Setup

Unlike most other particle beams, PSI provides a continuous beam with an LHC-like bunch structure of $50\,\rm MHz$. Since LHC and the APVs are only clocked with $40\,\rm MHz$, a dedicated PLL-based NIM module has been developed, which derives a synchronous $40\,\rm MHz$ signal from the PSI clock. Moreover, this module produces a short SYNC pulse when both clocks are in phase, which occurs every $100\,\rm ns$ (fig. ). The SYNC pulse is used to select particle triggers which are in phase with the APV clock. By this synchronization, four out of five possible triggers are discarded, but it is ensured that passing triggers are in phase with the APV clock. Thus, the system environment is very close to what it will be in CMS.

Figure: The synchronization module derives a $40\,\rm MHz$ clock (cyan) and SYNC pulses (purple) out of the $50\,\rm MHz$ PSI clock (yellow).

Normally, the beam was set to low intensity at the order of $10\,\rm kHz\,cm^{-2}$ with a particle trigger derived from a scintillator with an area of $12\times 12\,\rm mm^2$, watched by two photomultiplier tubes. The photomultipliers were equipped with preamplifiers and operated at relatively low voltage to avoid saturation at high beam intensity. Several dedicated runs were taken at a particle rate of up to $2.5\,\rm MHz\,cm^{-2}$. At this intensity, it is not necessary to use the scintillator, because every bunch is filled with 2.5 particles in average. Thus, the SYNC pulse alone is good for triggering at high intensity. In that case, the measured beam profile is no longer restricted to the area covered by the scintillator, but reflects the actual beam spread.

Several silicon detector modules were tested at PSI, two of which were constructed at HEPHY. A module consisting of two 4" sensors with 1024 strips, read out by eight APV6 chips, was built within the framework of a module production milestone. Therefore, this
module was called ``Vienna Milestone'' (VM). The ``Vienna APV25'' (V25; fig. , p. ) module with two 6" sensors and APV25S0 readout was already presented in section , p. .

Several other institutes within the CMS collaboration joined these tests and provided their detector modules. In the May test, a total of five APV6 and two APV25S0 modules were tested, while in December, all six modules were read out with the new APV25S1 chip. Tab.  gives an overview of the properties for the modules tested at PSI. The module order corresponds to the arrangement in the beam as seen by the particles (from top to bottom).

Table: Properties of the detector modules tested at PSI in May and December 2000. The silicon sensor resistivity is divided into low (LR, $1.4 \ldots 3.5 \,\rm k\Omega\,cm$) and high (HR, $4 \ldots 8 \,\rm k\Omega\,cm$) regimes.

Date	Name	Built by	Sensors	Res.	Thickness	Pitch	Irradiated	Readout
					$[\rm\mu m]$	$[\rm\mu m]$	$[\rm cm^{-2}]$
May 2000	V25	HEPHY	$2\times 6''$	HR	320	140	no	$3\times \rm APV25S0$
	KA1	Karlsruhe	$1\times 6''$	LR	320	140	no	$3\times \rm APV6$
	PD1	Padova	$2\times 4''$	HR	300	61	no	$2\times \rm APV6$
	PD3	Padova	$2\times 4''$	LR	300	61	no	$2\times \rm APV6$
	PD4	Padova	$2\times 4''$	LR	300	61	$2\cdot10^{14}\,\rm n$	$2\times \rm APV6$
	VM	HEPHY	$2\times 4''$	HR	300	61	no	$8\times \rm APV6$
	PD25	Padova	$1\times 4''$	LR	300	61	no	$1\times \rm APV25S0$
Dec. 2000	BA1	Bari	$2\times 4''$	HR	300	61	no	$1\times \rm APV25S1$
	KA2	Karlsruhe	$2\times 4''\,\rm$	LR	300	61	$10^{14}\,\rm p$	$1\times \rm APV25S1$
	PD27	Padova	$2\times 4''\,\rm$	LR	300	61	$10^{14}\,\rm p$	$1\times \rm APV25S1$
	BA2	Bari	$2\times 4''$	HR	300	61	$10^{14}\,\rm p$	$1\times \rm APV25S1$
	PG	Perugia	$2\times 4''\,\rm$	LR	300	61	no	$1\times \rm APV25S1$
	PD26	Padova	$2\times 4''\,\rm$	LR	300	61	no	$1\times \rm APV25S1$

Some of the silicon detectors were previously irradiated with CMS-like doses to study their performance in comparison with virgin sensors. This required an ambient temperature of $-10^{\circ}\,\rm C$, which was provided by the cooling box, shown in fig. .

Figure: The fully equipped cooling box in the PSI beam area where the particle beam enters from the left. The rod to the right contain the trigger scintillator with two photomultipliers. The distribution board on the top of the box provides power, clock, trigger, reset and $\rm I^2C$ signals to the repeater boards, which are located above the fan outside of the cooled environment, but connect to the detector modules inside.

The main results of the PSI module tests are presented in the following sections, while additional information is available at [12].