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SEcondary Electron TRAnsmission Monitor



At relativistic energies, the usual method to determine heavy-ion beam intensities by measuring the electric current in a beam stop is not easily applicable due to the long range of charged secondary reaction products that emerge from the stopping process. At the FRS, a secondary-electron transmission monitor (SEETRAM) is used to survey the beam intensity almost without influencing the beam quality. Most of the secondary electrons have energies of a few eV. The number of electrons per ion at 1 A GeV is roughly 
ne » Z2 / 40. As the secondary electrons originate from a very thin surface layer so that even at high beam intensities, space-charge effects are not expected. Therefore, the secondary-electron current is assumed to be exactly proportional to the primary-beam current.

SEETRAM and the associated equipment installed installed at the FRS target station provide valuable information on the following topics:

Spill structure 
For most experiments it is important that the primary-beam intensity is distributed as homogeneously as possible over the extraction time in order to avoid unnecessary pile-up rates and dead-time losses. A fast monitoring of the beam intensity over the extraction time allows determining the extraction profile. This information helps finding the optimum tuning of the SIS extraction.
Extraction efficiency
The extraction losses lead to activation in the extraction zone. Therefore, the radioprotection service may demand a reduction of the beam intensity if the extraction losses exceed a certain level. The extraction efficiency can be determined by comparing the absolute intensity of the extracted beam, integrated over one spill, with the current determined inside SIS before extraction. For this purpose, the beam monitor needs to be calibrated.
Normalisation of production cross sections
For determining absolute production cross sections one needs to know the total number of projectiles. For this purpose it is necessary to register the beam intensity with a calibrated beam monitor continuously during the whole experiment.


Detector description 

SEETRAM operation is based on the emission of secondary electrons from thin metal foils by the passage of the projectiles. It consists of one titanium foils of 10 microm thickness sandwiched between two aluminium foils of 14 microm thickness each (see Figure 1). Each foil has an diameter of 11.5 cm. They are mounted perpendicular to the beam axis. The outer foils are connected to a voltage of +80 V. They and their supporting aluminium rings form the detector housing. The middle foil is supported by two Teflon rings and is insulated against other parts of the detector. The foils are curved in order to reduce the sensitivity to mechanical vibration of the beam line. Secondary electrons emitted from the middle foil are collected by the two outer foils. The created current in the middle foil is measured by a Current Digitiser CD1010 developed at GSI [1].  The sensitivity of the Current Digitiser is computer-controlled from the control room with the use of  the NODAL programme[1]. SEETRAM sensitivity ranges from 10-4 to 10-10 Ampere full scale. The full scale current produces 1 V at the monitor output and 10 KHz at the digitised output. This means that 1 count corresponds to a charge of Q = sensitivity×10-4 Coulomb.



Figure 1. Layout of SEETRAM [1]. The outer foils are supported by aluminium rings (1, 2, 5, 6). The inner foil is supported and insulated by two Teflon rings (3, 4).

The detector device name in the beam diagnostic program (SD-Anwahlprogramm) is TS1DI4SP. 


Limits of application 

The lower limit of the SEETRAM application is given by the condition that the produced current must be higher than 10-12 Ampere in order to distinguish the signals from the positive offset of the Current Digitiser (the same is true for the current signal from the IC01). The corresponding beam intensity depends on the projectile charge and energy, see figure 20 in ref. [1]. There is no upper limit for the SEETRAM current. The SC01 is applicable up to  counting rates of 105 particles/s where saturation effects start to be higher than 1 %. The particle counting of the IC01 is limited to 104 particles/s due to pile-up in the preamplifier. Also, the particle-counting with the IC01 is not suitable for ions with Z < 10, because the energy-loss signals could not be distinguish from the electronic noise. The upper limit of the current signal from the IC01 is 10-7 Ampere where the recombination losses become higher than 10 %.  


Possible problems during the measurements

- If the beam is not centred on the target, there is a question if all the projectiles are measured by the SEETRAM, SC01 and IC01. One should check the beam position using the current grids before the calibration.

- SEETRAM can only be calibrated in the slow extraction mode. In the case of the fast extraction, the time structure of spills cannot be seen, but the integration of the secondary-electron current should work. Moreover, there is also a danger that the Current Digitiser is saturated because of the presence of short, high intensity pulses. For this reason, in the case of fast extraction, one should insert a filter with a time constant in the order of 1 s between the detector and the input of the  Current Digitiser.

- The offset of the Current Digitiser must be tuned high enough to ensure that the digital output of the Current Digitiser never stops. If this happens, any information on the magnitude of the current during this time is lost.

- One should compare the number of particles on SEETRAM with those in SIS (ask operators for beam transformator value). For longer runs it has to be at least 70%, otherwise the radioprotection service may demand a reduction of the beam intensity because of too high extraction losses.



[1] Christine Ziegler, ‘Aufbau und Einsatz eines Sekundärelektronen-Transmissions-Monitors zur Messung de absoluten Teilchenstroms am Fragmentseparator’, Diplomarbeit TH Darmstadt, October 1992.

[2] B. Jurado, K.-H. Schmidt, K.-H. Behr, ‘Application of a secondary-electron transmission monitor for high-precision intensity measurements of relativistic heavy-ion beams’, Nucl. Instr. Meth. A 483 (2002) 603.

[3] A. Junghans, H.-G. Clerck, A. Grewe, M. de Jong, J. Müller, K.-H. Schmidt, ‘A self-calibrating ionisation chamber for the precise intensity calibration of high-energy heavy-ion beam monitors’, Nucl. Instr. Meth. A 370 (1996) 312.


[1] How to use the NODAL programme:

1) load the programme NODAL
2) run INTMON
3) choose the detector type (3)
4) choose the SEETRAM name (3)
5) choose the virtual accelerator (8)
6) choose 20 to check
7) choose 22 to change the SEETRAM sensitivity.



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