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APR 2 4 1970 NBS TECHNICAL NOTE 523

Experimental

Techniques for

Electron Scattering

Investigations

U.S.

>ARTMENT

OF

OMMERCE

National

Bureau

of

Standards
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NBS TECHNICAL NOTE 523

ISSUED APRIL 1970

Nat. Bur. Stand. (U.S.), Tech. Note 523, 46 pages (Apr. 1970)

CODEN: NBTNA

Experimental Techniques for

Electron Scattering Investigations

Samuel Penner

Linac Radiation Division

Center for Radiation Research

National Bureau of Standards

Washington, D.C. 20234

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FOREWORD

This report was originally prepared for presentation at a Seminar

on the subject "Nuclear Investigations on Linear Electron Accelerators

at Low and Medium Energy^' held at the P. N. Lebedev Physical Institute,

Moscow, USSR, December 10-16, 1969. The paper presents a fairly extensive review of modern techniques used in the performance of electronnucleus scattering experiments with electron linear accelerators in the

50 to 1000 MeV range, and particularly of the NBS facility which has not

been fully described elsewhere. The paper is therefore reproduced here

in its original form in order to be available to a wider audience.

in
CONTENTS

Page

1. Introduction 1

2. Spectrometer Design 2

3. Detector Systems 7

4. Current Monitoring 12

5. Background Considerations 14

6

.

Data Handl ing with On-Line Computer Sys terns 16

7. Theoretical Considerations 19

8. Electron Scattering at 180° 23

9

.

Summary 24

10. References 26

IV
Experimental Techniques for Electron Scattering Investigations

S . Penner

Modern experimental techniques for high energy electron

scattering are discussed. Subjects included are: high

resolution spectrometer design, the energy loss spectrometer concept, detector ladder systems, beam current

monitoring techniques, suppression and measurement of

background, the use of on-line computer systems, theoretical considerations and the analysis of data, and

180° scattering.

Key words : background; current monitoring; detector ladders;

electron scattering; experimental techniques; line-shape

fitting; on-line computer systems; radiative tails;

resolution; spectrometer design.

1. Introduction

For many years it has been said that electron scattering is a

powerful tool for the study of nuclear structure, because quantum

electrodynamics is a very accurate theory and because the weakness of

the electromagnetic interaction allows accurate theoretical interpretation of experimental results. In spite of these advantages, electron

scattering has in the past contributed little to our understanding of

the nucleus, in contrast to the vast amount of information obtained by

reaction studies employing nuclear particles (protons, alphas, etc.).

This situation has changed greatly in recent times, and electron scattering is at last proving its great value as an experimental technique

for the study of nuclear properties.

As a result of recent improvements in experimental techniques and

apparatus we are now able to perform detailed and accurate experiments

which yield valuable information on nuclear structure. For example,

figure 1 shows a recently-obtained spectrum of electrons inelastically

1/* 48-

scattered from Ti compared to an inelastic alpha scattering experi-

*Literature references at the end of this paper.

1
ment on the same nucleus. We can see from figure 1 the the resolution,

peak-to-background ratios, and the states excited in the two experiments

are quite comparable.

The main reasons that we are now able to perform experiments of this

quality are: (1) the development of the modern electron linear accelerator which provides high current, small beam energy spread, good beam

emittance, and improved duty cycle compared to earlier machines; (2)

better understanding of the principles of beam transport which allows us 2/

to obtain well resolved, stable electron beams of high quality51

; (3)

improvements in the design and construction of magnetic spectrometers to

obtain high resolution and good acceptance (solid angle times useful

momentum range along the focal plane) ; (4) the development of multichannel "ladder" detector systems; (5) improved methods of beam current

monitoring; and (6) the use of on-line computer systems, enabling us to

handle the enormous quantity of data obtained and to effectively control

and monitor the progress of the experiment.

In this paper we will report on the present state-of-the-art in the

experimental techniques mentioned above. We will also refer briefly to

developments in electron scattering theory as they apply to the interpretation of experimental data, and discuss some current problems such

as the treatment of backgrounds.

2. Spectrometer Design

The properties of an electron scattering facility are largely determined by the parameters of the magnetic spectrometer which is used. We

will begin our discussion by listing the major design requirements for a

spectrometer. First, good resolution is essential. As shown in Table I,

2
many factors contribute to the overall resolution of the system. Not

only must the intrinsic resolution of the spectrometer be good, but

large dispersion is required so that detectors and beam spots of reasonable dimensions can be used. Second, because low cross sections are an

ever-present problem, large solid angle and momentum acceptance are

needed. Third, the spectrometer should be useable over a broad range of

both momentum and scattering angle to permit measurement of both longitudinal and transverse form factors over an extended range of momentum

transfer. Fourth, the geometry of the spectrometer must allow for good

shielding of the detector system because we must measure very small

cross-sections in the presence of very high backgrounds.

Ten years ago the standard instrument for electron scattering was

the 180 n=l/2 double focussing spectrometer- . It provided a resolution

of 0.1 to 0.27o with a solid angle of order .005 steradians and, since the

double-focussing property results in a small detector volume, it was

relatively easily shielded. With the advent of the high current electron

linac, we could afford to define a smaller momentum bin in the beam

transport system, use thinner targets to reduce energy-loss straggling,

and still have reasonable counting rates. It was then necessary to

design spectrometers to take advantage of the better resolution capability ty thus available. We chose the "magic angle spectrometer"- because it

provides good resolution while retaining all the desirable properties of

the 180 spectrometer. This spectrometer, shown in figure 2, is now in

use at NBS. Similar spectrometers are in use at Amsterdam-, Tohoku

(Japan), and Glasgow. A spectrometer of slightly modified design providing smaller aberrations in the transverse (scattering) plane and a larger

3
6/ useful momentum acceptance is being installed at Saclay—

.

The resolution obtainable with the magic angle spectrometer is

demonstrated in figure 3, which shows inelastic scattering data from the

first two excited states of from a 35 mg/cm beryllium oxide target-.

The overall resolution of about 60 keV (full width at half-maximum)

results from a total instrumental resolution of 48 keV (0.08 percent of

the incident energy) plus a contribution due to energy loss straggling.

I do not want to give the impression that the magic angle spectrometer is the only design which is suitable for high resolution electron

scattering work, because there are in fact many other possibilities. I

want to mention here one entirely different design, the "energy loss

8/ spectrometer" being built at MlT~". The MIT spectrometer is a uniformfield, 90 deflection split -pole type, designed for a momentum resolution

4 capability of about one part in 10 over a 27 interval, and a solid angle

9/ of about 5msr—. The important difference of the MIT design from conventional spectrometers is that the transverse optics provides "point-toparallel" rather than "point-to-point" focussing. The reason for this

choice is that for large momentum transfers and light target nuclei the

kinematic broadening of the resolution for a reasonable acceptance angle

in the scattering plane is large. By employing point-to-parallel focus-

. sing, knowledge of the scattering angle is preserved, and thus a correction can be made for the kinematic broadening. A possible disadvantage

of this approach is that one of the methods for handling the "instrumental background" is not possible here. This will be further discussed in

section 5.

While we are discussing the MIT design, I would like to comment on

4
the energy-loss system which the MIT group plans to use. The basic

principle is illustrated in figure 4, which is taken from reference 8.

In this system, the beam transport arrangement provides a momentumdispersed (but spacially focussed) beam at the spectrometer target location. If this dispersion is properly matched to the imaging and dispersion properties of the spectrometer then all electrons which have lost a

given amount of energy in the target arrive at a single point in the

spectrometer focal plane, independently of their initial energy. Since

good resolution in energy loss is the essential requirement in electron

scattering, the energy loss system allows one to use a larger portion of

the current from the linac while maintaining good resolution. Thus the

advantage of this system is that it permits much higher counting rates

whenever beam current is the limiting factor. I want to emphasize that

the energy loss system does not allow a relaxation in the resolution

requirements of either the beam transport system or the spectrometer. In

fact, it may be more difficult to obtain the needed resolution because

some spectrometer aberrations, which are not significant when the beam

spot size at the target is small, are important for the larger spot size

of the dispersed beam.

There are some general principles of spectrometer design which are

important regardless of the specific optics. In order to obtain the

optical properties called for in the design it is necessary to obtain the

required magnetic field configuration to a high degree of accuracy— .

Obtaining the desired field configuration is particularly difficult in

electron scattering because the spectrometer must be useable for a large

range of particle momenta, and thus a large range of field strengths.

5
Differential saturation of the iron can distort the field distribution

drastically when the field strength is changed. One method of minimizing

this effect is to use the "Rogowski profile11 for the edge of the magnet

poles—. To simplify the construction problem, it may be adequate to

approximate the Rogowski profile by one or two plane segments at the

edges. This is the procedure used in the NBS spectrometer, as shown in

figure 5. We have shown that the optical properties of this spectrometer

do not change appreciably in the range of field strengths between 0.9kG

and 13kG (in our case this corresponds to momenta between about 20 and

290MeV/c)

.

In order to fully use the high resolution properties of the spectrometer, the magnetic field must be stable to a small fraction of the

resolution width. Stabilities of one part in 10 or better are obtained

from conventional well regulated power supplies. Figure 6 demonstrates

the current stability of the NBS spectrometer. When taking data we

routinely measure the spectrometer current (actually the voltage across

a series "shunt" resistor held at constant temperature by water cooling)

along with our count rate measurements at intervals of typically one to

three minutes. We do not routinely measure the magnetic field in the

spectrometer directly as is done in most other laboratories. As a matter

of fact it is not obvious that a single point magnetic field measurement

should be more precisely proportional to electron momentum than is a current measurement. Our procedure is to follow a prescribed path along the

hystersis curve of the iron by "cycling" the spectrometer. This provides

a reproducibility of momentum for a given spectrometer current within

4 12/ one or two parts in 10 when the spectrometer is recycled—. When taking
typical inelastic scattering spectra, peaks corresponding to known energy

levels are observed to occur at the proper electron energy, typically

within ±5keV.

3. Detector Systems

The main requirements placed on detector systems for scattered electrons are: small size in the dispersion direction to take full advantage

of the resolution, stability of counting efficiency, good rejection of

background events, and the ability to operate at high instantaneous

counting rates. It is highly desirable to use many counting channels,

not only to improve the data accumulation rate, but to minize crosssection errors due to variations of beam energy, spectrometer magnetic

field, or beam current. The first multichannel detection system, or

"ladder counter" for electron scattering work was developed at Stanford

13/ by H. W. Kendall—. The basic idea is that a number of small detectors

are located along the focal plane of the spectrometer in order to detect

the position at which each electron intersects the focal plane. Thus

each detector corresponds to a slightly different momentum bin of scattered electrons for any given magnetic field in the spectrometer.

The ability to handle high counting rates (in each detector) is very

important in electron scattering for two reasons. First, it is often

necessary to obtain good statistical precision for the net area of a

small peak superimposed on a large continuum (e.g., due to the radiative

tail of the elastic peak). Secondly, it is often desirable to normalize

inelastic scattering cross sections to the elastic scattering cross section which may be many orders of magnitude larger. Better accuracy can

be obtained when it is possible to avoid making large changes in the

7
beam current between the elastic and inelastic measurements.

Several different types of detectors have been used successfully

in ladder systems, including plastic scintillators, semiconductors, and

multi-wire spark chambers or proportional counters. We will describe

these types of ladder counters briefly here and compare the advantages

and disadvantages of each type. We refer to the literature for more

detailed descriptions.

Plastic scintillator ladders offer the advantages of high count rate

capability due to the fast response and rapid recovery of the scintillator-phototube .combination. When a coincidence requirement between the

counters in the ladder and a "baek-up" detector (usually a large

Cerenkov Counter) is included in the system, the background rejection

properties are good. However, it has been demonstrated that if sufficient care is taken in reducing the background, plastic scintillator

14/ detectors can be used effectively with no coincidence requirement— (at

least when the electron beam energy is low). The main problem with

plastic scintillators in high resolution systems is that the detectors

must be very small, making it difficult to get enough light to the photo

tube to have a satisfactory signal-to-noise ratio. It is also difficult

to package a scintillator system compactly, because of the necessity for

light pipes and the relatively large size of phototubes which should be

as close to the scintillators as possible.

Counter ladders employing lithium-drifted silicon semiconductor

detectors were first used for electron scattering at NBS"—'—. A semiconductor ladder is also in use at the Tohoku linac, and a large semiconductor system is in preparation at Saclay. It is relatively easy to

8
make semiconductors very small (the dimensions of the present NBS detec3

tors are 1 x 1.25 x 60mm ), and pack them close together in a ladder.

Since appropriate preamplifiers can be made very small, expecially if

integrated circuits are used, the entire array is quite compact compared

to a scintillator-phototube system and therefore is easier to shield.

Semiconductor systems are in general not as fast as phototube systems,

but if the detectors are cooled to liquid-nitrogen temperature, charge

collection times of about 20ns can be obtained. The NBS counters have a

coincidence resolving time of 50ns and a dead time of 80ns. With the

aid of auxiliary circuitry to measure chance counts and dead-time corrections, our system is useable at high counting rates and in severe

background-to-true count rate situations —

*

. Two disadvantages of

semiconductor detectors are their sensitivity to radiation damage and to

surface contamination. Because of these effects, the detector efficiencies can change (over a period of some weeks or months), and eventually

the detectors must be replaced. This is not too serious because the

detectors themselves are inexpensive, and replacing them is not difficult

In the NBS system, the useful detector lifetime seems to be roughly one

to two years.

Spark chambers of various types have also been employed as ladders

counter—

'

. The spatial resolution of spark chambers is about the

same as semiconductor detectors; they are just as compact, and the electronic readout system is simpler. A great many channels can be built

since the cost per channel is relatively small. Counting rate limitations

and poor background rejection seem to be the main disadvantages of spark

chambers. I am not aware of any system of this type which is capable of

9
recording more than one count per linac beam pulse per channel, whereas

phototube systems and even semiconductors can record several events per

39/ pulse for beam pulse lengths of a few microseconds— .

A serious problem with any multichannel array is to determine the

detection efficiencies of the individual detectors. The problem is

severe with detectors of very small size because a substantial fraction

of the incident particles pass through or near the edges, thus producing

pulses which are smaller (and perhaps of a different shape) than the

pulses due to particles passing through the center of the detector.

Unless one is willing to be committed to a data taking procedure in which

all detectors in the ladder are eventually located at all electron momemtum values in the spectrum desired, a means must be found for accurately

intercomparing the efficiencies of all detectors. The usual procedure

for obtaining the efficiencies is to measure the counting rates due to a

flat, or at least reasonably smooth, spectrum of scattered electrons.

From data taken in all detectors at several closely spaced spectrometer

field settings, the relative efficiencies of the detectors can be deter19/ mined—. A different procedure can be used when provision is made for

mechanically moving the detectors along the focal plane. One simply

moves the ladder so that each detector occupies the position previously

. occupied by its neighbor and compares the counting rates of each pair of

adjacent detectors under identical experimental conditions—. By combining these two procedures it is possible to distinguish intrinsic differences between the detectors from a number of contributing effects such as

variation of solid angle and dispersion along the focal plane and loss of

efficiency due to multiple scattering of electrons in the small detectors

10
(resulting in some electrons not reaching the back-up detector, an effect

which can be important for low energy electrons).

The ability to move the detectors along the focal plane has other

useful features. When the resolution is good, only a few detectors see

a peak in the spectrum at one magnetic field setting. For an isolated

level, this is not very important since only the area of a peak is physically significant, but for closely spaced levels or small peaks superimposed on a continuum, many points closely spaced in energy are needed

to distinguish contributions from each level and/or the continuum. With

a movable ladder, it is easy to space data points very closely by small

changes in the detector position. Of course, such changes can also be

accomplished by changing the magnetic field but is seems to us to be

easier to mechanically move the detectors a precisely determined distance

reliably and reproducibly. In the NBS system, the precision of detector

motion corresponds to 2 parts in 10 in momentum, which would be very

difficult to achieve or measure with magnetic field changes.

A separate, but related, problem is the determination of the absolute efficiency of the detector system which is necessary if absolute

cross sections are to be measured. Absolute electron scattering cross

sections are rarely measured, but in fact can be traced back to the

assumed accuracy of the Rosenbluth formula for elastic electron-proton

20/ scattering together with the experimentally known proton form factors—

.

At NBS we have demonstrated the feasibility of a method for determining the

absolute efficiency of our detector system which involves the use of an

auxiliary "area" detector in the focal plane which is much larger in the

dispersion direction than the detectors in the ladder. The area detector

11
determines the area of a scattering peak without precise knowledge of

detector edge effects, since only a very small fraction of electrons in

the peak pass near the edges. The small detectors can be compared to the

area detector by a series of measurements in which the detectors are

moved along the focal plane. From this data the efficiencies of the

small detectors are determined relative to the area detector, and the

small influence of edge effects on the latter can be removed iteratively.

To complete the determination of absolute efficiencies, it is necessary

to study in detail the response of the area detector as well as the

plastic scintillators in coincidence with it. This is done by determining pulse height spectra, variations of count rates with detector collection voltage or phototube high voltage, and by other standard techniques.

4. Current Monitoring

Accurate beam charge measurement is essential to any cross section

determination. A Faraday cup together with an electronic integrating

circuit provides good absolute accuracy, but introduces several problems

if used during the course of the scattering measurements. The problems

are: (1) the cup is a very strong source of background; (2) at low

energies, multiple scattering in the target may prevent some of the electrons in the beam from reaching the cup; and (3) if used continuously,

the cup must be cooled to remove the heat due to the beam power. Because

of these difficulties, most laboratories employ other types of current

monitors, which must be calibrated against the Faraday cup. The two

most commonly used monitors are the secondary emission monitor (SEM),

and the ferrite pulse transformer. Although good results have been

21/ reported with the SEM— , we have not had much success with them at NBS.

12
We believe that our high beam current causes changes in the surface

properties of the foils in the SEM, resulting in erratic behavior and

substantial variations in calibration during data taking.

A ferrite-core pulse transformer provides a good measurement of

beam pulse current, and produces no background since the beam is not

intercepted. Electronic integration of the signal to obtain the beam

charge is more difficult than in the case of a Faraday cup because the

signals are smaller and A. C. coupled. Appropriate integrating circuits

22/ have been developed— which provide charge measurement precision of

about ±0.2 percent for currents of the order of 0.5|JiA or larger. The

limiting factor seems to be how well electrical pickup (primarily due

23/

to the linac modulators) can be suppressed—

.

An interesting technique for current measurement is in use at

Amsterdam, employing a ferrite pulse transformer together with a Faraday

24/

cup—. The Faraday cup is located far enough from the experimental target so that it can be well shielded. A pair of quadrupole magnets collects the multiple-scattered electron beam from the target to maximize

the fraction of the beam reaching the cup. A known multiple of the current collected by the cup is passed back through the ferrite so as to

25/ cancel the beam current pulse—. A measurement of the charge collected

by the Faraday cup together with a knowledge of the multiplying factor

which produces a null signal from the ferrite provides an accurate

measure of the beam charge delivered to the target. This method is

illustrated in figure 7.

A similar procedure is used at NBS to measure the absolute charge

collection efficiency of our Faraday cup. Measurements now being made

13
by J. S. Pruitt of our laboratory indicate that the cup, whose dimensions

26/

are based on the work of Kantz and Hofstadter—, is large enough to contain essentially all of the charged secondaries produced by the beam. He

also finds that ther is a small (<l7o) but energy-dependent loss of charge

due to backscattering of electrons through the entrance of the cup, and

that there is a small (~.5%) increase in charge collected in the cup due

to collection of secondary electrons generated by electron-electron scattering in the entrance vacuum window of the cup. By applying corrections

for these measured effects, we believe we can determine the absolute efficiency of our Faraday cup to an accuracy of better than 0.1%.

5. Background Considerations

Suppression of background counts in the detector system is essential

for accurate cross-section measurements. We distinguish three main

sources of background which must be considered.

First, there is the room background generated primarily at the place

the electron beam, having passed through the target, is finally absorbed.

This type of background is handled by shielding the Faraday cup (as

mentioned already in section 4), or by using a non- intercepting monitor

(or SEM) and carrying the residual beam to a well shielded "beam dump".

Shielding directly around the detectors is obviously necessary also. At

NBS our main shielding consists of 20cm of lead and 10cm of boron loaded

polyethylene. The room background which does penetrate the shielding is

mostly neutrons and thus produces many more singles counts in the individual detectors than it does coincidence counts which might be interpreted as good events. Thus the main effect of room background in our

case is to produce electronic dead times. This amounts to roughly 2%

14
dead time per microamp of beam current when the Faraday cup is in the

beam, and 0.57o per microamp when the cup is not in the beam. We feel

that we can make deadtime corrections to better than 20% of their value,

thus permitting 1% counting accuracy at 10|aA beam current.

A second source of background is degraded electrons and x-rays in

and around the main electron beam, which can scatter from the target and

27/ enter the spectrometer, besides contributing to the room background—

.

Suppression of this type of background depends mainly on good beam handling technique-.

The third and most serious type of background is that due to scattered electrons which enter the spectrometer but have an energy such that

they can not reach the detectors directly, but instead scatter from the

pole tips and vacuum chamber walls of the spectrometer. This "instrumental scattering" background is worst when attempting to measure a small

inelastic peak which is a few percent (in scattered electron energy) away

from a strong elastic scattering peak. This type of background can be

reduced by at least three approaches as follows: (1) A system of baffles

is installed in the spectrometer to reduce the number of electrons striking the vacuum chamber walls and to absorb the scattered electrons which

12/ do strike the walls—, as shown in figure 8. (2) Making use of the

double-focussing property of the spectrometer one moves the detectors

transversely (out of the spectrometer midplane) so that electrons coming

from the target do not pass through them, and measures the background

14/ directly—. Obviously this method is not available if the spectrometer

optics does not produce a transverse-plane image of the target in the focal

plane, as in the MIT system discussed above (in section 2) . (3) The

15
instrumental scattering is greatly reduced by using a thin-walled vacuum

chamber and moving the magnet return yoke away from the immediate vicin28/

ity of the magnet poles, as in the Mainz spectrometer—. A combination

of these methods should be better than any one of them. Therefore at

NBS, where we use the baffle technique, we plan to replace our present

detector ladder with a two dimensional array so that we can simultaneously measure good events and instrumental background.

6. Data Handling with On -Line Computer Systems

In high resolution electron scattering, enormous quantities of data

must be recorded and analyzed. In a typical experiment on the low-lying

levels of a light nucleus, one would spend perhaps six hours to obtain

a spectrum for a single incident energy. In order to obtain data at

sufficiently closely spaced momentum intervals and to obtain data for

several detectors at each momentum value (to be able to extract relative

detector efficiencies as discussed in section 3, and to make the results

relatively insensitive to the accuracy of the efficiency intercomparison)

we would use some 200-300 different combinations of magnetic field setting and detector ladder position. A typical spectrum of this type is

shown in figure 9. At NBS we have a 20 channel ladder, so that for each

of these 200 or so "runs" we must record 40 numbers (including such

quantitites as beam charge, run time, spectrometer shunt voltage, counts

in auxiliary channels for dead time corrections, etc., in addition to the

counts in 20 channels). I can say from experience that it would just not

be feasible to take data of this type without some type of automatic data

handling equipment.

In addition to collecting the data, the computer serves several

16
other important functions. It can be programmed to perform a set of runs

automatically. For example, the computer at NBS monitors the beam charge

continuously and terminates the run (by turning off a master counting

gate) when a predetermined charge has been collected. It then computes

the results of the run, types out the data and records them on magnetic

tape for further use, clears all temporary data storage, moves the detectors and/or changes the spectrometer field as desired, and turns on the

master counting gate for the next run. The experimenter, being freed of

all these routine operations, has time to study the results as they are

obtained and thus can intelligently determine the best way to proceed

with the experiment. In order to make these decisions, the experimenter

should have the results available on-line. This means that the computer

must analyze the data during and between runs. At NBS we can choose any

data field in the computer for observation as an oscilloscope display.

We might choose to look at the counts in the detectors of the ladder

displayed as a histogram while the data are being accumulated. More

often, we would watch one of several spectra, corresponding to different

targets or to analysis of one spectrum in various size momentum bins.

These spectra are updated by the computer after each run. The data of

each run are corrected for such things as counting rate effects and

detector efficiency differences and added to the previously-existing

spectra in units of counts per MeV per unit beam charge. A typical

spectrum of this type is shown in figure 10. These are actually the same

data as in figure 9 except that the results from all detectors has been

combined and divided into uniform momentum bins. This particular spectrum was obtained by off-line reanalysis of data on a different computer,

17
using as input a punched -paper tape record produced by the on-line system. Reanalysis is necessary because detector efficiencies and backgrounds are determined during the course of the experiment, so that the

on-line analysis relies on provisional values based on past experience

which may not be highly accurate. Several different computers have been

used for reanalysis, including the on-line system.

I do not want to describe any particular computer system in detail,

however I would like to mention some of the general features of the NBS

29/ system—. We chose a modular approach to the programming, as well as to

the hardware, in order to have a system with maximum flexibility. The

system is used for all the physics experiments done with our linac including neutron time -of -flight spectroscopy, charged particle electroproduction experiments (such as (7,p) and (y,d)), and several others

besides electron scattering. Very little programming must be done for

any particular experimental setup because with the modular concept most

programs (executive system, display of various kinds, etc.) are used by

all experiments. Most changes of experimental procedure or equipment

do not require additional programming. For example, when we converted

our detector ladder from 12 to 20 channels, no original programming was

needed, although several simple modifications in existing programs were

made to accomodate the additional channels of information. The multilevel priority interrupt feature of our system is extremely valuable. It

permits several experimenters to use the computer simultaneously.

(Although possible, this is not actually done very much because of memory

size limitations, and only one experiment is usually done with the linac

beam at one time.) More important, the interrupt feature permits assign18
ment of priorities to the many functions performed by the computer in

one experiment. For example^ all data taking functions are assigned

higher priority than the display function, and the magnetic tape data

recording is done at higher priority than the typing of results since the

tape record is more useful than the printout.

Finally, I should like to point out that on-line computers for

experiments with pulsed accelerators need not necessarily be extremely

fast because the time between beam pulses is available for computation.

The computer at NBS has a memory cycle time of 8p-s (the memory is 16,000

- 24 bit words). In electron scattering, about one millisecond is used

between beam pulses for data accumulation, and about 5 seconds between

runs for calculation of spectra, recording of data on tape, and changing

of experimental conditions.

7. Theoretical Considerations

Although the subject of this paper is experimental techniques, there

are certain theoretical questions which bear directly on the analysis

and interpretation of experimental data, and therefore should be mentioned. These questions are more urgent now than they would have been a few

years ago because of the increasing accuracy of recent experiments.

Radiative corrections are discussed in detail in the recent review

30/ article by Maximon—. From the experimentalist's viewpoint, it appears

that the corrections are well enough understood for the reliable interpretation of experimental data at the level of about one percent accuracy.

To go beyond this level of accuracy seems to be very difficult theoretically, and also difficult to verify experimentally. The reason it is difficult to verify the corrections experimentally is that even with the

19
best resolution presently available, the largest contribution to the correction occurs at values of the radiative energy loss within the experimental resolution width, and thus is not directly accessible experimentally. It will be interesting to test the validity of the corrections

with the improved resolution expected at the new Saclay and MIT facilities. (The only experimental test I know of is to measure the area of a

scattering peak as a function of the integration end point AE, calculate

the radiative correction for each value of AE, and check for consistency^

Dispersive corrections can be important in both elastic and inelas30/

tic scattering, particularly in the vicinity of diffraction minima—

.

I know of no unambiguous experimental measurements of dispersive effects,

although there is one experiment in progress which might turn out to

show an effect. Accurate elastic scattering measurements on the titanium

31/ 32/ isotopes (46, 48, 50) have been made at both NBS— and Stanford— , but

the momentum transfer corresponding to the first diffraction minumum was

obtained by quite different combinations of scattering angle and beam

energy in the two experiments. Dispersive effects are expected to be

energy dependent and might be expected to be particularly large in this

case because of the presence of very strong collective E2 transitions to

low-lying excited states in these isotopes. The inelastic cross section

reaches a peak at momentum transfer values near the first elastic cross

section minimum.

As we continue to improve experimental resolution capability, we

will attempt to measure the cross-sections for more closely spaced

nuclear levels. Thus we will continue to be faced with the problem of

determining the separate cross -sections of peaks in the spectrum which

20
are not fully resolved. To do this accurately requires a line-shape

analysis. I want to briefly describe the procedure developed by

7,33 /

J. C. Bergstronr^ . The calculation includes the effects of Landau

straggling (ionization energy losses), small angle bremsstrahlung, radiation emitted during the large angle scattering, and the experimental

resolution. The computer program which has been developed finds the

best fit to one or more peaks in the spectrum plus continuum (radiative

tails and background) . The parameters which can be found by the least

squares analysis are peak height, peak energy location, target thickness

(in terms of Landau straggling width), and resolution width (assumed to

be a gaussian function). Alternatively any one or more of these parameters may be fixed, allowing the program to find best values for the

remaining ones. For example, if the difference in energy of two levels

is accurately known, the peak heights can be determined more accurately.

In the case of levels which have a physical width comparable to or larger

than the resolution, the physical width can be determined separately if

the experimental resolution is known (say from the analysis of a nearby

narrow peak in the spectrum). The curves in figures 3 and 11 are generated by this line -shape program.

In order to determine inelastic scattering cross-sections the radiative tails due to all processes at lower excitation must be subtracted

from the observed spectrum. This subtraction is relatively unimportant

in the case of resolved peaks, but must be done very well to obtain

accurate cross section information in the continuum region (for examples,

the giant dipole resonance and quasi-elastic scattering). The best

34/ available radiative tail calculations— are performed in Born approxi21
mation. These calculations give excellent agreement with experimental

results for the first few MeV of the spectrum in the case of carbon

targets with incident energies of 50 to 150 MeV, and scattering angles

1 / O "7 / as large as about 160 degrees

*— . When thick target effects

(bremsstrahlung and electron-electron scattering) are included, the

agreement between calculation and experiment is within about 37o in the

above-mentioned limited range of parameters. More important is the

question of whether the calculated tail is correct for the entire spectrum. Figure 12 illustrates a case where the agreement appears to be

better than 10% for the entire spectrum. In this example the absolute

scale of the calculated tail is based solely on the observed elastic

scattering cross section; there is no arbitrary normalization. This

spectrum was measured with an incident energy of 69MeV from a carbon

target at a scattering angle of 75 . At higher energies and more

12/ backward angles the agreement gets progressively poorer— . This effect

is not necessarily due to a failure of the radiative tail theory but

could just as well be caused by the increasing importance of backgrounds

and the relatively greater importance of the nuclear continuum cross

section. Very little is known about the validity of radiative tail

calculations in heavier nuclei, where the Born approximation may not be

very good. More work in this area, both theoretical and experimental,

is needed in order to understand the situation.

Until rather recently, electron scattering cross sections of light

nuclei were conventionally analyzed in the framework of the Born approximation using plane-wave electron wave functions. (The failure of the

Born approximation per se is discussed above where the effect is refer22

!
red to as dispersive corrections.) Even in the lightest nuclei however

the plane wave approximation is only good to about ten percent. Futhermore the distorted wave corrections depend on the multipolarity of the

transition and thus do not cancel in the ratio of inelastic to elastic

cross sections. A number of investigators have studied the problem of

35/ distorted wave effects—, and accurate computer programs have been

developed to perform the calculations by phase-shift analysis using

relativistic coulomb wave functions, for both elastic and inelastic cross

sections. In comparing experimental results, one must be sure to understand how each experiment was analyzed. We should be very careful when

presenting experimental data to state very explicitly when and how

distorted wave corrections are made.

8. Electron Scattering at 180°

In my opinion there has been a tendency among some of our colleagues

to overestimate the value of 180 scattering experiments. The reason

for this is basically that the cross sections due to transverse effects

(magnetic or transverse electric) do not increase at backward angles

(at fixed momentum transfer the cross section actually decreases by about

30% between 150 and 180 ) ; thus when the scattering angle is large

enough that errors due to subtracting continuum tails from the transverse cross section peaks are unimportant, there is no point in going to

more backward angles. The optimum angle depends on resolution, counting

rate, and background as well as on the ratio of transverse to coulomb

2 cross sections (which we know to be essentially tan 9/2). My point is

that we can do better in terms of resolution, counting rate, and background at conventional angles. The reasons are: (1) Up to about 150

23
targets can be oriented for transmission geometry rather than the reflection geometry needed for 180 scattering, giving for a fixed target

thickness, a factor of nearly four in the effective thickness (hence,

counting rate), and simultaneously better energy resolution because in

transmission geometry only the Landau straggling contibutes to the

energy spread, instead of the spread in energy loss corresponding to

twice the target thickness which occurs in reflection. (2) It is usually

necessary to severely limit the solid angle in 180 experiments.

Scattering experiments at 180 are valuable as a part of a more

general program, for those special cases where transverse and large

coulomb cross sections are present at the same excitation energy. I refer

36/ here particularly to magnetic elastic scattering—, and certain special

cases of inelastic scattering such as the Ml contribution to the electrodisintegration of the deuteron near the breakup threshold.

9 . Summary

We have tried to summarize in this report the advances in experimental techniques which have led to major recent improvements in the

accuracy and reliability of electron scattering data. In our opinion

the most important of these advances are those in the areas of resolution,

detector systems, beam current monitoring, background suppression, the

use of on-line computers, and higher counting rates (resulting largely

from the development of the high-current electron linac). In many

experiments a limiting factor at present is the difficulty of obtaining

suitable (in terms of thickness, uniformity, and stability) target

materials, especially in the case of separated-isotope targets. I

believe that in the very near future we will see the results of the

24
recent advances in experimental techniques, in the form of the greatly

improved understanding of nuclear structure which can be gained by

electron scattering studies.

25
10. References

1. D. L. Blum, J. W. Lightbody, Jr., and S. P. Fivozinsky, experiment

in progress at NBS.

2. S. Penner "Beam Handling Techniques for Electron Linear Accelerators"

NBS Technical Note (to be published).

3. E. E. Chambers and R. Hofstadter, Phys . Rev. 103 , 1454 (1956).

R. Belbeoch, P. Bounin, and G. Proca, J. Phys. Radium 21, 489 (1960).

4. S. Penner and J. W. Lightbody, Jr., Proc . Int. Symposium on Magnet

Technology, 154, Stanford Linear Accelerator Center, Stanford,

California, CONF-650922 (1965).

5. C. W. DeJager, F. Th. Duoma, P. J. T. Bruinsma, and C. DeVries, Nucl.

Instr. and Methods 74, 13 (1969).

6. J. B. Bellicard and Ph. Leconte, Revue de Phys. Appliquee 4, 207

(1969).

7. J. C. Bergstrom, W. Bertozzi, S. Kowalski, X. K. Maruyama, J. W.

Lightbody, Jr., S. P. Fivozinsky and S. Penner, Phys. Rev. Letters

24, 4, 152 (1970).

8. S. Kowalski, W. Bertozzi, and C. P. Sargent, in Medium Energy Nuclear

Physics with Electron Linear Accelerators , 39, LNS-MIT Report

TID-24667 (1967).

9. S. Kowalski, private communication.

10. Harald A. Enge, Proc. Int. Symposium on Magnet Technology, 84,

Stanford Linear Accelerator Center, Stanford, California, CONF650922 (1965).

11. C. M. Braams, Nucl. Instr. and Methods 26_, 83 (1964).

12. J. W. Lightbody, Jr., PhD Thesis, University of Maryland, 1970

(unpublished)

.

13. H. W. Kendall, Trans, on Nucl. Sci. of Inst. Radio Engrs. NS-5, 190

(1958).

14. A. Goldmann, Dissertation, Technische Hochschule, Darmstadt, 1969

(unpublished)

.

15. J. W. Lightbody, Jr., and S. Penner, IEEE Trans. Nucl. Sci. NS-15,

419 (1968).

16. W. R. Dodge, J. A. Coleman, S. R. Domen, and J. K. Whittaker, Rev.

Sci. Instr. 3_Z, 1151 (1966).

26
17. J. Jeanjean, J. Lefrancois, J. Perez -Y-Jorba, Report L.A.L. 1143,

Orsay (1965).

18. G. Charpak, R. Bouclier, T. Bressani, J. Favier, and C. Zupancic,

Nucl. Instr. and Methods 62., 262 (1968).

19. H. L. Crannell and L. R. Suelzle, Nucl. Instr. and Methods 44, 133

(1966).

20. R. Engfer and D. Turck, Zeit. fur Physik 205 , 90 (1967).

21. D. B. Isabelle and P. H. Roy, Nucl. Instr. and Methods 20, 17 (1963).

22. J. L. Menke, Nucl. Instr. and Methods 64, 61 (1968).

23. J. L. Menke, IEEE Trans. Nucl. Sci. NS-16., 921 (1969).

24. J. A. Jansen, G. J. Veenhof, and C. DeVries, Nucl. Instr. and Methods

74, 20 (1969).

25. J. F. Hague, R. E. Jennings, and R. E. Rand, Nucl. Instr. and Methods

24, 456 (1963).

26. Asher Kantz and Robert Hofstadter, Phys . Rev. 89, 607 (1953) and

Nucleonics 12_, 36 (1954).

27. Hans Dieter Wohlfahrt, Doctoral Dissertation, Johannes Gutenberg

Universitat, Mainz, 1969 (unpublished).

28. Reiner Neuhausen, Doctoral Dissertation, Johannes Gutenberg

Universitat, Mainz, 1969 (unpublished).

29. J. B. Broberg,IEEE Trans, on Nucl. Sci. NS-13, 192 (1966).

J. M. Wyckoff,IEEE Trans, on Nucl. Sci. NS-13, 199 (1966).

30. L. C. Maximon, Rev. Mod. Phys. 41_, 193 (1969). A good bibliography

of the subject of radiative corrections is included in this review

article.

31. E. Romberg, N. S. Wall, D. Blum, J. Lightbody, Jr., and S. Penner,

Bull. Am. Phys. Soc. 14, 12, abstract FC14, (Dec. 1969).

32. D. G. Ravenhall, private communication.

33. J. C. Bergs trom, in Medium Energy Nuclear Physics with Electron

Linear Accelerators, 251, LNS-MIT Report TID-24667 (1967).

34. L. C. Maximon and D. B. Isabelle, Phys. Rev. 133 , B1344 (1964).

L. C. Maximon and D. B. Isabelle, Phys. Rev. 136 , B674 (1964).

Y. S. Tsai, Int. Conf. on Nucleon Structure, Stanford Univ. Press,

Stanford, California (1965).

27
35. S. T. Tuan, L. E. Wright, and D. S. Onley, Nucl. Instr. and Methods

60, 70 (1968).

C. R. Fischer and G. H. Rawitscher, Phys. Rev. 135 . B377 (1964).

D. Drechsel, Nucl. Phys. A113 . 665 (1968).

T. H. Schucan, Phys. Rev. 171. 1142 (1968).

36. R. E. Rand, R. Frosch, and M. R. Yearian, Phys. Rev. Letters 14, 234

(1965).

J. Goldemberg, L. A. L. 1140, Orsay (1965).

37. R. A. Schrack, Nucl. Instr. and Methods 45, 319 (1966).

38. H. Crannell, C. J. Crannell, F. J. Kline, J. T. O'Brien, S. P.

Fivozinsky, and S. Penner, Bull. Am. Phys. Soc. 14 , 12, abstract

FC 10 (Dec. 1969).

39. Note added in proof : Very recent work at Saclay indicates that

the count rate limitation of multiwire proportional chambers can

be improved by the use of electronegative gases in the chambers.

This group has observed resolution times near 30ns and counting

rates of the order of 10' per second. See: "Multiwire Proportional

and Semiproportional Counter with a Variable Sensitive Volume"

C. Grunberg, L. Cohen, and L. Mathieu, preprint, Service de Physique

Nucleaire a Haute Energie, Centre d'E tudes Nucleaires de Saclay.

We thank Prof. W. Bertozzi for calling this paper to our attention.

28
Table I

EFFECTS CONTRIBUTING TO OBSERVED RESOLUTION

The resolution factors listed are deduced from experimental data

obtained with the NBS electron scattering facility. The percentages are

given as full width at half maximum (FWHM) values relative to the incident beam energy. The effects listed should be added in quadrature to

obtain the overall resolution (.07 percent, minimum). The starred

entries are the FWHM resolutions of gaussian distributions which would

37/ give the same contributions as the actual rectangular distributions— . 2

The target referred to here is 20 mg/cm polyethylene, (CH ) , and the

values quoted are for incident energies 60-100 MeV and scattering

angles 90-145 degrees.

FACTOR PERCENT

I. Beam Transport

Entrance Aperture .04-. 09*

Energy Slits varies

Intrinsic .04

II. Spectrometer

Detector Size .03*

Intrinsic .03

Beam Spot Size .02

III. Target

Kinematics .01-. 02*

Energy Loss Straggling .01-. 02

29
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energy scales for the two figures have been made equal. Vertical scales are arbitrary. The (pt3 a

% ) data is taken from the

paper of A. M. Bernstein,et al., Nucl. Phys. A115, 79 (1968).

30
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THE NATIONAL ECONOMIC GOAL

Sustained maximum growth in a free market economy, without inflation, under conditions of full employment

and equal opportunity

THE DEPARTMENT OF COMMERCE

The historic mission of the Department

is "to foster, promote and develop the

foreign and domestic commerce" of the United States. This has evolved, as a

result of legislative and administrative

additions, to encompass broadly the re- sponsibility to foster, serve and promote

the nation's economic development and

technological advancement. The Depart- ment seeks to fulfill this mission through

these activities:

MISSION AND

FUNCTIONS

OF THE

DEPARTMENT OF

COMMERCE

"to foster, serve and

promote the nation's

economic development

and technological

advancement"

Participating with

other government

agencies in the

creation of national

policy, through the

President's Cabinet and its subdivisions.

• Cabinet Committee

on Economic Policy

• Urban Affairs Council

•Environmental

Quality Council

Promoting progressive business policies and

growth.

• Business and

Defense Services

Administration

• Office of Field Services

Assisting states, communities and

individuals toward economic progress.

• Economic

Development

Administration

• Regional Planning Commissions

• Office of Minority Business Enterprise

NOTE: This schematic is neither an organization chart nor a program outline for budget purposes. It is a general statement

of the Department's mission in relation to the national goal

of economic development.

Strengthening

the international economic position

of the United

States.

• Bureau of International Commerce

• Office of Foreign Commercial

Services

• Office of Foreign

Direct Investments

• United States

Travel Service

• Maritime

Administration

Assuring effective use and growth of the

nation's scientific and technical resources.

• Environmental Science Services

Administration

• Patent Office

• National Bureau of Standards

• Office of Telecommunications

• Office of State

Technical Services

Acquiring, analyzing and disseminating

information concerning the nation and

the economy to help achieve increased

social and economic

benefit.

• Bureau of the Census

• Office of Business Economics

JULY 1969

NBS TECHNICAL PUBLICATIONS

PERIODICALS NONPERIODICALS

JOURNAL OF RESEARCH reports National

(Bureau of Standards research and development in

physics, mathematics, chemistry, and engineering.

Comprehensive scientific papers give complete details of the work, including laboratory data, experimental

procedures, and theoretical and mathematical analyses. Illustrated with photographs, drawings, and

charts. Published in three sections, available separately: • Physics and Chemistry

;Papers of interest primarily to scientists working in these fields. This section covers a broad range of physical and chemical research, with major emphasis

|Dn standards of physical measurement, fundamental

constants, and properties of matter. Issued six times a year. Annual subscription: Domestic, $9.50; for- eign, $11.75*. • Mathematical Sciences

Studies and compilations designed mainly for the mathematician and theoretical physicist. Topics in mathematical statistics, theory of experiment design,

numerical analysis, theoretical physics and chemistry, logical design and programming of computers

and computer systems. Short numerical tables. Issued quarterly. Annual subscription: Domestic,

$5.00; foreign, $6.25*. • Engineering and Instrumentation

Reporting results of interest chiefly to the engineer

and the applied scientist. This section includes many

of the new developments in instrumentation resulting

from the Bureau's work in physical measurement,

data processing, and development of test methods.

It will also cover some of the work in acoustics,

applied mechanics, building research, and cryogenic

engineering. Issued quarterly. Annual subscription:

Domestic, $5.00; foreign, $6.25*.

TECHNICAL NEWS BULLETIN

The best single source of information concerning the Bureau's research, developmental, cooperative and

publication activities, this monthly publication is designed for the industry-oriented individual whose

daily work involves intimate contact with science and

technology

for engineers, chemists, physicists, re- search managers, product-development managers, and

company executives. Annual subscription: Domestic,

$3.00; foreign, $4.00*.

• Difference in price is due to extra cost of foreign mailing.

Applied Mathematics Series. Mathematical tables, manuals, and studies. Building Science Series. Research results, test methods, and performance criteria of building materials, components, systems, and structures. Handbooks. Recommended codes of engineering

and industrial practice (including safety codes) de- veloped in cooperation with interested industries,

professional organizations, and regulatory bodies. Special Publications. Proceedings of NBS conferences, bibliographies, annual reports, wall charts,

pamphlets, etc. Monographs. Major contributions to the technical

literature on various subjects related to the Bureau's

scientific and technical activities. National Standard Reference Data Series.

NSRDS provides quantitive data on the physical

and chemical properties of materials, compiled from

the world's literature and critically evaluated. Product Standards. Provide requirements for sizes, types, quality and methods for testing various industrial products. These standards are developed cooperatively with interested Government and industry groups

and provide the basis for common understanding of product characteristics for both buyers and sellers. Their use is voluntary.

Technical Notes. This series consists of communications and reports (covering both other agency and

NBS-sponsored work) of limited or transitory interest. Federal Information Processing Standards Publications. This series is the official publication within

the Federal Government for information on standards

adopted and promulgated under the Public Law

89-306, and Bureau of the Budget Circular A-86

entitled, Standardization of Data Elements and Codes

in Data Systems.

CLEARINGHOUSE

The Clearinghouse for Federal Scientific and

Technical Information, operated by NBS, supplies

unclassified information related to Government-generated science and technology in defense, space, atomic energy, and other national programs. For

further information on Clearinghouse services, write: Clearinghouse

U.S. Department of Commerce

Springfield, Virginia 22151

Order NBS publications from: Superintendent of Documents

Government Printing Office

Washington, D.C. 20402
U.S. DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20230

OFFICIAL BUSINESS POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE