ThGEM Housing

a convolution of both Gaussian and Lorentzian distribution as shown in equation 2.10 below.

GL(x;F,E,m) =exp(−4ln2(1−m)(x−E) 2

F2 )/(1+4m

(x−E)2

F2 ) (2.10)

This line shape is the general form used for all peaks fitted in this thesis unless stated otherwise. A general nomenclature used is GL(x) where the number in the bracket shows the mixing where GL(100) is a pure Lorentzian line-shape and GL(0) is a pure Gaussian line-shape.

As mentioned previously if the sample to be measured has insulating properties, such as MgO & ZnO which are both wide band gap insulators, charging up of the surface can cause the binding energies in the spectra to be shifted. To account for this a charge correction can be done by taking a peak with well known B.E. and shifting the whole spectrum by the difference in the actual measured B.E. and the expected B.E. The adventitious Carbon 1s peak is most commonly used for this purpose.

As well as using binding energies to highlight the chemical states of various elements the modi- fied Auger parameter (α’) can also be used. Originally defined by Wagner [97], this parameter

is equal to the binding energy of the component of interest plus the kinetic energy from its re- spective Auger peak. As the kinetic energy of the Auger electron is independent of the photon energy, this parameter is unaffected by surface charging so therefore can be used as an effective marker for chemical states, especially when used in tandem with the binding energy measured. Another feature present in the spectrum is peak splitting due to energy differences between the singlet and triplet states via interaction of the spin and orbital magnetic moments. This feature occurs when there are unpaired electrons in valence shells. If the unpaired electron is parallel to the valence electron it results in a lower binding energy, or alternately a higher energy for anti-parallel spin. The relative intensity of the 2 peaks of spin-orbit couplet is determined by the 2J+1 multiplicity of the levels. For example for a 2p spectra, the area ratio for the two spin orbit peaks (2p1/2:2p3/2) will be 1:2 which corresponds with the fact there are 2 electrons in the 2p1/2 level and 4 electrons in the 2p3/2level.

2.4

ThGEM Housing

ThGEMs and other gaseous detectors need to be sealed in a noble gas or noble gas mixture. For most of our experiments a source of pure argon(99+%)gas is used. As such, a chamber needs to be made to hold and seal the GEM element whilst allowing UV light to enter without much attenuation. A UV transparent window is needed to allow UV photons through without much attenuation and the GEM element needs to have 2 connections: one to ground and one to a high voltage (HV) input.

One such chamber, which we will call the standard chamber, consists of two separate sections; a metal box to house the electronics and a front section which can be easily removed with a viton seal to keep the gas mixture in the chamber constant. On the back of the electrical box exists

2.4. THGEM HOUSING

an electrical connection which feeds into a data acquisition (DAQ) unit and gas entry to fill the front chamber with gas and subsequently seal it off using a tap. This setup is shown in figure 2.6.

Fused Silica was chosen as the material for the optical window into the chamber as it has high

Figure 2.6: Standard chamber for mounting and testing ThGEM elements

Figure 2.7: Transmission through fused silica window as function of incident wavelength transmission (>90%) for wavelengths above 220nm. The standard transmission spectrum of fused silica is shown in figure 2.7, as provided by Thorlabs.

2.4. THGEM HOUSING

at the top which feeds the induced current output into the amplifier is used to mount the ThGEM element into the chamber. The table needs to be highly insulating and high density PTFE is used. Different tables are available for different thickness ThGEMs, with length and width of 15mmx15mm. The HV input and current output lead into a DAQ which is controlled by a com- puter.

To investigate larger ThGEMs another chamber was also constructed. Effectively operated in the same way as the previous chamber, this chamber also consists of a quartz window allowing UV light to enter without much attenuation. The HV in this case is controlled by a separate programmable NIM panel HV source. The amplifier used is the same circuit as that used in the standard chamber. The ThGEMs can either be free standing directly soldered onto wires or in a table similar to the one on the standard setup just on a larger scale.

2.4.1 Amplifier Circuit

Figure 2.8: Basic Circuit diagram for amplifier circuit

As the output current is low, an amplifier is required to convert the small current from photo- generated electrons in the ThGEM to a readable voltage, it also needs to be resistant to sparks as the likelihood of one occurring can be quite high. The basic design is a current to voltage non-inverting amplifier as shown in figure 2.8, this basic circuit design is used in all of the dif- ferent chambers. As the input impedance for the Op-amp is very high most if not all the input current flows into the resistor R0, giving a voltage across the non-inverting input of the op-amp ofV+ =IinR0. Therefore we use a large resistance for R0 (1GΩ). And the ratio of the resis- tors R1 and R2 control the output voltage such thatVout=V+(R1+R2)/R1. For this setup the gain in the amp is calibrated to convert 1pA of current into a output signal of 10mV, therefore (R1+R2)/R1=10.