ORTEC DSPEC Pro Instructions for use

ORTEC
®
DSPEC Pro ®
Digital Gamma-Ray Spectrometer
Hardware User’s Manual
Printed in U.S.A. ORTEC Part No. 794380 0914
Manual Revision D

Advanced Measurement Technology, Inc.
(“AMT”)
WARRANTY
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no case exceed the price allocable to the item or service furnished or any part thereof that gives rise to the claim. In the event
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exclusive remedy shall be release of the buyer from the obligation to pay the purchase price. In no event shall AMT be liable
for special or consequential damages.
Quality Control
Beforebeingapprovedfor shipment,each AMTinstrument mustpass a stringent set of qualitycontrol tests designed to expose
any flaws in materials or workmanship. Permanent records of these tests are maintained for use in warranty repair and as a
source of statistical information for design improvements.
Repair Service
If it becomes necessary to return this instrument for repair, it is essential that Customer Services be contacted in advance of
its return so that a Return Authorization Number can be assigned to the unit. Also, AMT must be informed, either in writing,
bytelephone [[phone]] or by facsimile transmission [[phone]], of the nature of the fault of the instrument being
returned and of the model, serial, and revision (“Rev” on rear panel) numbers. Failure to do so may cause unnecessary delays
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control tests that are used for new-production instruments. Instruments that are returned should be packed so that they will
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AMT repair center. The address label and the package should include the Return Authorization Number assigned. Instruments
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will provide a quotation.
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Copyright © 2014, Advanced Measurement Technology, Inc. All rights reserved.
ORTEC®is a registered trademark of Advanced Measurement Technology, Inc. All other trademarks used herein are the property of their
respective owners.
NOTICE OF PROPRIETARY PROPERTY — This document and the information contained in it are the proprietary property of
AMETEK Inc. It may not be copied or used in anymanner nor may any ofthe information in or upon it be used for any purpose without the
express written consent of an authorized agent of AMETEK Inc.

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ADDITIONAL WARRANTY STATEMENT
Please note that the DSPEC Pro contains no user-serviceable parts.
Breaking the seal on the case voids your warranty.The DSPEC Pro
should be opened only by ORTEC-authorized service personnel.

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TABLE OF CONTENTS
WARRANTY ................................................................ ii
ADDITIONAL WARRANTY STATEMENT ...................................... iii
SafetyInstructionsandSymbols................................................ viii
CleaningInstructions......................................................... viii
1. INTRODUCTION .......................................................... 1
1.1. WhytheDSPECPro? ................................................... 1
1.1.1. DSPEC Pro Technologies .......................................... 1
1.1.2. Proven Technologies of the DSPEC Family ............................ 2
1.2. Enhanced Throughput Mode .............................................. 3
1.3. ResolutionEnhancementModeCorrectsforChargeTrapping ................... 4
1.4. TheLowFrequencyRejector(LFR)Filter ................................... 5
1.5. Ballistic Deficit and Adjusting the Flattop Duration ........................... 8
1.6. ZeroDead-Time(ZDT)Mode............................................. 9
1.6.1. Off—UncorrectedSpectrumOnly .................................. 10
1.6.2. NORM_CORR—ZDTandUncorrectedSpectraStored................. 11
1.6.3. CORR_ERR—ZDTandErrorSpectraStored......................... 11
1.6.4. Choosing a ZDT Mode ............................................ 12
1.6.4.1. The NORM_CORR Diagnostic Mode ........................ 13
1.6.4.2. ToSummarize ........................................... 13
1.6.4.3. MoreInformation......................................... 14
1.7. DSPEC Pro SMART-1 HPGe Detector Support .............................. 14
1.7.1. SMART-1HPGeDetectorConstruction .............................. 15
1.7.2. UsingOlderDetectors ............................................ 15
1.7.3. HowStateofHealth(SOH)isMonitored ............................. 15
1.8. HostComputerandSoftwareRequirements ................................. 16
1.9. About this Manual ..................................................... 16
2. GETTING STARTED ...................................................... 17
2.1. TheDSPECPro ....................................................... 17
2.1.1. FrontPanel ..................................................... 17
2.1.1.1. TheDisplay ............................................. 17
2.1.1.2. Controls ................................................ 18
2.1.2. RearPanel...................................................... 18
2.2. SoftwareandHardwareInstallation ....................................... 19
Installation — page 19

DSPEC Pro®Digital Gamma-Ray Spectrometer User’s Manual 794380D / 0914
vi
2.2.1. Step 1: Install the CONNECTIONS DriverUpdate ....................... 20
2.2.2. Step2: InstallMAESTRO......................................... 20
2.2.3. Step3: ConnecttheDSPECPrototheComputer ...................... 21
2.2.4. Step4: RunMCBConfigurationtoCommunicateWithYourMCBs....... 21
2.2.4.1. ConfiguringaNewInstrument .............................. 22
2.2.4.2. CustomizingIDNumbersandDescriptions .................... 22
2.2.5. AttachingMoreThanOneDSPECPrototheComputer ................. 23
2.2.6. ConnectingtoandDisconnectingfromtheComputer ................... 24
2.3. DSPECProMCBPropertiesinMAESTRO................................. 24
2.3.1. Amplifier....................................................... 24
2.3.2. Amplifier2 ..................................................... 26
2.3.2.1. InSightMode ............................................ 27
MarkTypes ......................................... 29
2.3.3. AmplifierPRO .................................................. 30
2.3.3.1. “Training”theResolutionEnhancer .......................... 32
ToEnableandTraintheEnhancer ....................... 32
ToTurntheEnhancerOff.............................. 33
2.3.4. ADC .......................................................... 33
2.3.5. Stabilizer....................................................... 34
2.3.6. HighVoltage ................................................... 35
2.3.7. About ......................................................... 36
2.3.8. Status ......................................................... 36
2.3.9. Presets......................................................... 39
2.3.10. MDAPreset ................................................... 40
2.4. SettingtheRiseTimeinDigitalMCBs..................................... 42
2.5. Troubleshooting ....................................................... 43
2.5.1. MAESTRODoesNotConnectwiththeDSPECPro .................... 43
3. SPECIFICATIONS......................................................... 45
3.1. DSPECPro........................................................... 45
3.1.1. ElectricalandMechanical ......................................... 47
3.1.2. Connectors ..................................................... 48
3.2. HPGe High-Voltage Detector Interface Modules ............................. 48
3.3. FeatureMaskBits ..................................................... 49
4. FIRMWARE COMMANDS AND RESPONSES ................................. 53
4.1. CommandFormat...................................................... 53
4.2. ErrorCodes .......................................................... 53
4.2.1. Dollar Response Records .......................................... 54
4.2.2. MCBCommands ................................................ 55

794380D / 0914 TABLE OF CONTENTS
vii
APPENDIXA. STATE-OF-HEALTHBITDEFINITIONS........................... 91
APPENDIXB. LISTMODEINTHEDSPECPRO ................................. 93
B.1. ListModeData ....................................................... 93
B.1.1. SpectrometerDataFormat......................................... 93
INDEX..................................................................... 97

viii
CAUTION To prevent moisture inside of the instrument during external cleaning, use only
enough liquid to dampen the cloth or applicator.
Safety Instructions and Symbols
This manual contains up to three levels of safety instructions that must be observed in order to avoid
personal injury and/or damage to equipment or other property. These are:
DANGERIndicates a hazard that could result in death or serious bodily harm if the safety instruction is
not observed.
WARNING Indicates a hazard that could result in bodily harm if the safety instruction is not observed.
CAUTION Indicates a hazard that could result in property damage if the safety instruction is not
observed.
In addition, the following symbols may appear on the product:
DANGER – Hazardous voltage
ATTENTION – Consult the manual in all cases where this symbol is
marked in order to determine the nature of the potential hazards and any
actions that must be taken to avoid them
Protective earth (ground) terminal
Please read all safety instructions carefully and make sure you understand them fully before attempting to
use this product.
Cleaning Instructions
To clean the instrument exterior:
!Disconnect the instrument from the power source.
!Remove loose dust on the outside of the instrument with a lint-free cloth.
!Remove remaining dirt with a lint-free cloth dampened in a general-purpose detergent and water
solution. Do not use abrasive cleaners.
!Allow the instrument to dry completely before reconnecting it to the power source.

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1
1. INTRODUCTION
The DSPEC Pro®is the next generation in digital signal processing-based gamma-ray spectro-
meters from ORTEC. Perhaps the most versatile digital spectrometer on the market today, the
DSPEC Pro features all of the spectral acquisition, MCA emulation, and computer control you
expect from our DSPEC family of instruments, plus specialized features for more challenging
gamma-ray spectroscopy applications. The DSPEC Pro offers superior performance for more
applications than any other gamma-ray spectrometer, delivered across a high-speed USB con-
nection. Whether your application is in nuclear power, non-destructive analysis, research
physics, homeland security, real-time monitoring, or nuclear safeguards, the DSPEC Pro gives
you exceptional ease of use, flexibility, rock-solid count-rate and peak-position stability, and
impressive overall performance.
1.1. Why the DSPEC Pro?
ORTEC digital signal processing (DSP) offers more options in optimizing the output signal of
high-purity germanium (HPGe) detectors. The DSPEC Pro builds on our DSP techniques and
combines them with the proven technologies of the DSPEC family in one compact, economical
package with high-speed USB connectivity — no other instrument comes close for HPGe
applications.
1.1.1. DSPEC Pro Technologies
Resolution Enhancer In ORTEC DSPEC products, the flattop width parameter of the trape-
zoidal filter automatically corrects for ballistic deficit. Unfortunately, this doesn’t compensate
for resolution loss due to charge trapping, which results in variations in charge collection effi-
ciency over the crystal volume. The DSPEC Pro introduces our new Resolution Enhancer, which
enables you to correct for charge trapping by “training” the spectrometer to accommodate the
particular performance of your detector.
Enhanced Throughput Mode This signal processing method allows the spectrometer to begin
sampling the next pulse before the preceding one has returned to baseline. The effect is optimum
throughput while maintaining resolution and peak shape.
List Mode In List Mode, the DSPEC Pro records and stores the pulse value with a time-stamp
for every pulse generated by the detector. With List Mode, you can write your own programs
that can reconstruct histograms for any time segment without dead time between histograms, or
make histograms for overlapping time slices. Data can be stored easily for reconstruction of any
time frame needed. If your application requires real real-time monitoring, no other instrument
comes close for HPGe applications.

DSPEC Pro®Digital Gamma-Ray Spectrometer User’s Manual 794380D / 0914
1Patent pending.
2Patent number 6,327,549.
2
1.1.2. Proven Technologies of the DSPEC Family
High-Speed USB Interface Connect the DSPEC Pro via high-speed USB to any computer
running under Microsoft®Windows®8, 7, or XP®SP3. A nearly unlimited number of DSPEC
Pros (as well as any combination of other ORTEC USB instruments) can be connected to your
computer via USB hubs (and across the network). As an ORTEC CONNECTIONS multichannel
buffer (MCB), DSPEC Pro works in both standalone and networked configurations.
Low-Frequency Rejector (LFR) Filter Uniquely designed for mechanically cooled HPGe
detectors, our Low-Frequency Rejector1digital filter surpasses all signal processing methods for
reducing the effects of microphonics, ground loops, and virtually all other sources of periodic
noise for HPGe and NaI(Tl) spectrometry.
Zero Dead-Time Correction The DSPEC Pro features our innovative ZDT™ mode of opera-
tion, an alternative to the classical “extended-live-time clock.” The ZDT method produces accu-
rate results in all situations and completely overcomes some of the shortcomings of extended
live-time methods. Moreover, the new ZDT method overcomes some limitations in previous
“loss-free” dead-time correction methods. Most notably, the ZDT mode does not require any
manual adjustments and is capable of computing the statistical uncertainty of the corrected
spectrum.2
Single-Cable Connection to Detectors DSPEC Pro uses the unique ORTEC Detector Interface
Module (DIM) for single-cable connection between the DSPEC Pro and the detector. The DIM
is designed to supply bias close to the detector so the cable carries only signal and low-voltage
power — no longer is high-voltage bias, and its associated dangers, carried over long distances.
SMART-1™ Support Detectors equipped with the SMART-1 technology have their recom-
mended bias values preset at the factory. No more shuffling through paperwork or looking for
tags on the detector to find the right bias setting. Simply turn on the electronics and the
SMART-1 detector senses the detector temperature and applies the right high-voltage bias.
Our SMART-1 detectors also allow our more recent MCBs, such as the DSPEC®-50 and digi-
DART, to monitor and display detector state of health (SOH) during acquisition, including
detector temperature, preamplifier power, bias overrange, and bias on/off state. This continuous
SOH monitoring ensures the integrity of the acquired data. A quick check of the SOH flag in the
SMART-1 detectors shows if any parameter deviated from specification during the measure-
ment. This is vitally important for environmental samples that must be counted for long periods
of time and regulatory-driven samples where chain-of-custody integrity means everything.

794380D / 0914 1. INTRODUCTION
3Patent number 5,872,363.
4Patent number 5,912,825.
5Ron Jenkins, R. W. Gould, and Dale Gedcke, Quantitative X-Ray Spectrometry (New York: Marcel Dekker,
Inc.), 1981, pp. 266–267.
3
Sample Changer Connections and Controls The DSPEC Pro connects easily to many types
of automatic sample changer.
Compact, Convenient Connectivity A small footprint means that the DSPEC Pro can sit right
on the desktop. Lightweight but rugged, multiple units can be stacked on top of one another via
their interlocking cases without fear of sliding or tipping. The easy-to-read, backlit LCD screen
displays the instrument ID, name, preset count conditions, current live and real time, dead-time
percentage (if active), input count rate, HV status and value, and, the serial number of SMART-1
detectors.
Use a USB hub to quickly connect multiple instruments to your computer. ORTEC CONNEC-
TIONS software supports any combination of USB devices (to a total of 127) connected to a
particular computer. In addition, any number of other ORTEC multichannel buffers (USB and
other interfaces) can be connected to the same system.
Standard DSPEC-Family Features The DSPEC Pro also offers the InSight™ Virtual Oscillo-
scope, which allows you to optimize detector performance for a given application from the com-
puter; our easy automatic pole-zero adjustment3and automatic baseline restorer4; and the highly
accurate Gedcke-Hale extended live-time correction method.5
1.2. Enhanced Throughput Mode
To optimally process an input pulse stream, and thereby obtain the best spectral resolution, the
signal processing device should allow the input signal to return to baseline before beginning to
process the subsequent pulse. However, in cases where some loss of resolution is acceptable, it
is possible for a DSP to begin processing a subsequent pulse before the first pulse has returned
to the baseline, provided the first pulse has returned to baseline when the peak-detect of the
second pulse occurs.
The ORTEC Enhanced Throughput Mode takes advantage of this feature of digital signal pro-
cessing by allowing you to adjust the delay between the peak-detect and the start of processing
of the next pulse. The dead time for a conventionally processed pulse is the sum of the pulse’s
rise time, flattop, and fall time. By contrast, the dead time in ORTEC’s Enhanced Throughput
Mode can be as little as the rise time plus the flattop. These two scenarios are illustrated in
Fig. 1. The DSPEC Pro lets you set a protection time (PT), following a peak-detect, that blocks

DSPEC Pro®Digital Gamma-Ray Spectrometer User’s Manual 794380D / 0914
4
subsequent peak-detects until the protection time has elapsed. Note that the conventional dead
time illustrated in Fig. 1 is the same as the maximum protection time in Enhanced
Throughput Mode.
The protection-time settings with the low-frequency rejector (LFR) filter off range between:
!Minimum PT (highest throughput) = (1 × rise time) + (1 × flattop)
!Maximum PT = (2 × rise time) + (1 × flattop)
With LFR on, the protection-time settings range between:
!Minimum PT = (3 × rise time) + (2 × flattop)
!Maximum PT = (6 × rise time) + (3 × flattop)
1.3. Resolution Enhancement Mode Corrects for
Charge Trapping
When a gamma ray interacts with a germanium detector, mobile electrons and holes are gen-
erated. The electrons and holes are swept to the detector electrodes by the electric field inside
the detector. If all the electrons and holes travel the complete path to the detector electrodes, a
signal is produced proportional to the energy deposited in the detector by the gamma ray. If
some of the electrons or holes do not reach the electrodes, the signal produced will be smaller
than expected. The process of interrupting the movement of an electron or hole is referred to as
“charge trapping.”
If charge trapping did not occur, the resolution of a reasonably low-noise germanium detector
for the 1.33-MeV gamma ray from 60Co would be about 1.5 keV FWHM. Real detectors typi-
cally have a resolution of 1.8 to 2.2 keV FWHM. ORTEC’s Charge Trapping Corrector (CTC)
helps reduce this energy resolution loss caused by charge trapping, yielding the DSPEC Pro’s
Resolution Enhancer Mode. The controls are accessed via the Acquire/MCB Properties...
command in the accompanying MAESTRO®Multichannel Analyzer Emulation Software (A65-
BW) and other ORTEC CONNECTIONS applications.
Charge trapping is caused by various defects and impurities in the germanium crystal, and thus
varies from detector to detector. The amount of charge lost due to trapping also depends on the
distance the charge carrier (electron or hole) has to move to reach the collecting electrode. If the
carrier must travel a long distance, it is more likely to be trapped. If some of the charge is
trapped, it does not contribute to the signal. The reduced signals have a different rise time than
the full signals. The relationship between rise time of the signal and charge trapping is the basis
of the Charge Trapping Corrector.

794380D / 0914 1. INTRODUCTION
5
Figure 1. Demonstration of the DSPEC Pro’s Minimum and Maximum Protection
Times in Enhanced Throughput Mode. At the maximum protection-time setting, the
DSPEC Pro processes pulses in the conventional way.
The digital filter in the DSPEC Pro measures the amount of charge collected for each event and
uses the result to increment the spectrum memory location associated with that charge measure-
ment. The Charge Trapping Corrector also measures the pulse rise time for the event. The pulse
rise time is used to adjust the very fine gain. For each pulse, the measured charge is increased by
the appropriate fine gain factor to correct for the signal lost due to carrier trapping.
1.4. The Low Frequency Rejector (LFR) Filter
In designing a multichannel buffer (MCB) that can be used in conjunction with mechanical
coolers, ORTEC has developed a new digital filter, the Low Frequency Rejector (LFR) filter,
capable of correcting the pulse output signal for changes in the baseline caused by cooler-
induced microphonics. In many ways digital filters are easier to understand than their analog
counterparts. Figure 2 shows the voltage step output produced at the preamplifier by the collec-
tion of charge produced by absorption of a gamma-ray and the resulting trapezoidal weighting
function in a digital spectrometer. The difficulty in the measurement is to precisely determine
the height of the step pulse because the baseline contains noise. A fairly obvious estimate of the

DSPEC Pro®Digital Gamma-Ray Spectrometer User’s Manual 794380D / 0914
6
Figure 2. Typical Trapezoidal Weighting Function
(right) Arising from Detector Preamplifier Output
Signal (left).
Figure 3. Example of Weighting Function Output
Resulting from the Positive Slope Due to Low-
Frequency Noise (shown as a sine wave).
step signal is obtained by averaging the
digitized samples of the signal before and
after the step. M samples immediately after
the event are first ignored, to allow for a
maximum rise time of M times the sample
interval. N samples of the baseline prior to
the step pulse are averaged and then sub-
tracted from the average of N samples
taken after the step pulse. This simple
procedure produces a trapezoidal weight-
ing function with a rise time of N sample
intervals and a flattop of M sample intervals. The maximum value of the trapezoid output, occur-
ring at the end of the flattop, is the best estimate of the step height and therefore the gamma-ray
energy. With a proper selection of M and N, this filter is very nearly the optimum filter for a
system with noise arising only from the detector leakage (parallel noise) and the FET current
(series noise).
The trapezoidal filter is essentially inde-
pendent of dc offsets, since the averaging
and subtracting removes the dc component
of the signal. Unfortunately, it is just as sen-
sitive as analog filters to slowly varying
signals such as those produced in micro-
phonic noise. Figure 2 shows the output of
the trapezoidal filter is equal to the slope
of the baseline signal multiplied by the
full width at half maximum (FWHM) of
the trapezoid. If a step pulse were to be
measured on such a base-line, the filter
output value would be too great by an
error equal to the difference between the
average values A1 and A2. Since the
microphonic noise component in a signal
is approximately a sine wave, as illus-
trated in Fig. 3, the error induced can be
positive, negative, or zero. This error
signal adds to the width of the spectral
lines, appearing as degraded resolution
performance from the detector, and can
in many cases be a dominant noise
source, especially at lower energies.

794380D / 0914 1. INTRODUCTION
7
Figure 4. LFR-Enabled Digital Filter.
The ORTEC LFR filter removes most of the microphonic noise by estimating the microphonic-
induced error signal on a pulse-by-pulse basis and subtracting the estimated error signal from the
trapezoid output. As noted above, the error signal is proportional to the slope of the baseline
during the energy measurement. If the slope is known, then so is the error introduced by the
microphonics. An excellent estimate of the slope can be obtained by using the trapezoidal filter
itself to measure the slope both before and after the energy measurement. Since the digital filter
is always sampling the input signal, it is only necessary to store (1) the values measured before
the event is detected, (2) the gamma-ray energy measurement, and (3) the values measured after
the event is detected. The modified trapezoidal digital filter for LFR from an InSight Virtual
Oscilloscope trace is shown in Fig. 4. A suitably weighted and averaged value of the before and
after slope measurement is then subtracted from the energy measurement producing a measure-
ment essentially free of microphonic noise. Although the inherent increase in the pulse proces-
sing time increases the dead time of the system, the resolution can be greatly enhanced when
periodic noise is present.

DSPEC Pro®Digital Gamma-Ray Spectrometer User’s Manual 794380D / 0914
8
To switch to LFR mode, click the Amplifier PRO tab under Acquire/MCB Properties..., and
mark the Low Frequency Rejector checkbox (see Section 2.3.3). Note that you cannot optimize
or pole-zero the DSPEC Pro while in LFR mode. The Optimize feature should be used with the
LFR filter off (checkbox unmarked). Subsequent measurements can then be taken with the LFR
filter on.
1.5. Ballistic Deficit and Adjusting the Flattop Duration
In germanium detectors it takes a variable amount of time to collect all of the charge released in
the detector diode when a gamma ray deposits energy in the detector. The duration of this charge
collection time depends on the dimensions of the detector, the geometry of the electrodes, and
the points at which energy is deposited in the detector. A small, planar detector has charge col-
lection times that vary from 50 ns to 100 ns, whereas a large coaxial detector is characterized by
charge collection times that vary from 100 ns to 700 ns. The variability of the charge collection
time within a specific detector is the source of the resolution degradation described by the
ballistic deficit effect.
The ballistic deficit becomes a significant source of resolution degradation when very short
shaping times are selected in order to reduce dead time and improve the high counting-rate limit.
For a digital filter having the shape of a symmetrical triangle in the time domain, the output
pulse begins to rise as soon as the gamma ray is detected. It continues to rise until it reaches a
peak amplitude at a time specified by the currently selected rise/fall time (this is the Rise Time
parameter entered on the Amplifier 2 tab under Acquire/MCB Properties...; see Section 2.3.2).
Beyond this peak-amplitude time, the pulse falls back to the baseline to allow a subsequent
gamma ray to be processed. If all the charge has not been collected by the detector by the desig-
nated time for peak amplitude, there will be a deficit in the measured peak amplitude and a
broadening of the pulse width. The deficit in amplitude is called the ballistic deficit. Obviously,
events that result in a faster charge collection time will suffer less deficit and less increase in
pulse width than those yielding a slower charge collection time, even though the deposited
energy was identical. Thus, the ballistic deficit resulting from variable charge collection times
broadens the energy resolution for that gamma ray.
If the longest charge collection time is negligible compared to the rise time of the filter pulse, the
ballistic deficit will be imperceptible, and the energy resolution will not be degraded. Thus, at
the 10- to 20-µs rise times typically selected for optimum signal-to-noise ratio (i.e., minimum
peak FWHM at low energies) the ballistic deficit problem can be ignored.
Long rise times create higher dead times and depress the maximum counting-rate limit. If it is
necessary to accept higher counting rates, the rise/fall times must be shortened accordingly. For
this case, the DSPEC Pro includes a flattop feature for the filter that can eliminate the resolution
broadening caused by ballistic deficit. Instead of a symmetrical triangle with a sharp point at the

794380D / 0914 1. INTRODUCTION
9
peak amplitude, the rising and falling edges are separated by a flattop to form a trapezoid. The
width of the flattop is adjusted to be long enough to match the longest charge collection time for
the detector employed. This allows time for the longer-charge-collection pulses to reach the
same maximum pulse amplitude as the faster-charge-collection pulses from gamma rays of the
same energy. Consequently, the effect of ballistic deficit is reduced, and the energy resolution is
dramatically improved at these short pulse widths.
Note that the selection of shorter pulse widths inevitably degrades the signal-to-noise ratio com-
pared to the optimum achieved at longer pulse widths. Thus, operating at shorter pulse widths
will compromise the energy resolution of low-energy gamma rays, for which the preamplifier
noise makes a significant contribution to the energy resolution. The signal-to-noise degradation
is independent of the ballistic deficit effect.
The flattop and other shaping controls are on the Amplifier 2 tab under Acquire/MCB Proper-
ties...; see Section 2.3.2 for a more detailed discussion.
1.6. Zero Dead-Time (ZDT) Mode
An extended live-time clock increases the collection time (real time) of the acquisition to correct
for input pulse train losses incurred during acquisition due to system dead time. This corrected
time value, known as the live time, is then used to determine the net peak count rates necessary
to determine nuclide activities.
As an example, consider the case where the spectrometry amplifier and ADC are 60% dead
during the acquisition. the elapsed real time will be:
If the Ncounts in the gamma-ray peak in the spectrum are divided by the elapsed live time, the
resulting counting rate, is now corrected for dead-time losses. The standard
deviation in that counting rate is .
Unfortunately, extending the counting time to make up for losses due to system-busy results in
an incorrect result if the gamma-ray flux is changing as a function of time. If an isotope with a
very short half-life is placed in front of the detector, the spectrometer might start out with a very
high dead time, but the isotope will decay during the count and the dead time will be zero by the
end of the count. If the spectrometer extends the counting time to make up for the lost counts, it

DSPEC Pro®Digital Gamma-Ray Spectrometer User’s Manual 794380D / 0914
6Patent number 6,327,549.
10
Mode Uncorrected
Spectrum ZDT Corrected Spectrum ZDT Error Spectrum
Off (ZDT Disabled) Yes No No
NORM_CORR (ZDT–LTC Mode) Yes Yes No
CORR_ERR (ZDT–ERR Mode) No Yes Yes
Table 1. ZDT Modes.
will no longer be counting the same source as when the losses occurred. As a result, the number
of counts in the peak will not be correct.
When a supported ORTEC MCB operates in ZDT6mode, it adjusts for the dead-time losses by
taking very short acquisitions and applying a correction in real time — that is, as the data are
coming in — to the number of counts in the spectrum. This technique allows the gamma-ray flux
to change while the acquisition is in progress, yet the total counts recorded in each of the peaks
are correct. The resulting spectrum has no dead time at all — in ZDT mode, the data are correc-
ted, not the acquisition time. Thus, the net counts in a peak are divided by the real time to deter-
mine the count rate.
ZDT mode has a unique feature in that it can store both the corrected spectrum and the uncor-
rected spectrum, or the corrected spectrum and the uncertainty spectrum. Therefore, supported
MCBs allow you to choose between three ZDT Mode settings on the ADC tab under MCB
Properties...: Off, NORM_CORR, and CORR_ERR.
Table 1 shows which spectra are collected in the three possible ZDT modes.
1.6.1. Off — Uncorrected Spectrum Only
In this mode, only the uncorrected spectrum (live time and real time with dead-time losses) —
also called the live-time-corrected or LTC spectrum — is collected and stored in the
.SPC file.
The LTC spectrum can be used to determine exactly how many pulses at any energy were pro-
cessed by the spectrometer. The corrected spectrum gives the best estimate of the total counts
that would have been in the peak if the system were free of dead-time effects. The uncertainty
spectrum can be used to calculate the counting uncertainty, channel by channel, in the corrected
spectrum.