Nuclear Electronics Lab
PH 488 : Spring AY7-2
Purpose: To obtain a basic understanding of the electronics used in nuclear physics research.
Objectives: 1)Describe the function of an oscilloscope and be able to make simple voltage and time measurements with an oscilloscope.
2)Explain how a surface barrier detector operates.
3)Draw the schematic for a charged particle detection system based on a surface barrier detector.
4)Explain what is meant by a NIM module and NIM bin and describe the functions of each module used in a charged particle detection system.
5)Build and troubleshoot a charged particle detection system using a surface barrier detector and NIM modules.
6)Perform an energy calibration of a charged particle detection system using a radioactive source and a pulser.
Introduction:
I.Electronic Detection System
Many different physical quantities are determined in nuclear physics experiment. For instance, the energies of the particles is of primary importance in some experiments while other experiments are concerned with the time that has elapsed between two or more events. However, regardless of the physics involved in the experiment, electronic circuits deal only with two quantities: 1) voltage and 2) current. Thus, any nuclear detection system can be described by the following function diagram:
No matter how complicated an electronic detection system may appear, it can be broken into the functional blocks shown above.
a) Detector
The detector converts the appropriate physical quantity (energy, time, momentum, etc.) in to one or more voltage or current signals. Thus, the type of detector that is used in a particular detection system will depend on the type of physical quantity that is being measured. The detector used in this lab is a surface barrier detector and is primarily used to detect the energy of charged particles.
b) NIM Electronics
A detailed knowledge of electrical engineering was required for a physicist to design an electronic detection system prior to the 1950’s. Fortunately, such knowledge is no longer required due to modern integrated circuit and transistor technology. Today, most nuclear detection systems can be built using standardized electronic modules. These modules are called NIM (Nuclear Instrument Modules) modules and have been designed to perform specific logic functions or calculations without requiring the physicist to understand how the task was accomplished. An analogy of this process is a student’s use of a calculator to multiply two numbers. The student doesn’t have to know how the calculator performs the operation but only which buttons he or she must push. A qualitative sketch of a standard NIM module is shown below:
The front and back of a NIM module contains connections for providing input voltage signals to the module and for obtaining output voltage signals. The front of the NIM module also contains knobs and switches by which the operator can adjust the operation of the module. For instance by turning the Course Gain knob on the front of the amplifier, the operator can change the height of the output voltage pulse created by the module.
Electrical power is supplied through connections in the bottom of the back of the NIM module by sliding the module into a NIM bin (crate) shown below. Using AC power from a 110 wall outlet, the NIM bin will produce the +/-6,+/-12 and +/-24 volt DC power needed by the transistors and integrated circuit components in the electronic modules. This method of standardizing all of the electronic power connections was developed during the early 1960’s by NASA for instrumentation on the unmanned lunar probes.
There are several advantages to using modular electronics like NIM. First, very sophisticated electronic systems can be designed and built by individuals with limited electronics knowledge. Secondly, modular system are more reliable and cheaper to build and repair. One disadvantage of the NIM electronic modules is that they do not generally allow for computer interfacing as personal computer technology was not available in the 1960’s when the NIM standard was developed. However, special interface modules can be purchased to allow computers to communicate to newer NIM modules through the GPIB (IEEE-488) interface or through the RS232 voltage or RS232 current interfaces. Several newer electronic module systems (CAMAC, VXI, etc..) have also been developed to allow for easier access by computers and for faster data transfer, but these systems are very expensive compared to NIM and unnecessary for many applications.
c) Multichannel Analyzers and Counter/Timers
The data output units on most nuclear electronic system are either Multichannel analyzers or Counter/Timers. Multichannel analyzers provide the scientist with a graphical picture of the data as well as the numerical information. Counter/Timers are more convenient when you are only interested in a specific number (like how many particles were counted) and are less expensive than Multichannel analyzers. You have used Multichannel analyzers to collect energy spectra of photons during labs in both PH 365 and PH 486. A Counter/Timer is now used during the “Radioactive Decay Lab” in PH 365.
II.Types of NIM Electronic Signals
The rules (specifications) of NIM electronics state that all signals entering or leaving NIM modules are voltage signals and must be one of two types: 1) linear pulses or 2) logic pulses.
a)A NIM standard logic pulse is a rectangular pulse that follows the following specification:
Type / Output (Must Deliver) / Input (Must Respond To)Logic 1 / +4 to +12 V / +3 to +12 V
Logic 0 / +1 to -2 V / +1.5 to -2V
Because of the prevalence of TTL (Transistor-Transistor Logic) circuitry in digital integrated circuits, most NIM modules have adopted a +5V pulse for logic 1 with 0V representing zero. The width of the pulse is not defined in the NIM specification, but for standard logic modules it is usually at least 1 s. These pulses can be counted by Counter/Timers or used to trigger other electronic devices. Although there is also a NIM standard for fast logic pulses for use with timing circuitry, we will not need this specification in this lab.
Although technically not a logic pulse, a gate or enable signal is a level signal which can turn on or off the input of an MCA or NIM module. When a +5 volt gate signal is applied to a NIM module, input signals are allowed to pass into the module. However, when the gate signal goes to 0V then the input signal is blocked. Thus, the gate signal acts as if it is opening and closing a gate to the input, hence its name.
b)In the NIM standard, alinear pulse is defined as any signal that contains information through its amplitude or its shape. The term linear (as compared to digital) has to do with the linear electronic circuits used to generate and manipulate these pulses and not the actual shape of the pulses. When the information from a detector is contained in the height of a linear pulse, the experiment is called “Pulse Height Analysis (PHA).” Most of our lab experiments will be PHA experiments as they are the most common type of experiment done in nuclear physics. The NIM modules used in PHA analysis with usually modify the shape of a linear pulses obtained from a detector. This occurs because the modules contain filters to reject electronic noise from the system so that the pulse height can be measured as accurately as possible. NIM defines three types of linear pulses: 1) fast linear pulse, 2) tail pulse, and 3) shaped linear pulse. The fast tail pulse is a voltage signal that is created when you collect a current signal from a detector using a collection circuit whose timeconstant is very small. The polarity of a fast tail pulse may be positive or negative. Fast linear pulses are useful in detection systems where timing information is important. If the current is collected using a circuit with a larger time constant, the corresponding voltage signal is called a tail pulse. The term tail pulse comes from the long fall time of the pulse which is caused by the large time constant of the collection circuit. The very fast rise time of the pulse is determined by the type of detector that is used. This type of pulse more accurately relates the height of the voltage pulse to the current from the detector than does fast linear pulses. Thus, most preamplifiers (the first stage of most PHA electronic detection systems) produce tail pulses. A shaped linear pulse is a tail pulse whose width has bee drastically reduced. The most common type of linear pulse is the shaped linear pulse that are produced by amplifiers. Because the voltage signals produced by detectors are very small (V to mV), the signals must be amplified in order to be measured accurately. However, many millivolt noise sources (thermal, shot, ground loops, etc.) also are present in the system. Thus, a tail pulse must be amplified without amplifying the noise signals. By including a differentiation circuit followed by several integration circuits in the amplification process, it is possible to filter out the noise while amplifying the tail pulse. However, this process does modify the shape of the tail pulse. The resultant shaped linear pulse is usually a gauassian or triangular pulse with an amplitude of 0-10 V. Unipolar shaped linear pulses are provided to a Multichannel analyzers for PHA applications while bipolar pulses are used for slower timing applications.
III. NIM Modules and Detector Electronics
a) Surface Barrier Detector
Surface barrier detectors are the most common type of detector used in charged particle detection systems. A surface barrier detector is created by using an n-type silicon crystal and then creating a shallow p-type region using either ion implantation or by etching the silicon with acid followed by exposure to air in order to oxidize the surface. Thus, the surface barrier detector is a p-n junction similar to those you studied in PH 365. Two electrodes are then connected to the p-n junction to allow for biasing of the p-n junction to complete the device as shown in the diagram.
In order to use the p-n junction as a charged particle detector, it is necessary to reverse bias the diode with a large external electric field supplied by a DC power supply so that the depletion region of the device will encompass the entire p-n junction. When properly biased, the p-n junction contains no free charge carriers in the detector and the only current flow is due to the very small thermal current from electron and hole pairs created by the absorption of thermal energy. However, when a charged particle strikes the detector as shown below, the particle will give up its kinetic energy by creating electron-hole pairs in the detector. The strong electric field inside the detector will prevent the electrons and holes from recombining and will sweep the electrons and holes in opposite directions toward the two electrodes. Thus, a charged particle can be detected by measuring the current or charge pulse at a single electrode. Furthermore, since it takes approximately 3.6 eV of energy to create a single electron-hole pair in silicon, the energy of the incoming particle can be determined by the magnitude of the charge pulse (number of electron-hole pairs created).
b) Charge Preamplifier (PreAmp)
As mentioned earlier, the signals from detectors are very small and must be amplified and filtered in order to reduce the effects of noise in the detector system and in order to accurately measure the height of the pulse. Thus, it might seem logical that a NIM amplifier module should be connected directly to the output of the detector. However, for most detector systems, a preamplifier must be inserted between the amplifier and the detector. The purpose of the preamplifier is to match the high output impedance of the detector to the low impedance of the amplifier. Although a study of transistor amplifier systems is beyond the scope of this course, the basic principle involved in connecting the detector to a second NIM module can be understood by analyzing the PH 202 circuit shown in the following diagram. In this diagram, the detector produces an output voltage Vdet which we want to amplify. However, the voltage signal that will be amplified is the input voltage seen by the second NIM module, Vin, is smaller due to voltage drop across the output resistance of the detector. In fact, Vin will only be equal to Vdet when the input impedance of the second module, Rin, is much greater than the output impedance of the detector. Since many detectors have output impedance’s of hundreds of kiliohms to several Gigaohms and most amplifier circuits have input impedance’s of < 10 k, this is rarely the case.
Using a preamplifier stage solves this impedance matching problem due to the fact that a preamplifier has a very high input impedance and a very low output impedance.
There are two types of preamplifiers used in nuclear detection systems: 1) charge preamplifiers, and 2) voltage preamplifiers. The choice of preamplifier depends on the type of output signal produced by the detector. NaI and photon detectors generally produce voltage pulses and require voltage preamplifiers. Surface barrier detectors produce a charge pulse and therefor require a charge preamplifier to collect the charge pulse and to convert the pulse into a voltage pulse for use with other NIM modules.
c) Amplifier (Amp)
A linear amplifier is used in a nuclear detection system to increase the height of the voltage signals and to reduce the effects of electronic noise sources.
d) Pulser
A pulser is a NIM module that can supply tail pulse type test signals that are electronically equivalent to the signals created by the detection of radiation. Pulsers are used to test the electronic circuit in order to ensure that the system is operational and are also used to perform energy calibrations. Remember, electronic circuits don’t know about energy just voltage and current so we must determine how much energy is equivalent to a given voltage (1V). This is called an energy calibration.
e) Single Channel Analyzer (SCA)
A single channel analyzer is a NIM logic module. The SCA accepts an linear voltage pulse as its input signal. Using the knobs on the front of the module, the operator selects a voltage range (for example 1.0-1.2 volts). If the amplitude of the input signal is within the selected voltage range then the SCA will produce a +5 V NIM output logic pulse otherwise the module produce no (0 V) output signal. A counter/timer can then be connected to the output
of the SCA to count the number of particle’s with the correct energy to cause this voltage signal.
f) Multichannel Analyzers (MCA)
Originally MCA’s were composed of many single channel analyzers connected together such that the window of each SCA was successively higher than the window of the previous SCA. Example: The first SCA has a voltage window of 1.0-1.2V so the next SCA would have a voltage window of 1.2-1.4V and so on. Although MCA’s still work on the same principle, they consist of an analog to digital converter which converts the voltage input signal into a binary number. The binary number is then used to address a specific location in computer memory. The numerical value stored at this location is then increased by one to indicate that an event has occurred. Using computer technology, MCA’s are now built with as many as 16,000 channels which is equivalent to 16,000 SCA modules and 16,000 counters!!!
g) Bias Supply
All electronic detectors require a DC power supply in order to operate. Such power supplies are called bias supplies. In the case of surface barrier detectors, it is easy to see why this name was chosen since the DC supply provides the necessary voltage to reverse bias the p-n junction of the detector.
III. Oscilliscope
During the labs in PH 488, you will need to operate an oscilliscope in order to build and troubleshoot your detector electronics. The specific instructions on how to operate an oscilliscope is dependednt on the model of oscilliscope that is used and will be provided in lab. However, a general understanding of the oscilliscope can be obtained if you simply remember that the function of an oscilliscope is to graph one or more voltage signals as a funtion of time. Thus, it is just electronic graph paper and the oscilliscope controls simply let you determine which signal you want to graph, when you want to start graphing, and what x-y axis range will be used.
IV. Charged Particle Detection System
During lab, you will build a charge particle detection system similar to the one shown in the following diagram.
Lab Work:
I. Oscilloscope
A. Your first task is to turn on the Tektronix 2235(A) Oscilloscope.
1)Make sure that the oscilloscope is plugged into a 110 volt AC outlet.
2) Push the Power Button to the ON position.
3)The Green Power Indicator Light should be Litif you were successful.
B. Your second task is set-up the oscilloscope to graph a single input voltage as a function of time. The 2235 Oscilloscope is a dual trace oscilloscope which means that it is capable of graphing two different input voltage singles either independently or concurrently. Thus, you must tell the oscilloscope which input you wish the oscilloscope to plot on the vertical axis and when you want the oscilloscope to start the graph. We will use channel #1 for our input so we must set up the oscilloscope accordingly.