GEM Hardware Exercise at the CMS Upgrade School in DESY (CUPS)

Introduction

Multiwire proportional chambers were once the workhorse of particle detection. Being one of the first "electronic" detectors developed at the end of the '60 of the previous age, they are now still around in modified versions: the Cathode Strip Chambers used in both ATLAS and CMS. These detectors have a high spatial resolution (~100um) and moderate rate capability (~kHz/cm2). Gas Electron Multipliers are modern Gas detectors developped to improve the rate capability to the level of (~MHz/cm2), by splitting the amplification of the signal inside the gas in several gaps. This reduced the screening effect caused by postively charged ions (hence the name "Electron Multiplier").

Goal

The goal of the GEM hardware exercise is to familiarize students with a lab environment, work with very sensitive detectors, a high voltage system, readout electronics and oscilloscopes. We will try to readout the signals induced by a radioactive source in a prototype GEM detector and to determine the gain of the signal inside the gas volume, which is an important quality test before detectors are accepted.

Hardware Exercise

The hardware exercise is based on the gain calibration test. This test is described in details in this twiki page and hence this page will be used as the main guideline to do the exercise. This test tries to measure the gain (amplification) of the signal inside the gas. By slowly increading the HV on the foils of the GEM detector, one will see the detected particle rate (due to a radioactive source) increasing until a plateau is reached. In this plateau the maximum efficiency is reached, and knowing this efficiency (~97%) one can measure the rate of the radioactive source. We expect this curve to be a sigmoid. The turn on curve can be heavily affected by noise in the detector. One of the best ways to characterize the noise is to look at signals of a well known source. For this we will use a radioactive source and look at the energy spectrum. Understanding the signal we would like to capture from the energy spectrum we can adapt the threshold settings for the counting of the particles. In the meanwhile we will also measure the currents induced on the read out strips.

The Hardware exercise consist of 2 steps:

• investigation of the energy spectrum of the radioactive source;
• the actual gain calibration test: counting the amount of particles detected and measuring the current induced on the readout strips

In the first part we will use the oscilloscope to readout and record the pre-amplified signals coming from the GEM detector. In the second part we will make use of a very sensitive instruments to measure the currents of the readout strips, which are of the level of nano ohm.

More information

GEM Training School

The hardware exercise is just an introduction to a school completely dedicated to the understanding the working principles of GEMs and to give a thourough introduction to the construction of the GEMs and quality tests performed during and after construction. The school has been organized already twice in CERN in 2014 and will probably organized more in the future. The link to the exercises of the GEM Training School is given here.

Lectures

• Fundamentals of Particle Detectors and Developments in Detector Technologies for future Experiments by Werner Riegler Slides can be found at this indico page

Literature

More information on Gas detectors in general in Gas Electron Multipliers in special can be found here:

-- PietVerwilligen - 2014-11-10