A W thesis - Revision history

Foretony: /* Y-89 Activation */

2016-12-05T19:43:39Z

Y-89 Activation

Foretony: /* Y-89 Activation */

2016-12-05T19:43:13Z

Y-89 Activation

Foretony: /* Y-89 Activation */

2016-12-05T19:41:16Z

Y-89 Activation

Wellali3: /* Results and Conclusion */

2015-06-08T15:04:58Z

Results and Conclusion

Wellali3: /* Introduction */

2015-06-08T15:03:34Z

Introduction

Wellali3: /* Results and Conclusion */

2015-06-03T14:49:00Z

Results and Conclusion

Wellali3: /* Y-89 Activation */

2015-06-03T14:46:31Z

Y-89 Activation

Wellali3: /* Photon Activation Analysis (PAA) */

2015-06-03T14:45:34Z

Photon Activation Analysis (PAA)

Wellali3: /* Neutron Activation Analysis (NAA) */

2015-06-03T14:44:16Z

Neutron Activation Analysis (NAA)

Wellali3: /* Introduction */

2015-06-03T14:43:33Z

Introduction

@@ Line 114: / Line 114: @@
 The only natural occurring isotope of Yttrium is Y-89. During photon activation of a Yttrium foil, a neutron is removed from the nucleus in a reaction denoted as <math>{}_{39}^{89}\! \mbox{Y}_{50}(\gamma,n) {}_{39}^{88}\!\mbox{Y}_{49}</math>.
-<math>{89 \atop\;50 }Y (\gamma,n){88 \atop \; }Y</math>
+<math>{89 \atop\;39 }Y (\gamma,n){88 \atop \; }Y</math>
 The resulting Y-88 nucleus is radioactive with a half-life of 106.63 days. Table~\ref{tab:photon_lines} gives the prominent photon energies emitted by an excited Y-88 nucleus. The 898 keV and 1836.1 keV lines have been observed to be the most prevalent with relative intensities of 93.7$\%$ and 99.2$\%$.

@@ Line 113: / Line 113: @@
 ==Y-89 Activation==
-The only natural occurring isotope of Yttrium is Y-89. During photon activation of a Yttrium foil, a neutron is removed from the nucleus in a reaction denoted as <math>{}_{39}^{89}\! \mbox{Y}_{50}(\gamma,n) {}_{39}^{88}\!\mbox{Y}_{49}</math>. The resulting Y-88 nucleus is radioactive with a half-life of 106.63 days. Table~\ref{tab:photon_lines} gives the prominent photon energies emitted by an excited Y-88 nucleus. The 898 keV and 1836.1 keV lines have been observed to be the most prevalent with relative intensities of 93.7$\%$ and 99.2$\%$.
+The only natural occurring isotope of Yttrium is Y-89. During photon activation of a Yttrium foil, a neutron is removed from the nucleus in a reaction denoted as <math>{}_{39}^{89}\! \mbox{Y}_{50}(\gamma,n) {}_{39}^{88}\!\mbox{Y}_{49}</math>.
 Table~\ref{tab:coincidence_gamma} provides a list of Y-88 photons along with the gammas they are in coincidence with. Note that the 898 keV and 1836.1 keV lines are also in coincidence. This coincidence occurs when the 2734.1 keV state, with a 94$\%$ branching ratio, makes a transition after 0.7 ps to the 1836.1 keV state, emitting a 898 keV photon. The 1836.1 keV state will decay to the ground state after 0.154 ps and emit a 1836.1 keV photon. A diagram of the photon energies and their transitions can be seen in Figure~\ref{fig:Y-88_decay}. The total lifetime of the transitions is less than a picosecond~\cite{times}. For our experimental setup, discussed in section~\ref{Discrimination}, the coincidence timing window was set to 200 ns, so for effective purposes the 898 keV and 1836.1 keV photons are considered in coincidence and will be used for our PAA analysis.

@@ Line 113: / Line 113: @@
 ==Y-89 Activation==
-The only natural occurring isotope of Yttrium is Y-89. During photon activation of a Yttrium foil, a neutron is removed from the nucleus in a reaction denoted as ${}_{39}^{89}\! \mbox{Y}_{50}(\gamma,n) {}_{39}^{88}\!\mbox{Y}_{49}$. The resulting Y-88 nucleus is radioactive with a half-life of 106.63 days. Table~\ref{tab:photon_lines} gives the prominent photon energies emitted by an excited Y-88 nucleus. The 898 keV and 1836.1 keV lines have been observed to be the most prevalent with relative intensities of 93.7$\%$ and 99.2$\%$.
+The only natural occurring isotope of Yttrium is Y-89. During photon activation of a Yttrium foil, a neutron is removed from the nucleus in a reaction denoted as <math>{}_{39}^{89}\! \mbox{Y}_{50}(\gamma,n) {}_{39}^{88}\!\mbox{Y}_{49}</math>. The resulting Y-88 nucleus is radioactive with a half-life of 106.63 days. Table~\ref{tab:photon_lines} gives the prominent photon energies emitted by an excited Y-88 nucleus. The 898 keV and 1836.1 keV lines have been observed to be the most prevalent with relative intensities of 93.7$\%$ and 99.2$\%$.
 Table~\ref{tab:coincidence_gamma} provides a list of Y-88 photons along with the gammas they are in coincidence with. Note that the 898 keV and 1836.1 keV lines are also in coincidence. This coincidence occurs when the 2734.1 keV state, with a 94$\%$ branching ratio, makes a transition after 0.7 ps to the 1836.1 keV state, emitting a 898 keV photon. The 1836.1 keV state will decay to the ground state after 0.154 ps and emit a 1836.1 keV photon. A diagram of the photon energies and their transitions can be seen in Figure~\ref{fig:Y-88_decay}. The total lifetime of the transitions is less than a picosecond~\cite{times}. For our experimental setup, discussed in section~\ref{Discrimination}, the coincidence timing window was set to 200 ns, so for effective purposes the 898 keV and 1836.1 keV photons are considered in coincidence and will be used for our PAA analysis.

@@ Line 645: / Line 645: @@
 =Results and Conclusion=
-This thesis quantified the detection limit improvement of coincidence photon activation analysis over the standard photon activation analysis method. Background is one of the most important contributions to establishing a detection limit. One way that a detection limit can be improved is to reduce the background. This can be accomplished by requiring the detection of two photons in coincidence, since the background is typically composed of random events. The detection limit was defined as the point at which the signal could not be distinguished from background. In Chapter 3, the time when the background subtracted signal could no longer be measured ($t_{final}$) was determine for both PAA and CPAA measurements of the same sample. Using this final time in the nuclear decay equation~\ref{eq:nuclear} calculates the lowest number of activated nuclei, the detection limit, that PAA and CPAA can measure.
+This thesis quantified the detection limit improvement of coincidence photon activation analysis (CPAA) over the standard photon activation analysis method (PAA). Background is one of the most important contributions to establishing a detection limit. One way that a detection limit can be improved is to reduce the background. This can be accomplished by requiring the detection of two photons in coincidence, since the background is typically composed of random events. The detection limit was defined as the point at which the signal could not be distinguished from background. In Chapter 3, the time when the background subtracted signal could no longer be measured ($t_{final}$) was determined for both PAA and CPAA measurements of the same sample. This final time, along with a calculated initial value ($N_0$), is used to find the lowest number of activated nuclei, the detection limit, that PAA and CPAA can measure.
 ==Efficiency for Converting Y-89==

@@ Line 48: / Line 48: @@
 =Introduction=
-Photon Activation Analysis (PAA) is a multi-elemental analysis method that performs photon spectroscopy on materials activated by high energy (MeV) photons to measure the concentrations of elements in a material.  The method measures the energy of photons emitted by decaying nuclei using a high purity Germanium detector that has a typical photon energy resolution of 1 keV.  Although the observed photon energy may be used to construct a finite list of decaying nuclei, the specific decaying nuclear isotope is usually uniquely identified once a half life is measured.  Many activated nuclei de-excite to the ground state by transitioning through several intermediate states and in the process emitting photons within a picosecond or less timescale of each other.  This cascade of gamma rays may be used to improve the identification of the decaying nucleus and reduce the background from other decaying nuclei by requiring the detection of at least two photons in coincidence, a method that will be referred to as Coincidence Photon Activation Analysis (CPAA).  This thesis quantifies the improvement achieved by CPAA in terms of the lowest number of activated nuclei, the detection limit, that PAA and CPAA can measure.
+Photon Activation Analysis (PAA) is a multi-elemental analysis method that performs photon spectroscopy on materials activated by high energy (MeV) photons to measure the concentrations of elements in a material. The method measures the energy of photons emitted by decaying nuclei using a high purity Germanium detector that has a typical photon energy resolution of 1 keV. Although the observed photon energy may be used to construct a finite list of decaying nuclei candidates, the specific decaying nuclear isotope emitting the photon is usually uniquely identified once a half-life is measured. PAA is able to identify the presence of several nuclei by detecting photons emitted by decaying nuclei.
 In addition to PAA, there are several other analytical techniques used to quantify the elemental composition of materials. They each have their own associated advantages and disadvantages. For example, AAS and ICP methods chemically process a sample, usually by removing a portion of the material and destroying the sample. This is not an ideal technique when investigating materials such as artifacts and antiques, where cutting away pieces for a sample is not desireable. Additionally, there is a danger of introducing contamination during preparation procedures. However, unlike some techniques that are limited to surface studies, these two methods can be used for volume or bulk analysis. Further explanations on some of the more readily available techniques and their differences may be found in Appendix A.

@@ Line 643: / Line 643: @@
 =Results and Conclusion=
-This thesis quantified the detection limit improvement of coincidence photon activation analysis over the standard photon activation analysis method. Background is one of the most important contributions to establishing a detection limit. One way that a detection limit can be improved is to reduce the background. This can be accomplished by requiring the detection of two photons in coincidence, since the background is typically composed of random events. The detection limit was defined as the point at which the signal could not be distinguished from background. In Chapter 3, the time when the background subtracted signal could no longer be measured ($t_{final}$) was determine for both PAA and CPAA measurements of the same sample.
+This thesis quantified the detection limit improvement of coincidence photon activation analysis over the standard photon activation analysis method. Background is one of the most important contributions to establishing a detection limit. One way that a detection limit can be improved is to reduce the background. This can be accomplished by requiring the detection of two photons in coincidence, since the background is typically composed of random events. The detection limit was defined as the point at which the signal could not be distinguished from background. In Chapter 3, the time when the background subtracted signal could no longer be measured ($t_{final}$) was determine for both PAA and CPAA measurements of the same sample. Using this final time in the nuclear decay equation~\ref{eq:nuclear} calculates the lowest number of activated nuclei, the detection limit, that PAA and CPAA can measure.
 ==Efficiency for Converting Y-89==
- present and past tense
+The concept of photon activation analysis relies on activating a nucleus by using a high energy photon to eject one or more nucleons (protons or neutrons). The nuclei that have had one or more nucleons removed are typically unstable and decay. It is well understood that radioactive materials decay as an exponential function of time. The measured signals of the Y-88 sample were fit to an exponential curve to find the initial number of activated atoms in the foil. The exponential fit was extrapolated back to the initial time $t=0$ when the foil was activated. The half-life of Y-88 is well known, 106.63 days, and was used to calculate the decay constant $\lambda$. Using the relationship between activity and the number density, $A = \lambda N$, the initial value, $N_0$, can be found for time $t=0$.
-The Yttrium foil's mass was found to be 0.0288 +/- 0.0002 g. Using Avogadro's number, $N_a$, and the isotopic mass of Y-89, $m_i=88.905848 \frac{g}{mol}$, the number of atoms (N) is found using:
+The Yttrium foil's mass was found to be 0.0288 +/- 0.0002 g. Using Avogadro's number, $N_a$, and the isotopic mass of Y-89, $m_i=88.905848 \frac{g}{mol}$, the number of atoms (N) was found using:
 \begin{equation}
@@ Line 660: / Line 656: @@
 \end{equation}
-\noindent
+The foil contained $(195.1 \pm 1.35)\times 10^{18}$ atoms of Y-89. Taking the ratio of $N_0$ and the number of atoms gives the accelerator beam's efficiency at activating the sample. Using the above number of atoms and using the average of $N_0$ for singles and coincidence events, the ratio was $(24.42 \pm 5.26)\times 10^{-12}$ using singles and $(65.28 \pm 5.41)\times 10^{-12}$ using coincidence. For every $10^{12}$ atoms of Y-89, one  atom of Y-88 was produced.
 ==Minimum Number of Y-88 Atoms Detectable==
- After activating a source, a time will be reached when the sample has decayed past the point where the signal can be identified from the background.
+After activating a source, a time will be reached when the signal is no longer observable due to background. The lowest activity of Y-88 that the apparatus could measure was predicted by extrapolating the measured signal to noise ratio (SNR) forward in time until it was zero. Details on finding the signal to noise ratio can be found in Chapter~\ref{Data Analysis}. Beyond this point in time ($t_{final}$), the signal will not be distinguishable from the background noise and measurements will no longer be possible with this apparatus. The average value of $N_0$, $(8.75 \pm 2.02)\times 10^9$ nuclei, and the time when the SNR reached zero were used in the exponential decay function $N(t) = N_0 e^{-\lambda t}$ to find the lowest measurable activity, N($t_{final}$), using singles and coincidence counting. The 898 keV energy line resulted in a N($t_{final}$) of $(9.77 \pm 2.35)\times 10^8$ nuclei after 337.209 days using singles counting and $(3.77 \pm 0.954)\times 10^8$ nuclei after 483.60 days using coincidence counting. For the 1836.1 keV energy line, N($t_{final}$) was $(11.77 \pm 2.93)\times 10^8$ nuclei after 308.569 days with singles counting. Using this apparatus, coincidence counting can measure half of the singles minimum number of detectable atoms. The detection limit can be improved by at least a factor of two using coincidence counting over standard PAA.
 =Appendix: Methods=

← Older revision		Revision as of 14:46, 3 June 2015
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	==Y-89 Activation==		==Y-89 Activation==

−	The only natural occurring isotope of Yttrium is Y-89. During photon activation of a Yttrium foil, a neutron is removed from the nucleus in a reaction denoted as ${}_{39}^{89}\! \mbox{Y}_{50}(\gamma,n) {}_{39}^{88}\!\mbox{Y}_{49}$. The resulting Y-88 nucleus is radioactive with a half-life of 106.63 days. Table~\ref{tab:photon_lines} gives the prominent photon energies emitted by an excited Y-88 nucleus. The 898 keV and 1836.1 keV lines have been observed to be the most prevalent with relative intensities of 93.7$\%$ and 99.2$\%$. Table~\ref{tab:coincidence_gamma} provides a list of Y-88 photons along with the gammas they are in coincidence with. Note that the 898 keV and 1836.1 keV lines are also in coincidence. This coincidence occurs when the 2734.1 keV state, with a 94$\%$ branching ratio, makes a transition after 0.7 ps to the 1836.1 keV state, emitting a 898 keV photon. The 1836.1 keV state will decay to the ground state after 0.154 ps and emit a 1836.1 keV photon. A diagram of the photon energies and their transitions can be seen in Figure~\ref{fig:Y-88_decay}. The total lifetime of the transitions is less than a picosecond~\cite{times}. For our experimental setup, discussed in section~\ref{Discrimination}, the coincidence timing window was set to 200 ns, so for effective purposes the 898 keV and 1836.1 keV photons are considered in coincidence and will be used for our PAA analysis.	+	The only natural occurring isotope of Yttrium is Y-89. During photon activation of a Yttrium foil, a neutron is removed from the nucleus in a reaction denoted as ${}_{39}^{89}\! \mbox{Y}_{50}(\gamma,n) {}_{39}^{88}\!\mbox{Y}_{49}$. The resulting Y-88 nucleus is radioactive with a half-life of 106.63 days. Table~\ref{tab:photon_lines} gives the prominent photon energies emitted by an excited Y-88 nucleus. The 898 keV and 1836.1 keV lines have been observed to be the most prevalent with relative intensities of 93.7$\%$ and 99.2$\%$.
		+
		+	Table~\ref{tab:coincidence_gamma} provides a list of Y-88 photons along with the gammas they are in coincidence with. Note that the 898 keV and 1836.1 keV lines are also in coincidence. This coincidence occurs when the 2734.1 keV state, with a 94$\%$ branching ratio, makes a transition after 0.7 ps to the 1836.1 keV state, emitting a 898 keV photon. The 1836.1 keV state will decay to the ground state after 0.154 ps and emit a 1836.1 keV photon. A diagram of the photon energies and their transitions can be seen in Figure~\ref{fig:Y-88_decay}. The total lifetime of the transitions is less than a picosecond~\cite{times}. For our experimental setup, discussed in section~\ref{Discrimination}, the coincidence timing window was set to 200 ns, so for effective purposes the 898 keV and 1836.1 keV photons are considered in coincidence and will be used for our PAA analysis.
		+
		+	PAA utilizes photon spectroscopy to measure the concentration of elements in a material. It is predicted that by requiring the detection of two photons in coincidence, CPAA will allow the signal background to be decreased and measure a lower detection limit. The primary objective of this thesis will be to quantify the improvement of CPAA in the detection limit using the activation of Y-89. A determination of the detection limit for PAA and CPAA was made using the apparatus described in the following chapter.

@@ Line 105: / Line 105: @@
 In photon activation analysis (PAA)~\cite{Segebade}, nuclei in the sample material are excited into radioactive meta-stable states through exposure to high-energy (MeV) photons. PAA has detection limits of 0.01$\rightarrow$100 ppm~\cite{Segebade}~\cite{Lutz}. The photon source is typically produced by accelerating electrons onto a target layer of metal with a high atomic number, such as Tungsten. The de-acceleration of electrons passing through the target (radiator) produces electromagnetic radiation. Such radiation is referred to as bremsstrahlung radiation. If the bremsstrahlung radiation strikes a sample nucleus and results in the removal of a nucleon from the nucleus, the nucleus will more often than not be left in an excited state. The resulting nucleus is usually unstable and de-excites by emitting beta or gammas. The energy of the emitted $\gamma$ radiation is usually characteristic of the nuclide. A measurement of the gamma energy can thus be used to identify the nuclear isotope. Similar to NAA, gamma radiation is commonly measured with radiation sensitive crystals, such as thallium-doped sodium iodide (NaI(Tl)), or semiconductor type, such as HPGe detectors. For coincidence photon activation analysis, two detectors are used to require the detection of two photons decaying in coincidence.
-Concerning the choice of sample that would be activated for PAA study, it was desirable to activate an isotope with half-lives of days to allow measurements of coincidence high and low energy gamma lines. Photon spectroscopy suffers from an increase in background noise when the photon energy decreases below 1 MeV. A detector system can require the presence of two photons from the decay in coincidence in order to separate low energy photon signals from the background noise. From the resources conveniently available, Yttrium met the high and low energy line requirements and had some additional characteristics that were favorable for testing. The half-life of activated Yttrium (Y-88) is 106.63 days. Photon activated materials with such long half-lives allow a waiting period for the short lived contaminates to decay away and still have a signal that could be measured. Details of Yttrium activation and decay process are described below.
+When performing nuclear activation on a sample, all nuclei are affected. Contaminates present in a sample will also be activated and contribute to the signal background. It was desirable to activate an isotope with half-lives of days to reduce this background and study the impact of CPAA. Photon activated materials with such long half-lives allow a waiting period for the short lived contaminates to decay away and still have a signal that could be measured. It would also allow multiple measurements and systematic studies could be performed. Of the materials available, Yttrium met this requirement. The half-life of activated Yttrium (Y-88) is 106.63 days. Details of Yttrium activation and decay process are described below.
 [[File:Photon_Activation_Analysis_1971_Lutz.pdf]]

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 ===Neutron Activation Analysis (NAA)===
-Neutron activation analysis (NAA) is a non-destructive technique used to measure trace elements in materials to within detection limits of 0.01 $\rightarrow$ 10 ppm~\cite{Pollard}. In NAA, the incident radiation particle is a neutron. Several different neutron sources are available, but only a few that offer the high neutron fluxes useful for NAA. These include reactors and accelerators. Since they have high fluxes ($10^{13} \frac{n}{cm^2 \cdot s}$) of neutrons from uranium fission, nuclear reactors offer the highest available fluxes~\cite{Glascock}. The sequence of events taking place during the most common type of nuclear reaction for NAA, namely neutron capture (n,$\gamma$), is illustrated in Figure~\ref{fig:Neutron_Capture}.
+Neutron activation analysis (NAA) is a non-destructive technique, similar to PAA, used to measure trace elements in materials to within detection limits of 0.01 $\rightarrow$ 10 ppm~\cite{Pollard}. In NAA, the incident radiation particle is a neutron. Several different neutron sources are available, but only a few that offer the high neutron fluxes useful for NAA. These include reactors and accelerators. Since they have high fluxes ($10^{13} \frac{n}{cm^2 \cdot s}$) of neutrons from uranium fission, nuclear reactors offer the highest available fluxes~\cite{Glascock}. The sequence of events taking place during the most common type of nuclear reaction for NAA, namely neutron capture (n,$\gamma$), is illustrated in Figure~\ref{fig:Neutron_Capture}.
 [[File:Neutron_Capture.png | 400 px]]

@@ Line 52: / Line 52: @@
 In addition to PAA, there are several other analytical techniques used to quantify the elemental composition of materials. They each have their own associated advantages and disadvantages. For example, AAS and ICP methods chemically process a sample, usually by removing a portion of the material and destroying the sample. This is not an ideal technique when investigating materials such as artifacts and antiques, where cutting away pieces for a sample is not desireable. Additionally, there is a danger of introducing contamination during preparation procedures. However, unlike some techniques that are limited to surface studies, these two methods can be used for volume or bulk analysis. Further explanations on some of the more readily available techniques and their differences may be found in Appendix A.
-The focus of this thesis is on the detection limits of PAA and CPAA. For comparison, Table 1.1 lists several existing elemental analysis methods along with current detection limits. The detection limits  range from 0.1 to 100 parts-per-million (ppm or <math>\mu \mbox{g}/\mbox{g}</math>).   One elemental analysis method that is similar to PAA is Neutron Activation Analysis (NAA).  NAA uses neutrons to excite the nucleus instead of photons and, as seen in Table 1.1,  has a detection limit of 0.01 ppm that is similar to PAA.  Our particular focus is on improving PAA's detection limit, however, the coincidence method is easily used for NAA as well.  The physics of nuclear activation using PAA or NAA is described below.
+The focus of this thesis is on the detection limits of PAA and CPAA. For comparison, Table 1.1 lists several existing elemental analysis methods along with current detection limits. The detection limits  range from 0.1 to 100 parts-per-million (ppm or <math>\mu \mbox{g}/\mbox{g}</math>).   While the particular focus is on improving PAA's detection limit, the coincidence method can easily be used on a complimentary nuclear method, Neutron Activation Analysis (NAA). The physics of nuclear activation is described below.
 table of detection limits -vs- Method