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Introduction

Today nuclear spectroscopy involves more than an accurate analysis of a sample. Concerns for worker safety, protection of the environment, compliance with regulatory guidelines, and the need for electronic record keeping creates complex needs. Some of these needs are difficult to satisfy using traditional nuclear counting systems.

Fortunately the progress in electronic data storage, computer networking technology, and breakthroughs in microelectronics have opened new avenues. Powerful multitasking operating platforms and enhanced graphics capabilities of computers made it easier to address today's diverse needs in nuclear spectroscopy.

Taking advantage of new technology and in keeping pace with the complex needs in the industry, a new generation of Nuclear Instrument Modules (NIM) entered the nuclear counting arena. These are programmable signal processing devices based on the Canberra Instrument Control Bus (ICB) — addressed by the acronym "ICB NIM."

This new generation of nuclear signal processing subsystem offers unique and distinct benefits to its users. This application note discusses these benefits in view of current needs in the industry.

The application note starts with a definition of ICB NIM and the various members of the ICB NIM family. A brief summary on the functions of traditional pulse processing electronics follows. The intent of this section is to distinguish the capabilities of the ICB NIM from that of traditional signal processing electronics. The main body of this document deals with the various needs in today's nuclear industry and how the ICB NIM addresses these needs. Discussion of detailed specifications of individual modules and system configurations have been kept to minimum. Such details are available in specification sheets and user manuals. The application note concludes with a summary of the benefits and areas of application of the ICB NIM.

Figure 1
Traditional NIM Amplifier with Manually Adjustable Knobs and Dials.

Figure 2
Front Panel of ICB NIM with all computer control

ICB NIM—What Is It?

Nuclear Instrument Modules based on the Canberra Instrument Control Bus (ICB) are computer controlled, programmable NIM. The ICB NIM is a new generation of Nuclear Instrument Modules consisting of an entire family of nuclear signal processing electronics.

The ICB NIM family consists of the following modules: 1) two fast Analog to Digital Converters(ADCs), 2) one Spectroscopy grade Amplifier, 3) two highly stable, High Voltage bias supplies, and 4) a Remote Parallel Interface Module (RPI) module, used for multiplexer and sample changer control. All of these modules are in the "Standard" NIM format.

One can easily see the distinction between a traditional NIM and the ICB NIM. Figure 1 shows a traditional NIM module. Its front panel is a complex array of knobs and dials. In the case of ICB NIM the complex front panel knobs are removed (Figure 2), and most adjustments are set from a computer.

A good example of computer set adjustment is setting the amplifier gain. In the traditional NIM amplifier, it is adjusted by turning several front panel controls (coarse gain, fine gain and super fine gain). The ICB NIM concept allows a user to remotely set and lock settings from the computer.

In addition to the computer control of operating controls, the desired operating parameters may be set and saved to a configuration file. Opening this file will automatically program all front end adjustments to the desired settings.

The ICB NIM works through an Acquisition Interface Module (AIM). The "AIM" is an Ethernet network based computer controlled MCA, in standard NIM format. It is the "Master Controller" of the entire ICB family, and allows the system to operate in an Ethernet Local Area Network (LAN).

The user interface and control is via the Canberra Genie-ESP software operating under the DEC (Digital Equipment Corporation) VMS operating system, or from the Genie-PC software under the IBM "OS/2" operating system.

Standard Functions of Pulse Processing Electronics

All nuclear counting and measurement systems have pulse processing electronics. In nuclear spectroscopy systems, these electronics are between the Multichannel Analyzer and the detection unit (Figure 3). Referred to as a "Front End Subsystem," they typically include the preamplifier, amplifier, high voltage bias supply, and the Analog-to-Digital-Converter (ADC).

Figure 3
Diagram of a Signal Chain.

Preamplifier:

The preamplifier is located with the detector. Its main function is to pre-process the incoming pulses. The preamplifier functions are the following:

1. Converting a charge pulse pro- duced in the detector to a voltage pulse
2. Pulse amplification
3. Matching the impedance of the detector to that of the electronics.

Amplifier:

The next component in the front end subsystem is the Spectroscopy Amplifier. It consists of a series of integrating and differentiating circuits. The amplifier functions are the following:

1. Increasing the amplitude of the pulses
2. Pulse shaping (Triangular, Gaussian)
3. Rejecting pulse pile-up and nearly coincident pulses
4. Supplying power to the preamplifier

Analog To Digital Converter:

The basic functions of the ADC are as follows:

1. Converting an analog pulse to a digital data
2. Providing signals to the MCA to track the live data collection time

High Voltage Bias Supply:

The High Voltage Supply provides a stable voltage to the detector for optimum collection of charges. Most spectroscopy grade units allow positive or negative biasing in ranges from 1000 volts up to 6000 volts.

Benefits of Using ICB NIM

The programmability of the ICB NIM provides distinct user benefits in terms of secure operation, flexibility in locating equipment and integrity of the counting system.

The following discussion explains these benefits in light of current needs in the industry.

1. Security and integrity of data

A key concern in many controlled counting laboratories is the security and the integrity of data. This concern for data integrity goes beyond the issue of experimental errors and deals with potential misadjustments in hardware set up parameters. Stringent quality control (QC) procedures are not sufficient to ensure that the adjustments made during a controlled calibration did not change at some later time.

The example of changing the amplifier gain is appropriate in this context. A slight change in gain will offset the spectrum collected by a large number of channels and invalidate the system calibration. Even if such inadvertent changes are corrected during the next calibration period, all the data analyzed since the previous calibration becomes questionable. Yet there are no easy ways to verify this during QC or to prevent its occurrance in the first place.

The ICB NIM reduces the possibility of such inadvertent change in system adjustments. The absence of knobs on the front panel eliminates this possibility. Only an authorized user can change previously adjusted parameters such as the amplifier gain, high voltage, ADC offsets, etc., via appropriate menus in the software (see Figures 4, 5, 6).

Figure 4
ICB NIM dialog is organized so that the user can make adjustments and immediately see the results in the spectrum.

 

Figure 5 The Amplifier Adjust Dialog.

 

Figure 6 The ADC Adjust Dialog.

 

Password and inherent security protections are standard features of the VMS operating software. The Genie-ESP software contains several layers of security functions. These security layers allow tailoring specific user accounts, and restricting sensitive activities to only those who are qualified.

In the Genie-PC environment any changes affecting calibration will be flagged, and the user will be notified of the changes. Thus unauthorized changes are difficult and do not go unnoticed by the system.

2. Electronic verification of data to regulators/auditors

When a sample is counted, all sorts of data are stored on paper or disk. This data includes — the spectrum itself, the elapsed timing information, the description of the sample, the calibration information, and so forth. These are in addition to the analysis results and raw data.

This sort of information becomes relevant when counting laboratories are under close scrutiny of Government Regulators and Auditors. Data verification often involves a massive paper trail of graphs, control charts, operating personnel identity and other forms of paperwork. The purpose of this record keeping is to assist in direct verification of results. Among a host of requirements regulators and auditors expect the labs to provide proof that the system setup did not change, including the front end configuration. This leads to manual record keeping, much of which could be avoided by replacing traditional front end with ICB NIM.

All front end setup information is stored in a single file structure called a "CAM" file. "CAM" stands for "Configuration Access Method" and is a unique file format created for Canberra Systems. The CAM files store all the data produced when a sample is counted. This includes the spectrum itself, calibration information, and acquisition setup parameters.

Thus the AIM/ICB combination under the auspices of the CAM, provide the ability to maintain proper records and verify system integrity. These include all setup parameters, the instrument model, as well as serial numbers with each sample record. It is easy to verify the settings for each sample counted because they are automatically stored with the sample record.

3. Ability to operate detectors with multiple setup parameters

Labs with high sample throughput count various types of samples, in different geometries, on numerous detectors. The MCA may be used in more than one gain/range configuration for different experiments. Sample characteristics, changes in detector, or MCA memory group selection often require changes in the setup. All these changes must be recorded on paper or memory for future references. To further complicate this task the changes are implemented manually by turning a morass of knobs and dials. The process is tedious, slow and susceptible to mistakes.

The ICB NIM solution provides support for multiple setup files for the same or different detectors. The computer controlled NIM allows multiple configuration files on each detector eliminating the need for new adjustments on the front end. The user simply calls up the appropriate configuration file at the click of a mouse button. There is no need to remember appropriate settings or implement changes manually.

4. Improved signal quality

Quantitative and qualitatively analysis of complex spectra requires very high resolution germanium detectors. Better spectral resolution is a prime concern today. Users invest large sums of moneys for high resolution detectors. However, the performance of these expensive devices are subject to a number of operating conditions. Among these conditions the level of electronic noise and cable quality are at the top of the list.

A well-known nuisance in nuclear counting is the degradation of spectral resolution due to unwanted electronic noise. A common source of this noise is from the "ringing" in signal cables used between the analyzer (computer) and the electronics.

The user often positions the signal processing subsystem away from the detector. This is done to bring the adjustments on the front end within the reach of the operator. As a result, the front end subsystem becomes distant from the detector. Lengthy cables produce increased noise in the system resulting in degraded spectral resolution.

Using the ICB NIM, the electronics are positioned next to the detector with the analysis station positioned at any remote location. In this case signal quality is not degraded.

Therefore, the ICB NIM improves the resolution of a system by decreasing unwanted interference resulting from lengthy cables.

5. Easy access to hardware from any node on the network.

The AIM/ICB combination is essentially an Ethernet based MCA. One of the key benefits of this type of MCA system is its ability to connect multiple spectroscopy systems and workstations along a single communications medium.

Large laboratories have multiple counting rooms or workstations networked together. These laboratories could benefit from the intrinsic redundancy of a network based acquisition subsystem. The ICB NIM is accessible from any workstation on a local area network. One can monitor live data acquisition from more than one PC at a time. Furthermore, the user can save or analyze data files at one node while acquisition is in progress at other nodes.

Since a user can operate the system from any node, breakdown of one computer (PC or VAX) at any station does not "down" the system. If one computer (in a specific counting station) fails, a second one can pick up its complete operation from a remote node. For the example in Figure 7, if PC #2 fails, PC #1 can pick up its acquisition and analysis functions.

Figure 7 ICB NIMs In Integrated Systems.

 

Figure 8

 

Figure 9 Typical Setup of a Large Networked Counting Facility

6. Counting operation with minimum exposure of personnel to "hot" areas

Reducing exposure to count room personnel is in concert with "ALARA" (As Low As Reasonably Achievable) efforts. This is particularly important for facilities with high activity samples. Fuel processing plants, weapons facilities, and nuclear power plants fall in this category.

ICB NIM controls all detector subsystems remotely, with the operator safely away from high exposure areas. It minimizes exposure by enabling a sample count start/stop, adjustments of hardware, calibration and other functions remotely. Additionally it provides access to multiple counting stations from a central monitoring/supervisory location.

ICB NIM Components

The ICB NIM family consists of the Model 9615 programmable amplifier, Model 9633/35 ADCs, Model 9641/45 HV bias supplies, and Model 554 Remote Parallel Interface (RPI). The Model 556 Acquisition Interface Module (AIM) controls all ICB NIM units. (See Figure 8). All ICB NIM accept programming information over the 8-bit ICB BUS. While each of these components serves all the traditional functions of front end subsystem electronics described earlier, they bring unique and advanced capabilities to the Nuclear Spectroscopy System.

Acquisition Interface Module:

The Acquisition Interface Module (AIM) acts as a local or remote host to signal processing NIM. A single AIM has two inputs and technically controls a whole BIN (12 slots) of programmable NIM, for a total computer control of front end configuration.

The Model 556 Acquisition Interface Module is a single width NIM with built-in Ethernet (LAN) interface conforming to IEEE 802.2/802.3 communication standards. The AIM has two high speed ADC ports and an Instrument Control BUS for the programmable front end ICB NIM. Residing on an Ethernet network, the AIM allows either local or remote data acquisition from any computer in the network. The AIM can acquire data from either ADC port independently. The AIM contains 64K, 32-bit channels of local data acquisition memory and can acquire data from both ADCs at an aggregate rate of 1 MHz.

9615 Programmable Amplifier:

The Model 9615 ICB programmable amplifier, is a double wide NIM research grade amplifier with full computer controlled features. Vital controls including coarse gain, fine gain, super fine gain are software selectable. It features: differential inputs for common mode rejection, wide gain range, choice of semi-Gaussian or semi-Triangular pulse shaping to meet most detector applications. The 9615 features fully automatic pole/zero adjustment, eliminating the need/use of oscilloscopes for pole/zero adjustments, and allowing pole/zero adjustments to be set remotely.

Programmable ADC & HV:

Model 9633/35 are fast programmable ADCs. They are in single wide NIM format and used for moderate to high count rate applications. Programmable digital gain, range, and digital offset allow maximum use of MCA memory.

Model 9641/45 programmable HV bias supply allows computer control of high voltage enable/disable, as well range and polarity settings.

All adjustments of the programmable modules are through the Graphical User Interface of the Genie software environments (See Figures 4, 5 & 6).

Remote Parallel Interface

The Model 554 Remote Parallel Interface (RPI) module is used to control AMX multiplexers, sample changers, or similar digital devices. The RPI provides up to 32 TTL outputs and 32 TTL compatible inputs. These outputs may be connected to four Canberra AMX modules (Model 8224) to independently control up to 32 detectors.

ICB NIM Configurations

Figure 7 shows a multiple node ICB NIM configuration. Multiple counting stations are networked together by an Ethernet LAN, with a mix of PCs and VAX stations in the same network. The ICB NIM represents the front end electronic subsystem for all the stations.

Figure 9 is a sketch of a large laboratory with various counting rooms. This is typical in Nuclear Power Stations and large National Laboratories where counting/monitoring data is retrieved simultaneously from several locations.

Conclusion

The ICB NIM philosophy is based on the numerous needs in the highly regulated nuclear industry. Its unique advantages are summarized as the following:

1. Security and integrity of data.
2. Easy verification of data to Regulators/Auditors.
3. Ability to operate multiple or single detectors with multiple setup parameters.
4. Improved signal quality.
5. Easy access to hardware from any node in a network.
6. Counting operation with minimum exposure to "Hot Areas".

The application of the ICB NIM will be ideal for facilities having any of the following needs:

• Limited access to instruments due to its location in "Hot Areas".
• Need to eliminate degradation of signal quality due to cabling distance between detector and electronics.
• Need for highly stringent QA/QC.
• Need for data verification to Regulators and Auditors.
• Need for multi-detector systems, controlled and monitored centrally.
• Need for redundancy of counting capability to maximize sample throughput by use of networked spectroscopy work stations.

The Canberra ICB NIM holds tremendous potential for changing the roles of signal processing electronics in nuclear spectroscopy. This new role is in concert with current trends in the highly regulated nuclear industry.