JLAB Positron Source DOE Award DE-SC0002600

Award Info

8/15/09 - 8/14/2012

Funds Obligated: $381,509

Funds Requested: $411,509

Sponsored Project Proposal Number : 8-17

DOE Award Number : DE-SC0002600

Accounts set up on 10/14/09

Banner Index : RACL39 Account Numer : 684-143-40

Fund: X10165, Org code: 640000, Program : 040R0

Final Report

DOE Award #: DE-SC0002600

Project Title: The Development of a Positron Source for JLab at the IAC

Date: 8/1/13

Period covered: 8/15/09-8/14/2013 (Final Report)

Use the URL below to submit the final report

http://www.osti.gov/elink-2413

Accomplishments

The performance of positron source using a quad triplet collection system has been measured using the support from this award. The first year of research accomplished its goal of performing preliminary measurements of positron production and found that efficiency to be [math] 5 \pm 1 \times 10^{-15}[/math] positrons per incident electron. The second year of work focused on designing an achromat beam line along with a tungsten target to optimize the production of positrons with an energy dispersion [math] (\Delta E/E)[/math] less than 30 \%. The third year measured the performance of a linac based, achromatic positron source and observed a chromaticity of 60\% while improving the positron production efficiency measured in year one by more than a factor of two. Based on this work, we believe that the low positron efficiency observed can be greatly improved by placing the production target within a magnetic collection field instead of outside.

Initial Positron production measurement

An initial set of positron production measurements were performed at the Idaho Accelerator Center (IAC) to evaluate the signal to background level and detector performance. Figure ~\ref{fig:Year1BeamLine} depicts the beam line used for these measurements. A 25 MeV linac, pulsed at 300 Hz, was used to accelerate electrons to 10 MeV. The electrons were bunched into 100 ns wide pulses with a peak current of 40 mA and transported to a 2 mm thick tungsten target located between two dipoles. The second dipole was set to transport 3 MeV electrons or positrons to a shielded cell that housed an HpGe and NaI detector.

The beam line used for an initial set of positron production measurements	A picture of the HgPe detector entrance and Farraday cup.

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

A positron transported to the shielded cell would impinge onto a 6mm thick Tantalum foil where it would annihilate and produce two photons. An high purity germanium (HpGe) detector and an Sodium Iodide (NaI) detector were place at 90 degree angles with respect to the Tantalum target in order to detect the characteristic 511 keV photon that would emerge back to back when a positron annihilates after thermalizing in the Tantalum. The analog output of both detectors was measured using a multi-channel analyzer, model MPA-3 manufactured by FAST ComTec Communication Technology GmbH, equipped with a 12 bit ADC.

Figure~\ref{fig:IACpositronSpec} shows the photon energy spectrum observed by each detector. A clear peak at a photon distribution was observed using the HpGe detector while the NaI spectrum was less clear. A permanent magnet was inserted to deflect charged particles away from the Tantalum annihilation target. Figure~\ref{fig:IACpositronSpec} shows the reduction in the 511 keV peak observed by the HpGe detector when the magnet is inserted. Although the HpGe detector calibration was off by 15 keV, the magnet indicated that positively charged particles were being produced and transported to the shielded cell. We did not have the diagnostic equipment needed to determine if those particles were from the Tungsten converter target or created somewhere along the beam line. Our next measurement would use retractable targets to determine the origin of the positively charged particles producing the 511 keV photons.

The observed photon energy distributions as measured by the HpGe (top) and NaI (bottom) detectors. There was no coincidence requirement using the two detectors.	The HpGe photon energy distribution before and after a sweep magnet is used to deflect charged particles away from the annihilation target.

The design of a linac based positron source

The first year's research experience was used to design a beam line specifically for use as a positron source. The first step was to add a system of quads and steering magnets to the High Repetition Rate Linac (HRRL) located in the Physic's beam lab. The second step was to measure the emittance of the linac in order to simulate the beam line and optimize positron transport.

Beamline Optimization

HRRL beamline for positron generation.	A picture of the HRRL positron beam line.

The HRRL's accelerator cavity was relocated to the position shown in the Fig.~\ref{fig:app-hrrl-line} to provide enough space for a beam line that can transport either positrons or electrons. The beam elements are described in Table~\ref{tab:app-hrrl-coordinates}. Quadrupole and dipole magnets were added to the new beam line as well as diagnostic tools like an optial transmission radiator (OTR) and YAG screens, Faraday cups and toroids were installed to measure the electron beam size and the current. Energy slits were installed to restrict energy/momentum spread of the beam after the first dipole. A retractable tungsten foil target (T1) was placed between the 1st and 2nd triplets and used produce positrons when the electron beam interacts with it. The room where the HRRL is located is divided by a wall into two parts; the accelerator side and the experimental cell. A beam pipe at the end of the 90 degree beamline goes through a hole on the wall and delivers the beam from the accelerator side to the experimental cell. The positron detection system consisting of two NaI detectors was placed at the end of the beamline in the experimental cell side as shown in the Fig.~\ref{fig:app-hrrl-line}.

HRRL emittance measurements

Images from the JAI camera were calibrated using the OTR target frame. An LED was used to illuminate the OTR aluminum frame that has a known inner diameter of 31.75~mm. Image processing software was used to inscribe a circle on the image and measure the circular OTR inner frame in units of pixels. The scaling factor can be obtained by dividing this length with the number of pixels observed. The result is a horizontal scaling factor of 0.04327~\pm~0.00016~mm/pixel and vertical scaling factor of 0.04204~\pm~0.00018~mm/pixel. Digital images from the JAI camera were extracted in a matrix format in order to take projections on both axes and perform a Gaussian fit. The observed image profiles were not well described by a single Gaussian distribution. The profiles may be described using a Lorentzian distribution, however, the rms of the Lorentzian function is not defined. The super Gaussian distribution seems to be the best option~[11], because rms values may be directly extracted.

Fig.~\ref{par-fit} shows the square of the rms (\sigma^2_{s}) vs k_1L for x (horizontal) and y (vertical) beam projections along with the parabolic fits using Eq.~\ref{par_fit}. The emittances and Twiss parameters from these fits are summarized in Table~\ref{results}.

Fig. Square of rms values and parabolic fittings. .

Fig. Square of rms values and parabolic fittings. .

Parameter	Unit	Value
projected emittance [math]\epsilon_x[/math]	[math]\mu[/math]m	0.37 [math]\pm[/math] 0.02
projected emittance [math]\epsilon_y[/math]	[math]\mu[/math]m	0.30 [math]\pm[/math] 0.04
[math]\beta_x[/math]-function	m	1.40 [math]\pm[/math] 0.06
[math]\beta_y[/math]-function	m	1.17 [math]\pm[/math] 0.13
[math]\alpha_x[/math]-function	rad	0.97 [math]\pm[/math] 0.06
[math]\alpha_y[/math]-function	rad	0.24 [math]\pm[/math] 0.07
micro-pulse charge	pC	11
micro-pulse length	ps	35
energy of the beam E	MeV	15 [math]\pm[/math] 1.6
relative energy spread [math]\Delta E/E[/math]	%	10.4

HRRL energy spread

Energy scan was done to measure the energy profile of HRRL at nominal 12 MeV. A Faraday cup was placed at the end of the 45 degree beamline to measure the electron beam current bent by the first dipole. Dipole coil current were changed by 1 A increment and the Faraday cup currents were recorded. The scan results with corresponding beam energies are shown in the table below. The relation between dipole current and beam energy is given in the appendix.

The energy distribution of HRRL can be described by two skewed Gaussian fit overlapping.

The beam energy spread does not follow Gaussian distribution, but the overlapping of two skewed Gaussian found to be the best description of beam energy profile Media:Beam_Distributions_Beyond_RMS.pdf.

parameter	Notation	First Gaussian	Second Gaussian
amplitude	A	2.13894	10.88318
mean	[math]\mu[/math]	12.07181	12.32332
sigma left	[math]\sigma_L[/math]	4.46986	0.69709
sigma right	[math]\sigma_R[/math]	1.20046	0.45170

Positron converter target

A picture of the rotating target motor with cooling lines.

Positron Production Performance

A drawing fo the NaI detectors used to measure 511 keV photons from the annihlation of positrons by T2.	A picture of detectors along with modified bases.	A calibrated energy distribution using a Co-60 and Na-22 button source.

run in: 3735
Singles spectra for each detector with and without the annihilation target inserted

This a normalized spectrum (no background subtraction).

run in: 3735
Coincidence spectra.

positron rate: [math] e^+_{rate} = 0.259 \pm 0.016 ~ Hz[/math].

Things not accomplished

No Positron workshop but rather a PePPO collaboration meeting

Used OTR to measure emmittance.

Positron current was too low for a FC measurement