Sadiq Proposal Defense

Abstract

I propose to measure the positron production efficiency for a positron source that uses a quadrupole triplet system to collect positrons from a Tungsten target that are produced when the target is impinged by electrons from the High Rep Rate Linac (HRRL) at Idaho State University's (ISU) Idaho Accelerator Center (IAC). Positrons were observed in May of 2008 at the IAC without the use of a quad triplet collection system. Positrons escaping the downstream side of the Tungsten target have a wide momentum spread of 0 to 2 MeV when using a 10 MeV electron beam and a large divergence of [math]\pi[/math] rad. A quad triplet collection system can focus the positron beam and as a result increase our positron production efficiency. I will install the collection system and associated beam line components to measure the positron production efficiency using the HRRL.

Introduction

I propose to measure the positron production efficiency for a positron source that uses a quadrupole triplet system to collect positrons from a Tungsten target that are produced when the target is impinged by electrons from the High Rep Rate Linac (HRRL) at Idaho State University's (ISU) Idaho Accelerator Center (IAC). Polarized Positron source, as a new probe to explore nuclear and particle physics at JLab, is being studied at the CEBAF injector. While their main mission is to measure and increase polarization, in ISU, we want to explore methods to increase positron production efficiency. On the other hand, positron beamline at ISU is also potential tool for more nuclear physics studies. I have 4 NaI detectors ready for positron test, and I have measured emittance of the electron beam for beamlien optimization. I will install the collection system and associated beam line components to measure the positron production efficiency using the HRRL.

Previous Measurements

Earlier measurements were conducted at Idaho Accelerator Center of ISU, May of 2008. Setup are shown in Fig.2. Accelerator was operated at 300 Hz repetition rate, and 10 MeV energy. As shown is Fig.2 , The electron was bent by first dipole, and sent to a 2 mm thick tungsten target. Positrons produced focused by the two quadrupoles and bent 45 degree by the second dipole which was set for 3 MeV positrons. Positrons then transported to the the end of the linac where they annihilated in the Ta target. 511 keV photons were observed in both HpGe and NaI detectors. In the Fig.2 and Fig.3, the spectrum was taken over 600 seconds.

Item	Description
Tantalum Foil	6 mm thick 20 mm x 20 mm area
Tungsten Foil	2 mm thick 20 mm x 20 mm area
Phosphorus Flag	1 mil aluminum backing
Media:HpGe_Crystal_GEM-60195-Plus-P.pdf	81.3mm Diameter, 55.5mm Length
NaI detector

Fig.1 Setup for 2009 run.

Fig.2 Spectrum from HpGe Detector and NaI detecotrs.

Fig.3

Proposed Beamline

I propose a measurement of the positron production efficiency using the High Repetition Rate Linac (HRRL) located at the Beam Lab of the Physics Department, at Idaho State University (ISU). HRRL is a S-band electron linac located in the Beam Lab of the Physics Department at Idaho State University (ISU). It is one the 15 small size linacs dedicated for nuclear application operated by the IAC. HRRL can provide electron beam with energies between 3 MeV and 16 MeV, and Maximum repetition rate of 1 kHz. HRRL beamline had recently been reconfigured to generate and collect positrons, while it still can provide electron beam with improved quality. More details about HRRL is shown in Table 1.

Beam Energy	Max Peak Current	Repetition Rate	Max Pulse Length
16 MeV	80 mA	1 kHz	250 ns (FWHM)

New beamline first designed by Dr. G. Stancari, it uses quadrupole triplet system to collect positrons[1]. The design further optimized by Dr. Y Kim, and J.Ellis. Beamline is constructed, as shown in Fig.1, in the Beam Lab which is located in the basement of ISU physical sciences complex. Beam Lab is divided into two parts by a L-shaped cement wall. The accelerator cell houses the cavity and magntic elements needed to transport electrons to an experimental cell. The experimental cell is located in an adjacent room to the accelerator cell. The HRRL beamline was reconfigured into an achromat by moving the accelerator cavity to accomdate two dipoles and a system of quadrupole magnets optimized for collecting positrons.

In the new beamline, the electron beam exits the HRRL cavity and passes through first the set of quadruple triplet magnets which will be used to focus the electron beam onto the positron target. Positrons produced from the positron target will be collected by second set of quadruple triplet that will be optimized to collect positrons. The first dipole magnet bends the positrons or electrons, depending on the polarity setting, by 45 degrees towards a second dipole magnet. The second dipole will bend the beam another 45 degrees, thus completes a 90 degree bend. A third quadruple triplet will be used focus the e-/e+ beam, as users desired.

Fig.4 HRRL beamline for positron generation.

Item	Description
T1	Positron target
T2	Annihilation target
EnS	Energy Slit
FC1, FC2	Faraday Cups
Q1,...Q10	Quadrupoles
D1, D2	Dipoles
NaI	NaI Detecotrs
OTR	OTR screen
YAG	YAG screen

Positron annihilation target at the end of the beamline is set up for the measurement of the positrons created. When positrons annihilated at the positron target, two 511 keV photons will be created. Photons go back to back, and isotropically. The two detectors placed closely to the to the target, will used to detect these 511 keV photons.

Preparation for the Positron Production Experiment

HRRL Emittance measurements

Emittance is an important parameter in accelerator physics. If emittance with twiss parameters are given at the exit of the gun, we will be able to calculate beam size and divergence any point after the exit of the gun. Knowing the beam size and beam divergence on the positron target will greatly help us study the process of creating positron. Emittance with twiss parameters are also key parameters for any accelerator simulations. Also, energy and energy spread of the beam will be measured in the emittance measurement.

Optical Transition Radiation (OTR) emittance measurement was carried out on HRRL on March of 2011 with a 10 [math]\mu m[/math] thick aluminium screen. Transition radiation is emitted when a charge moving at a constant velocity cross a boundary between two materials with different dielectric constant. Emitted photons are observed on the camera give us information about the size of the electron beam.

We used quadrupole scanning method to measure emittance. In this method we turn off all the quads except one we use for scanning, we change quad current and we observe beam spot changes. In the Fig.5-7 are shown beam spots for quadrupole coil currents.

Fig.5 OTR image of 0 Amp Q1 coil current.

Fig.6 OTR image of +1 Amp Q1 coil current.

Fig.7 OTR image of +2 Amp Q1 coil current.

I used the MATLAB to analyze the data. The results shows that:

[math] \sigma_x^2= (3.678 \pm 0.022) + (-4.17 \pm 0.22)k_1L + (5.55 \pm 0.42)(k_1L)^2 [/math]

[math] \epsilon_x = 0.417 \pm 0.023~mm*mrad ~\Rightarrow~ \epsilon_{n,x} = 11.43 \pm 0.64~mm*mrad[/math]

[math] \beta_x=1.385 \pm 0.065, \alpha_x=0.97 \pm 0.07 [/math]

[math]\sigma_y^2 = (2.843 \pm 0.044) + (1.02 \pm 0.52)k_1L + (3.8 \pm 1.2)(k_1L)^2 [/math]

[math] \epsilon_y = 0.338 \pm 0.065~mm*mrad ~\Rightarrow~ \epsilon_{n,y} = 9.3 \pm 1.8~mm*mrad[/math]

[math] \beta_y=1.17 \pm 0.19, \alpha_y=0.22 \pm 0.10 [/math]

Positron Detection using NaI crystal

To detect positrons created, I want a put Ta target at the end of 90 degree beamline as my Annihilation. When positrons hit W-target, 511 keV photons will be created. I want to use NaI detectors to detect these 511 keV photons.

I acquired some NaI crystals from Idaho Accelerator Center (IAC). I built our own PMT bases for them, since their own bases not working properly. I modified the design of model PA-14 from Saint-Gobain crystals & detectors ltd. Now these detectors are tested and calibrated, and ready to use for the measurement.

Fig. 8 The NaI detector and base built.	Fig. 9 Detector 3 calibrated Spectrum.

Even though now the 511 keV peak seems to be very wide and resolution is low, But this can be improved by doing coincidence of two detectors in the experiment.

Future Plan

We can achieve few improvements compared to the 2008 experiments

1. By applying a quadrupole triplet before tungsten target, we will have control over the beam size and divergence at the target, whereas we don't have control over beamsize and divergence.

2. Cryogenically converter will be able to take more beam power, increase positron yield.

3. Positrons are collected by the quadrupole triplet system, whereas before there were two quadrupoles.

4. Simulations will be conducted to optimize beam elements for positron collection.

5. Chromox target as annihilation target has better annihilation efficiency than Ta.

Positron Target Installation

The step motor is shown in the figure will hold 8 tungsten targets. This moter will be placed in a chamber currently being manufactured.

Fig. 10 Step motor.

Measurement of the Positron Production Efficiency

I will insert the first tungsten target and create positrons. I am expecting to collect part these positrons and transport them down the second tungsten target at the end of the 90 degree beamline. By doing these, 511 keV photons will be created and I want to detect them by our NaI detectors.

References

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File:Emittance.tex