A W thesis

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.

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

A_W_CAA_apparatus

Y-88 CAA

Run6980_Y88

Run7023_Y88

Run7108_Y88

Run7161_Y88

Run7204_Y88

Run7236_Y88

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 singles (blue). Coincidence events (red) that occurred within a 200 ns timing window. We can see that by having the detectors in coincidence, the noise is reduced and several peaks which no not have multiple photons in coincidence are removed. The coincidence can be further improved upon by the addition of energy cuts on the NaI detector. The graph on the right is an overlay of the coincidence signal before and after and energy cut is applied to the companion detector (the NaI detector). The blue line shows the signal before an energy cut. For the red line, a cut has been placed on the NaI detector around the high energy Y-88 peak. The noise seen at low energies is greatly reduced after including the cut

Why aren't there only two peaks in the coincidence data when you don't have an energy cut?

Compton scattering and accidentals.

Background subtraction

Do the fit:

get parameters for line

Then fill 1 histogram with line

Then subtract

TH1F *coin1=new TH1F("coin1","coin1",30,1800,1860);

ntuple->Draw("ADC7*0.604963-49.7001 >>coin1")

TH1F *lin1=new TH1F("lin1","line1",30,1800,1860);

for(int i=1800;i<1861;i++){
lin1->Fill(i,-2028+1.12*i)
}

TH1F *sub1=new TH1F("sub1","sub1",30,1800,1860);

sub1->Add(coin1,1);
sub1->Add(lin1,-1);

sub1->Draw();

A pdf of the Mathematica notebook used to calculate background area, gaussian area, and plot signal/noise vs. activity.

File:AW Background Noise custom4.pdf

All the ROOT fit parameters used to find the background and the resulting peaks.

File:Y-88 Fit Log 2.pdf

File:Y-88 Fit Log Norm&Cut.pdf

Integrating the gaussian of the HpGe detector signal.

File:AW Gaussian Integral2.pdf

898 keV Signal Table

[math] f(x)=A \textstyle \int_{\mu-2\sigma}^{\mu+2\sigma} e^{-\frac{1}{2}\left(\frac{x-\mu}{\sigma}\right)^2} \operatorname{d}\!x +B x +C[/math]

[math]T_{1/2}[/math]	Trig	Signal	BackG Subtracted	Fit Parameters					Signal Area	Noise Area	SNR
0.98	sing			[math]\mu=[/math] +/-	[math]\sigma=[/math]	A= +/-	B= +/-	C= +/-	+/-	+/-
	coin			[math]\mu=[/math] +/-	[math]\sigma=[/math]	A=	B=	C=	+/-	+/-	+/-
1.46	sing			[math]\mu=[/math] +/-	[math]\sigma=[/math]	A= +/-	B= +/-	C= +/-	+/-	+/-
	coin			[math]\mu=[/math] +/-	[math]\sigma=[/math]	A=	B=	C=	+/-	+/-	+/-
2.12	sing			[math]\mu=[/math] +/-	[math]\sigma=[/math]	A=	B=	C=	+/-	+/-
	coin			[math]\mu=[/math] +/-	[math]\sigma=[/math]	A=	B=	C=	+/-	+/-
2.27	sing			[math]\mu=[/math] +/-	[math]\sigma=[/math]	A=	B=	C=	+/-	+/-
	coin			[math]\mu=[/math] +/-	[math]\sigma=[/math]	A=	B=	C=	+/-	+/-

File:AW Run with Cuts Calculations.pdf

Ba-133 CAA

BO-08-22-13

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

AW_ADC_7_Histogram

AW_ADC_3_Histogram

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 [math]E_{expected} - E_{fit} -vs- E_{expected}[/math]

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

PAA_Research

A_W_thesis_old