Photon Counters SR400  Gated photon counter (2-channel) SR400 Gated Photon Counter The SR400 Dual-Channel Gated Photon Counter offers a · Two independent counting channels convenient, integrated approach to photon counting that avoids the complexity and expense of older counting systems. · Count rates to 200 MHz No longer is it necessary to mix and match amplifiers, discriminators, gate generators and counters. The SR400 combines all these modules into a single, integrated, · 5 ns pulse-pair resolution microprocessor-controlled instrument. Complex measurement tasks such as background subtraction, synchronous detection, · Gated and continuous modes source compensation, and pile-up correction can all be performed easily with the SR400. · Gate scanning for time-resolved counting Counters · Built-in discriminators The SR400 has two independent channels that can count at rates up to 200 MHz. Different counting modes allow you to · Gate and discriminator outputs count for a fixed amount of time, until a certain number of counts have been received, or for a fixed number of triggers. Each counting channel has its own gate generator which · GPIB and RS-232 interfaces provides counting gates as short as 5 ns or as long as 1 s. The gates can be set in a fixed position relative to the trigger signal, or scanned to measure lifetimes or recover time- varying waveforms. The actual inputs to the counters can be viewed as NIM level pulses from the discriminator outputs on the front panel. The discriminated pulses are negative going from 0 to −0.7 V. The DISC outputs are very useful when adjusting discriminator thresholds or gate timing. · SR400 ... $5350 (U.S. list) Stanford Research Systems phone: (408)744-9040 SR400 Gated Photon Counter Signal Inputs and Discriminators counting is disabled and data may be transferred or external parameters may be changed. The dwell output provides a Both analog signal inputs (A and B) are internally terminated TTL signal which is high during the dwell time. This can into 50 Ω. The inputs accept signals between ±300 mV and are be convenient for interfacing other instruments used in protected to ±5 VDC. Each input is followed by a DC to the experiment. 300 MHz amplifier, allowing detection of pulses as small as 10 mV. If additional sensitivity is required, a remote Outputs preamplifier (like the SR445A) can be used. 9 The front panel can display counts up to 10 . Results from Discriminators are provided for each channel with a selectable both counters can be displayed individually or combined as threshold from −300 mV to +300 mV in 0.2 mV steps. Pulse- A−B or A+B. A front-panel D/A output provides an analog pair resolution is 5 ns, and pulses of either polarity may be signal proportional to A, B, A−B, or A+B depending on the detected. Each threshold may be programmed to scan in either counting mode. The scale may be logarithmic (1V/decade) direction with selectable step size. This can provide a pulse or linear. height analysis (PHA) output and is useful for choosing photomultiplier tube bias or discriminator levels. Computer Interfaces Count Periods Built-in RS-232 and GPIB interfaces provide a convenient means of controlling the instrument and retrieving data. While The SR400 may be programmed to cycle from 1 to 2000 count the SR400 is scanning, each of the count values for the A and periods in a single scan. At the end of the programmed scan, B counters are stored in a 2000 point internal buffer. This the counters may be stopped or the scan may be restarted. buffer can be transferred on a point-by-point basis, or dumped Consecutive count periods are separated by a “Dwell Time” all at once through either interface. which can be set from 2 ms to 60 s. During the Dwell Time, 300 MHz DISCRIMINATORS AMPLIFIERS FAST LOGIC IN 1 A A DISC DISC SELECT A - COUNTER 50Ω CLR A GATE B B SELECT DISC DISC B - COUNTER 10.0 CLR MHz B GATE IN 2 T T DISC DISC S E PRESET OR T L DWELL COUNTER E TIME 50Ω LOAD C T DWELL TRIG OUTPUT Trig DUAL GATE GENERATOR DISC DISC LEVELS GATE CONTROL COUNTER INPUTS RS232 D/A OUTPUTS MICROPROCESSOR CONTROL GPIB SR400 Block Diagram A block diagram of the SR400 is shown above. There are three fast counters identified as counters A, B, and T. All three counters operate at rates up to 200 MHz. The input to each counter is selected from a number of sources which includes the two analog signal inputs, the 10 MHz crystal timebase, and the external trigger input. Counter T can be preset to determine the measurement period. For pulsed experiments, counters A and B may also be synchronized to external events via the two independent gate generators. The gate generator provides gates from 5 ns to 1 s in duration with a delay from an external trigger ranging from 25 ns to 1 s. Stanford Research Systems phone: (408)744-9040 SR400 Specifications Signal Inputs The stop key terminates the current count period and pauses the scan. If scanning, gates and disc levels are held at their Bandwidth DC to 300 MHz current values. Pressing the stop key while in a paused Input impedance 50 Ω condition will reset the scan, and all scanned parameters will Linear range ±300 mV (at input) return to their start values. Pressing the start key while paused Input protection ±5 VDC, 50 V for 1 µs resumes the scan by starting the next count period. Overload recovery 5 ns The dwell time may also be set to external. In this mode, count Discriminators periods begin with the start key or external start input (TTL rising edge). Count periods terminate with the preset Counters A, B, and T have independent discriminators when condition, the stop key, or the external stop input (TTL rising counting the signal inputs. All discriminator levels may be set edge). Pressing the stop key while not counting resets the to a fixed level or scanned. scan. Discriminator range −300 mV to +300 mV All count data is internally buffered for one scan. Data may be Discriminator slope Rising or falling read over the computer interfaces during or after a scan. Resolution 0.2 mV Input offset voltage <1 mV Display Mode Minimum pulse input 10 mV Pulse-pair resolution 5 ns Continuous Displays current counter value Discriminator outputs NIM levels into 50 Ω Hold Displays final count value Inhibit input TTL high stops count D/A Output Trigger Input The front-panel D/A output is proportional (linear or log) to A, Impedance 10 kΩ B, A−B or A+B, and is updated at the end of each count Threshold ±2.000 VDC in 1 mV steps period. There are two rear-panel D/A outputs: port 1 and port 2. Slope Rising or falling These outputs may be set or scanned from the front panel or Protection 15 VDC, 100 V for 1 µs via the computer interface. Gate Generators Full scale ±10 VDC Resolution 12 bits (5 mV) Both the A and B gates may be fixed in time or scanned. The Current rating 10 mA gate outputs show the positions of the gates with respect to the Output impedance <1 Ω discriminator outputs. Accuracy 0.1 % + 5 mV Insertion delay 25 ns General Maximum delay 999.2 ms Minimum gate width 5 ns Interfaces GPIB and RS-232 Maximum gate width 999 ms or CW Dimensions 16" × 3.5" × 13" (WHD) Resolution 0.1 %, 1 ns minimum Weight 10 lbs. Accuracy 2 ns + 1 % Power 35 W, 100/120/220/240 VAC, Jitter 200 ps rms +100 ppm 50/60 Hz Maximum trigger rate 1 MHz Warranty One year parts and labor on defects Gate view output NIM levels into 50 Ω in materials and workmanship Gate view error <2 ns Scan and Dwell The number of count periods or data points in a scan may be set from 1 to 2000. The duration of one count period is determined by the preset condition. SR400 rear panel The time between consecutive count periods is the dwell time and can be set from 2 ms to 60 s. The dwell output will be TTL high during the dwell time. This output can be used to trigger Ordering Information external devices. At the end of a scan (of 1 to 2000 count SR400 Gated photon counter $5350 periods), counting may be programmed to stop or start the SR445A 350 MHz preamplifier $1100 scan over again. The start key begins the first count period of the programmed scan. Stanford Research Systems phone: (408)744-9040 SR400 Gated Photon Counter Example 1: Boxcar Mode Example 2: Synchronous (Lock-In) Mode This experiment uses a scanning gate to measure the lifetime Very small changes in the flux of photons may be measured by of an excited state pumped by a pulsed laser. The SR400's gate synchronous detection. If a signal is fixed in frequency and generators are triggered by a photodiode when the laser fires. has a 50 % duty cycle, then synchronous photon counting, or Counter A counts photons which occur during a narrow gate, photon counting in a “lock-in” mode, can be used. An optical while counter B counts the photons during the entire decay. chopper is used to modulate a CW light source. The reference output from the chopper triggers the photon counter's dual Decay data is normalized to the source intensity by counting gate generator. The A gate is positioned to count photons until a preset value is reached in counter B. When B reaches during the open phase of the chopper, and thus counts the its preset condition, the D/A output is set to a voltage signal plus background. The B gate only counts the proportional to A's count, A's gate is stepped, and a new count background, counting pulses only during the closed cycle of interval begins. the chopper. The difference between the two counts, A−B, is the signal. Accumulating data over many cycles is required to By plotting the log of A's count value vs A's gate delay, the measure the signal since the background rate often far exceeds exponential decay curve is linearized, and the lifetime of the the signal rate. excited state can be determined from the slope on the chart recorder. If count rates are high and count periods short, then The D/A output is proportional to A−B. Since the background scans may be displayed on an X-Y scope. The D/A output is count is subtracted for each chopper cycle, only the signal the Y drive and port 1 or port 2 is the X drive. The dwell which is synchronous with the chopper will cause the output output should be used as the blanking pulse. of the photon counter to change. Experiment Timing Diagram SR400 Log(A) vs. Gate Pulsed Photon Counter Delay for Preset B Light Light Source Log(A) PMT PMT In 1 D/A Scanning A - Gate Photodiode Trig D/A B - Gate SR445 Preamp Boxcar Mode Timing Diagram Experiment Chopper SR400 C.W. Photon Counter Source A - Gate PMT In 1 B - Gate Trig ∝ D/A V A - B for preset T SR540 Chopper SR445 Preamp Lock-In Mode Stanford Research Systems phone: (408)744-9040