | 0.0062 | |||||
242 | 2.27 | sing | 0.2449 | N/A | ||
coin | 0.0016 |
The signal to noise ratio is
Days | Trig | SNR | |
105 | 0.98 | sing | 42.920 |
coin | 9.791 | ||
156 | 1.46 | sing | 21.115 |
coin | 6.796 | ||
226 | 2.12 | sing | 5.282 |
coin | 10.288 | ||
242 | 2.27 | sing | 5.478 |
coin | 2.068 |
N_0 = xxxx
When SNR = 0 then t= 250.8 days (s) and 286.8 days (c)
N_{sing} = ZZZZ = lowest measurable activity using singles
N_{conic} = AAAA = lowest measurable activity using conic
Error analysis
1.) rate energy cuts
2.) Energy calibration error
3.) Linear background fit error
4.) Gaussian integration of Signal - background peak
5.) ignorable error from run length time
6.) Coincidence energy cut.
7.) dividing by average background for SNR
8.) correction ratio for 3rd data point
9.) error in half life to find N0 for the detection limit extrapolation
10.) error extrapolation efficiency to higher energy lines for N0 calculation
Half-life
898 keV Singles
Days | Trig | Energy Window | Energy Calibration | Linear Fit of Background | Gaussian Integration of Signal | Total Systematic Error | |
105 | 0.98 | sing | |||||
156 | 1.46 | sing | |||||
226 | 2.12 | sing | |||||
242 | 2.27 | sing |
898 keV Coincidence
Days | Trig | Energy Window | Energy Calibration | Linear Fit of Background | Gaussian Integration of Signal | Coincidence Energy | Total Systematic Error | Systematic Error without Coincidence Energy Cut | |
105 | 0.98 | coin | |||||||
156 | 1.46 | coin | |||||||
226 | 2.21 | coin | |||||||
242 | 2.27 | coin |
1836.1 keV Singles
Days | Trig | Energy Window | Energy Calibration | Linear Fit of Background | Gaussian Integration of Signal | Total Systematic Error | |
105 | 0.98 | sing | |||||
156 | 1.46 | sing | |||||
226 | 2.12 | sing | |||||
242 | 2.27 | sing |
1836.1 keV Coincidence
Days | Trig | Energy Window | Energy Calibration | Linear Fit of Background | Gaussian Integration of Signal | Coincidence Energy Cut | Total Systematic Error | ||
105 | 0.98 | coin | |||||||
156 | 1.46 | coin | |||||||
226 | 2.12 | coin | |||||||
242 | 2.27 | coin |
Signal to Noise Ratio
Energy (keV) | Trig | Backgrd(counts) | Backgrnt Rate(Hz) | Stat Error | Variance of Averaged Background |
898 | sing | ||||
coin | |||||
1836.1 | sing | N/A | |||
coin |
Efficiency
Days | Trig | Extrapolation of Efficiency to High Energy | |
105 | 0.98 | sing | |
coin | |||
156 | 1.46 | sing | |
coin | |||
226 | 2.12 | sing | |
coin | |||
242 | 2.27 | sing | |
coin |
Detection Limit Extrapolation
Days | Trig | Half-life Error to Find N0 | |
105 | 0.98 | sing | |
coin | |||
156 | 1.46 | sing | |
coin | |||
226 | 2.12 | sing | |
coin | |||
242 | 2.27 | sing | |
coin |
Singles Efficiency on 2/28/2014
Run | Source | Energy | Expected rate (Hz) | HpGe Det B Rate (ADC 7) (Hz) | HpGe Eff (%) |
7027 | Na-22 | 511 | 1589.218 +/- 1.842 | 18.07 +/- 0.111 | 1.137 +/- 1.066 |
7025 | Cs-137 | 661.657 | 2548.03 +/- 0.222 | 25.468+/- 0.172 | 0.999 +/- 0.1768 |
7029 | Mn-54 | 834.848 | 41.405 +/- 0.0426 | 0.485 +/- 0.014 | 1.171 +/- 0.0264 |
7026 | Co-60 | 1173.228 | 1876.619 +/- 0.2687 | 11.03 +/- 0.106 | 0.588 +/- 0.172 |
7027 | Na-22 | 1274.537 | 888.264 +/- 1.029 | 4.857 +/- 0.034 | 0.5468 +/- 0.5948 |
7026 | Co-60 | 1332.492 | 1878.167 +/- 0.2690 | 9.584 +/- 0.023 | 0.510 +/- 0.1562 |
Coincidence Efficiency on 2/10/2014
Run | Source | Energy | Expected rate (Hz) | HpGe Det B Rate (ADC 7) (Hz) | HpGe Eff (%) |
7027 | Na-22 | 511 | 1610.527 +/- 1.850 | ||
7026 | Co-60 | 1173.228 | 1888.999 +/- 0.2682 | ||
7027 | Na-22 | 1274.537 | 900.175 +/- 1.034 | ||
7026 | Co-60 | 1332.492 | 1890.557 +/- 0.2684 |
Ba-133 CAA
Useful commands
Converting CODA data file to ROOT
make sure the CODA and ROOT environmental variables are setup by source the following scripts
source ~/CODA/setup
source ~/ROOT/root/bin/thisroot.csh
Now change to the data subdirectory and execute the program to convert the data file to root
cd /data
~/CODA/CODAreader/ROOT_V5.30/V785V792/evio2nt -fr6994.dat > /dev/null
rename the file so it has the .root extension allowing ROOT to identify it in the browser
mv r6994 r6994.root
Calibration work
System's intrinsic err
Plot calibration parameters as a function of time
determine the variance of the parameters using several (>20) fits
Impact of higher order fits
Plot
Variance comes from several fits,.
Compare uncertainty when fit is E-vs-Channe to Channel-vs-E
Probably should use %error for the weighting
Concentration measurement
A comparison of the measure concentrations using singles and coincidence counting