This chapter deals with briefly account of experimental apparatus, the strategy of the technique which is used and the Accelerator.
3.1. Pelletron Accelerator:
Pelletron Accelerator Model 6SDH-2 is a 2 Million Volt tandem electrostatic gas pedal [ 1 ] with a horizontal accelerating column. It is capable of speed uping different assortments of ion species where wide scopes of energies are used for PIXE analysis, ion nidation and atomic natural philosophies experiments. The gas pedal consists of two ion beginnings one is ( RF ) Radio Frequency plasma beginning and other is ( SNICS-2 ) i.e Source of Negative Ions by utilizing Cesium Sputtering. Presently two beam lines are available and five more beam lines can be installed. Schematic of the Accelerator is shown is the Fig. 3.1.
FIGURE 3.1. Schematic of an electrostatic Pelletron S tandem gas pedal ( mfd. By National Electrostatics Corp. )
3.2. RF ( radio frequence ) Plasma Beginning:
RF plasma beginning ions are generated from gases. Hydrogen gas is used for singly charge energy and He gas is used for alpha atoms. The beginning is called an inductively coupled RF ion beginning [ 2 ] . For the extraction of the ions, the attack is used by Thonemann et Al. [ 3 ] and consists of a positive W electrode-called the investigation at close terminal of the discharge tubing as shown the Fig3.2. Operating the beginning positive ion beam is extracted from plasma produced in RF beginning and accelerated into the charge exchange cell where some portion of beam is converted to negative ions which are extracted by the gas pedal to the desired energy.
Figure3.2: RF Charge Exchange Ion Source
3.3. SNICS ( beginning of negative ions by caesium sputtering ) -2 Beginning:
SNICS-2 was developed at the University of Wisconsin [ 4 ] . SNICS-2 is a various beginning that produces negative ions of over 70 different elemental ions can be chosen from the periodic tabular array. It sputter cathode beginning through extractor and produces ion beam which form stable negative ion. Cs vapour comes out from caesium oven towards the enclosed country between the cooled cathode and the het ionizing surface as shown in the Fig3.3. Some of caesium condenses and some caesium ionizes by the hot surface. The ionised caesium accelerates towards the cathode by sputtering atoms from the cathode through the condensed caesium bed. Hence as a consequence some stuff will sputter negative ions. Other stuffs will preferentially sputter impersonal or positive atoms which pick up negatrons as they pass through the condensed caesium bed bring forthing negative ions.
Fig:3.3 Source of Negative Ion by Cesium Sputtering
The negative ions from the ion beginnings are foremost preaccelerated and so guided to the gas pedal entryway by the injector magnet. The einzel lens assembly preaccelerates the negative ions from the ion beginning and focuses them towards the Pelletron armored combat vehicle.
3.4. Pelletron Charging Chain:
This Pelletron armored combat vehicle consists of two pellet ironss of metal links with nylon spacers supplying a agency of set uping the speed uping possible ( see Fig.3.4 ) . A 50kV power supply is used to bear down an inductance, which pushes negatrons off the links to the grounded thrust block. The links are now positively charged and they move to the terminus shell, a hollow aluminium cylinder which is about 1ft in radius and 2 foot in length. Electrons move from the terminus shell to the links by obeying Gauss ‘s jurisprudence and go forthing a net positive charge on the terminus shell [ 5 ] . The terminus shell is located near the center of the gas pedal organic structure shown in Fig. 3.1.
FIGURE 3.4. Accelerator Charging System ( Beginning: Pelletroni?? Accelerator Charging System ; hypertext transfer protocol: //www.pelletron.com/charging.htm )
3.5. Main Accelerator Unit:
The Pelletron armored combat vehicle of the gas pedal consists of a figure of speed uping columns on each side of the terminus. Each column consists of a brace of hollow round aluminium casting supported by ceramic dielectrics. The cardinal portion of the tubing is the high-potential terminus. It is spherical in form and it is charged by motor driven ironss. The initiation electrodes induce the charge onto the concatenation at the base of the armored combat vehicle. This charge is so deposited on the terminus, thereby raising its possible. The high possible terminus is supported by insulating columns dwelling of two insulating home bases. The ions go throughing through the terminus are made to go through through the gas stripper which changes the negative ions into positive ions. The positive ions are farther accelerated in the gas pedal column raising the energy of the beam to ( 1+q ) V.
To stabilization the beam energy, the terminal electromotive force is stabilized by a feedback system. The provender back signal taken from the capacitive pickoff home base, on the control slit after the analysing switching magnet, is combined with the absolute electromotive force signal [ 6 ] . The complete gas pedal column that contains the charging system and gas pedal tubing is enclosed within a force per unit area vas filled with SF6 gas at a force per unit area of 80 pounds per square inch. The SF6 is chosen because of its first-class dielectric strength [ 7 ] .
As the ions leave the gas pedal tubing, they pass through a magnetic quadrupole lens, which focuses the beam to a little diameter ( 1cm or less ) . Then the ions enter the beam energy exchanging magnet.
The beam energy-analyzing magnet plants on the rule of flexing the flight of a charged atom traveling through a magnetic field. As we know, a traveling charged atom has kinetic energy ( Ep ) equal to A? ( mv2 ) , where m is the atom ‘s mass and V is the atom ‘s speed. A charged atom traveling through a magnetic field experiences a magnetic force ( Fb ) , which changes the way of its travel. This alteration of way creates a centripetal force ( Fc ) . By comparing the two forces, the look ( mv2 ) /r = qvB is obtained. Solving this equation and besides one for kinetic energy for V, we derive the equation, Ep = ( qrB ) 2/ ( 2m ) , where Ep is the energy of the atom, Q is the charge on the atom, m is the atom ‘s mass, B is the strength of the magnetic field, and R is the radius through which the atom ‘s flight is dead set. [ 8 ]
As the protons exit the shift magnet, they move into the beam line, an evacuated tubing connected to the mark chamber. The atoms moves with the proper flight can be determined by their kinetic energy, q/m ratio, and the magnetic Fieldss proceed down the centre of the beam line. Before go throughing through the mark chamber, the atom enters from perpendicular slit which is two horizontal insulated strips. Atoms with more or less energy move to the inner or outer borders, severally, of the beam line, meeting the slit strips in the procedure. The slit strips, therefore, command the beam energy divergency harmonizing to the breadth of the slit gap and besides controls the neutron splitting. The atoms so proceed to the reaction chamber where they bombard the samples to be analyzed.
3.6. Target Chamber:
The chamber is enclosed by the evacuated ( ~10-5 Torr ) country where the ion beam strikes the mark. It is made up of steel provided with Windowss and ports to give information about the charged atoms. The Si ( Li ) sensor is connected to the beam line through port of the chamber.Fig.3.5 shows the mark chamber of PIXE.
Fig.3.5. Target chamber of PIXE analysis.
The marks are normally placed at 45a?° to the beam way. Thick marks are mounted on the mark holder placed at the Centre of the chamber. The mark holder can be travel about its axis externally. Since the X raies are detected by the Si ( Li ) sensor, the chamber is so designed that by utilizing a rim, the terminal country of the sensor can be inserted through the rim so that the distance from the mark can be changed. Since the mark chamber is extremely evacuated ( ~10a?’5 Torr ) , it is preferred to fix a revolving mark holder on which many different samples can be loaded at any clip and one must be in a place to put a peculiar mark confronting the beam by traveling the mark holder from outside manually.
It is non necessary require to vent the vacuity chamber after each sample Run. The mark holder assembly is a pyramid type made of unstained steel. The place of the mark can be determined from the outer window. The mark holder could besides be manually rotated from exterior in order to point the marks at the coveted angle with regard to the beam way.
3.7. Detection of X raies
Therefore X raies are detected by Si ( Li ) energy diffusing system. Detector type is Nano hint equipped with NORVAR window. The active country of the sensor is 30 mm2 and Si crystal thickness is 3 millimeter. Aluminum coated window of thickness 0.04 Aµm is equipped with the sensor. X-ray pulsation processor is a complete sensor supply which is connected to the sensor with a individual overseas telegram and it besides controls temperature.
The ORTEC Amplifier is used for its first-class public presentation connected to the pulse processing unit. Data acquisition diagram in Fig.3.6 shows that it acquires the processing unit, pre amplifier and multi channel analyser ( MCA ) .
Fig 3.6: Block diagram of informations acquisition.
3.8. Gupix Software
The package bundle GUPIX [ 9 ] was used specifically to analyze the PIXE spectra from thick specimens. It provides nonlinear least-squares adjustment of the spectrum together with subsequent transition of the fitted X-ray extremum strengths to elemental concentrations with a defined standardisation technique affecting cardinal parametric quantities and a user determined instrumental invariable. Full history was taken of matrix effects and secondary fluorescence parts in both the spectrum suiting part and the computation of concentrations. The GUPIXWIN 2000 version is used in over research lab.
[ 1 ] R.Hellborg ( Ed. ) Electrostatic Accelerators Fundamentalss and Applications.
[ 2 ] B.Wolf: Handbook of Ion Sources ( CRC Press, Boca Raton, 1995 ) .
[ 3 ] P.C Thonemann et Al. : Proc. Phys. Soc. 61, 483 ( 1948 ) .
[ 4 ] G.T. Caskey, R.A.Douglas, H.T. Richards, H.V. Smithe Jr. : Nucl. Instr. Meth. 157, 1 ( 1978 ) .
[ 5 ] Pelletroni?? Accelerator Charging System, hypertext transfer protocol: //www.pelletron.com/charging.htm.
[ 6 ] H.R.Verma, Atomic and Nuclear Analytical Methods.
[ 7 ] A.H. Cookson: Proc. IEE A 128, 303 ( 1981 ) .
[ 8 ] Waldemar H. Scharf, Particle Accelerators- Applications in Technology and Research ( Research Studies Press Ltd. , Somerset, England, 1988 ) .
[ 9 ] J.A. Maxwell, J.L. Campbell and W.J. Teesdale. Nucl. Instr. and Meth. B 43 ( 1989 ) 218.
RESULTS AND DISCUSSIONS
As we know that experimental L-shell X-rays cross subdivision informations are scattered through single publications and some of those are reported merely in graphical signifier. This makes a systematic comparing with theories or among publications instead hard. Therefore, a complete and systematic survey of all available L shell cross subdivisions similar to that performed by Paul and colleagues [ 1-5 ] for the K shell, is severely needed.
The experimental informations are straight fitted to infer the empirical L X ray production cross subdivisions. A comparing is made between the empirical cross subdivisions reported in this work to the empirical 1s reported by “ Reis and Jesus ” [ M.A. Reis, A.P. Jesus, Atom. Data Nucl. Data Tables 63 ( 1996 ) 6 ] and besides with the corrected ECPSSR theory [ 7 ] .
In this field the first part is by Orlic et Al. [ 8 ] who reported empirical expression for the computation of empirical ionisation cross subdivisions for protons. Other of import part as mentioned earlier is reported by Reis and Jesus [ 6 ] who tried to execute the same process for the L X-ray production cross subdivisions as done by Paul [ 9 ] for the K lines. Strivay and Weber [ 10 ] reported empirical expression, which is used in this thesis based on the direct adjustment of experimental L X ray production cross subdivisions.
In this work, I report on the finding of the L X-ray production cross subdivisions by 0.5-3.5MeV proton impact on elements “ Tungsten and Gold ” . L shell X-ray spectrum of the Au is shown in the Fig4.1.
Fig.4.1. X-ray spectrum demoing L lines of Au at 1.5 MeV.
To find dependable, empirical L X ray production cross subdivisions, the ideal state of affairs is to execute the adjustment of the experimental information for each component individually at different proton energies. The database in the present work relies on the compiled experimental informations for proton impact is the same as that of Strivay and Weber.
To compare our consequences, those of Reis and Jesus and of ECPSSR intervention for the first portion we calculate them by the sets of coefficients reported by the writers and the 2nd portion D.D Cohen corrected ECPSSR values are taken. The divergences or alterations between the empirical one diagrammatically shown in Fig 4.2 and 4.3, which explains us the L X-ray production cross subdivisions normalized to their corresponding ECPSSR consequences as a map of proton beam energy. In this thesis, I have pointed out the different spread of the informations in each instance.
FIG 4.2.L X-ray production cross subdivisions from this work compared to those of Reis and Jesus [ 1 ] as a map of the proton energy for W. ( Black box symbol shows this work and trigon shows the Reis and Jesus. All these transverse subdivisions normalized to their corresponding ECPSSR theory.
FIG 4.3.L X-ray production cross subdivisions from this work compared to those of Reis and Jesus [ 1 ] as a map of the proton energy for Au.
The information Numberss which are used for the empirical cross subdivision which contains most of the LI± and LI? lines because of the fact that LI± and LI? are the outstanding L X ray lines and matching cross subdivisions are measured right instead comparing with the LI? lines.
By comparing the empirical attack in this work, I notice that the consequences reported by the expression agree by and large for the LI± , LI? and LI? lines for the elements W ( Z = 74 ) and Au ( Z = 79 ) . The consequences are compared with theory and with the empirical one are shown in Fig4.4 ( a ) , ( B ) , ( degree Celsius ) and ( vitamin D ) . The advantage of the empirical method is that it non requires an ECPSSR plan.
( I ) LI± subshell the understanding between our empirical cross subdivisions with Ries and Jesus and besides with the theoretical ionisation cross subdivisions is first-class for the whole scope of the proton energy for both elements W and Au.
( two ) LI? subshell the state of affairs is similar to that of the LI± subshell for the empirical cross subdivisions. The understanding between the Ries and Jesus of empirical cross subdivisions is rather satisfactory. In contrast, the divergence from the ECPSSR cross subdivisions increases with diminishing proton energy. The empirical cross subdivisions exceed the theoretical 1s at 0.5 MeV proton energy.
( three ) LI? subshell the consequences are less satisfactory as big disagreements are observed for both elements W and Au. The values of Ries and Jesus are consistently the highest and divert significantly for Au but are close to the W. On the other manus, our empirical values are closest to those of Ries and Jesus, which are expected to be the most dependable consequences. In this instance it is hard to pull any decision but I believe that the hapless understanding observed depends more strongly on the spread of the experimental informations than on the figure used in the adjustment.
( a )
( B )
FIG.4.4.Plots of LI± , LI? and LI? subshell ionisation cross subdivisions of elements W and Au. The empty boxes are ECPSSR informations and the curves are the fitted by the Eq. ( 2 ) . Fig ( a ) , ( B ) , ( degree Celsius ) and Fig ( vitamin D ) .
( degree Celsius )
( vitamin D )
This statement is confirmed by the fact that in malice of the about same Numberss of experimental information we used for the three L subshells, the consequences are less and less good when traveling from LI± to LI? subshell. It must be noticed once more that although the ECPSSR computations and the experimental informations, they still remain slightly far to acquire any understanding with either our empirical cross subdivisions or those reported by the other writers [ 8 and 12 ] . This ascertainment points out the demand for more surveies for better understandings.
In this thesis, one study on LI± , LI? and LI? subshells of empirical cross subdivisions for elements with atomic Numberss 74 and 79 for protons of energy 0.5-3.5 MeV. However, because of the sprinkling of the experimental information for the LI? subshell, the theoretical ionisation cross subdivisions can non be deduced every bit dependable as in the instances of the LI± and LI? subshells. But on the other manus, Reis and Jesus values for the LI± and LI? lines are in good understanding with the empirical cross subdivision values for the whole energy scope. But state of affairs for the LI? bomber shell is still rather unsure and more work should be done on its betterment.