SLAC-PUB-2993 August 1982 (1)

THE EFFECT OF HEAT ON RADIATION DAMAGE IN LARGE NaI CRYSTALS%

M.A. Van Driel,+ K.H. Kees,$ G. E. MasekBS J.C. Sens,+ J. R. Thompson,$ J. Weber,+ and J. T. White3

Stanford Linear Accelerator Center Stanford University, Stanford, California 94305

ABSTRACT

A project is described in which sixty large, heavily radiation-damaged

NaI crystals have been subjected to temperatures of up to 3OOOC. It is

shown that the treatment restores the uniformity of response of the

crystals.

Submitted to Nuclear Instruments and Methods

s Work supported in part by the Department of Energy, contract DE-AC03-76SF00515 and by the Foundation for Fundamental Research on flatter, the Netherlands.

t Permanent address : National Institute for Nuclear and High Energy Physics, Amsterdam.

$ Permanent address: University of California at San Diego, La Jolla, California 92093.

I

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May : “Oh, have the crystals faults like us?”

L : “Certainly, May. Their best virtues are shown in fighting their faults. And some have a great many faults; and some are very naughty crystals indeed.”

John Ruskin (1819-19001, The Ethics of the Dust, Ten Lectures to Little Housewives on the elements of Crystallisation, 1866 (London; Smith Elder).

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1. INTRODUCTION

One of the experiments currently mounted at PEP (SLAC, Stanford)

employs NaI crystals for the identification of secondary electrons and

positrons in processes of the type e+e- -) e+e-X, where X stands for

additional particles produced in the collision. The equipment consists

of two identical magnetic spectrometers, each equipped with 60 NaI

crystals closely surrounding the beam pipe at either side of one of the

PEP intersection points. Each crystal is 52 cm (20 radiation length)

long and hexagonal in cross section, with an area of 151 cmz. Figure 1

shows the layout of one of the two spectrometer arms.

The two spectrometers have been located in the PEP tunnel since

September 1980, shortly after PEP was first turned on. Since then the

NaI detectors have been subjected to background radiation associated with

the filling of the ring, the beam optimization studies and the steady-

state operation of PEP. For most of the time the crystals were shielded

against background radiation entering from the intersection side by two

remote-controlled 15 cm thick Pb shields (see fig. 1) which were pulled

out only during stable beam operation. This mode of operation provided

adequate shielding for most crystals as evidenced by the relative

constancy of the pulse height over the period November ‘80 - June ‘81.

In June 1981 the spectrometers had to be modified and reduced in

length, as a result of a relocation of the PEP quadrupole magnets nearest

to the intersections. The original Pb shields were removed for lack of

space. At that time, visual inspection of the array showed signs of

discoloration in a number of crystals. Twelve crystals were,returned to

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the manufacturer (Harshaw Chemical Co.), reannealed and reinstalled in

the enclosures. Up to that time rather heavy -beam losses had resulted in

only modest damage to the crystals. A four week period of continuous

monitoring (with temporary, fixed, Pb shields to protect the crystals)

indicated acceptable and steadily improving levels of background. It was

therefore decided to remove the shields from the set-up and begin the

data taking.

Measurements were made with a 6oCo source before and after a four week

period of PEP-operation with the Pb-shields removed. Figure 2 shows a

view of one of the NaI arrays, and indicates the numbering of the

crystals. The hole in the center is the passage for the beam pipe.

Figures 3 and 4 show the extent of the damage in the two arrays ("north"

and "south"). Plotted is the ratio of the pulse height for 6oCo as

measured before and after this period, versus cystal number (l-60). In

the North array which is facing the positron beam, the loss factors are

between 1.2 and 2.1 for most crystals, increasing to 3.5 for some of the

crystals of the inner most ring. In the South array, facing the usually

more intense electron beam, the loss factors are between -2 and 3.5,

except for some of the crystals of the inner ring. In the before/after

comparison a systematic error enters due to the difficulty of positioning

the source in a reproducible manner in the limited space available. This

may explain the fact that the observed loss factors appear to consist of

a common term plus an additional cyrstal-dependent term, both in figs. 3

and 4.

I

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Two days after the measurements referred to in figs. 3 and 4, November

13, 1981, the PEP electron beam was lost upon-injection before completing

one revolution around the ring. In successive trials extending over most

of the next 18 hours it was found that the beam was lost near the

intersection where the then unshielded crystals were located. Figures 5

and 6 show the pulse height ratio for 6oCo as measured just before

(November 11) and shortly after (November 18) the date of exposure. In

the North arm this ratio is close to one. In the South arm the ratio is

consistently larger than one, varying strongly from crystal to crystal,

and roughly independent of the distance to the PEP-beam pipe. The

combined effect of the damage resulting from the four weeks operation of

PEP with colliding beams and the 18 hours of exposure to the effect of a

“1 Ost” electron beam is illustrated in figs. 7 and 8, which show the

number of crystals (vertical scale) versus the overall ratio of the pulse

heights. While in the North array the overall effect for most crystals

is less than a factor two, the South array shows a wide distribution of

loss factors, extending from ~2 to ~18.

At this point the data taking was suspended and fixed Pb shields were

placed in front of the crystals. In February 1982, after the

installation of new remote-controlled shields, data taking was resumed.

Additional evidence was then obtained by triggering the spectrometers on

Bhabha scattering, e+e- + e’e-, at 14.5Y14.5 GeV, with the outgoing e-

(et) hitting collinear crystals in the South (North) array. Summing the

pulse height distributions over the solid angle accepted in the arrays

results in the data of fig. 9. Also shown are Bhabha data obtained in

June 1981. One observes a marked difference between the North and the

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South arm in the energy resolution of the 14.5 GeV Bhabha peaks in the

1982 data and a marked deterioration in the South array in the period

June 1981 - February 1982.

As a result of the radiation damage it was decided to remove the South

array at the earliest possible time (June 1982) and subject the crystals

to a heat treatment. As the possession of 60 large, heavily radiation-

damaged NaI crystals is a rather unique event it was decided to describe

this heat treatment and its effect in some detail. The project was

carried out at the Stanford Linear Accelerator Center (SLAC) in the

period March-September 1982. An important constraint limiting the

various options for assembling the oven facility has been the need to

complete the project in the period June 15 - September 15, 1982, admitted

by the shutdown schedule of the PEP storage ring.

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2. PREPARATION OF THE HEAT TREATMENT

2.1. I&J Room

Each NaI array consists of 60 crystals individually wrapped in white

reflective paper* and an outer layer of aluminized mylar, and placed in a

common hexagonally shaped Aluminum container. The cover plate has sixty

10 cm diameter holes in which fused quartz windows have been inserted.

Figure 10 shows an exploded view of the array. The weight of each array

is ~2.5 ton.

Since NaI is highly hygroscopic, the array was placed in a "dry room"

of 5.5 X 6 X 4 m3 in which a constant temperature and humidity was

maintained. The humidity was monitored with a hygrometer (see table 11,

the temperature was held constant at u20°C, the humidity corresponded to

a frost point of -4OOC. The dry room was equipped with 110, 208 and

480 V lines wired to be switched to stand-by diesel generators in case of

AC power failures during the heat cycles.

2.2. Ovens and Power Supplies

Three ovens were placed in the dry room. Their characteristics are

listed in table I. Each oven can contain up to four crystals at one

time. Their power supplies were designed for operation at constant,

preset temperature. In the present application the temperature must

increase and decrease slowly, however, in order to avoid thermal stress

in the crystals. The heating coils were therefore disconnected from the

*For commercial data on instruments and materials, see table I.

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supplies and connected instead to separate sources of AC power (listed in

table I) of which the output could be computer- controlled.

Ovens #l and #2 were mechanical convection ovens, equipped with

208 V/l phase/60 Hz power supplies, with inside dimensions 25X20X20

inches, and maximum temperature 316OC and 343OC above ambient temperature

respectively. The power supplies have four functions, i.e., to supply

power to the blower motor, to the over temperature control circuitry, to

the temperature control circuitry and to the heater element. The latter

two functions were bypassed and taken over by external devices. The

principle is illustrated in fig. 11. The HP power supply was designed

such that its mode of operation could be selected by making strapping

connections between given terminals on the terminal strips at the rear of

the unit. The mode selected was that of "remote voltage programming with

gain." The input/output ranges were O-6.2 V and O-230 V respectively.

For reasons of safety the currents were hardware and software limited to

16A, corresponding to (3.2 kW. The current was monitored via a series

resistor. The supplies were placed outside the dry room (to suppress

noise from their blowers) and connected to the oven through the

insulation of the wall.

Oven 83 was a mechanical convection oven equipped with a 440 V/

3 phase/60 Hz supply and maximum temperature 649OC. Here also the

temperature control and heater elements were disconnected from the

original supply. This function was taken over by a time proportioned

zero cross-over three-phase SCR power controller (see table I). In this

controller the firing circuit uses a 1.25 second time period to

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proportion power to the load. A current-to-time converter accepts a 4-20

mA control signal and generates a time-proportioned on/off signal, which

is transferred to the firing circuit via optical coupling. The oven has

three ranges depending on the settings of switches Sq and S2 in fig. 11

(either one, or both "on"). For the heat cycle selected only Sl was

liver was switched on. The maximum current the SCR controller could de

70 A.

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3. PROCESS CONTROL

On the basis of maximum temperature gradients specified in the

standard warranty document of the Harshaw Chemical Company it.was decided

to adopt a maximum rate of change of SOC/hour. We adopted a maximum

temperature of 250°C on the basis of tests with small radiation damaged

crystals.

The small crystals had been exposed to a 100 Curie 6oCo source for

several hundred hours. Spectral analysis of the transmission of light in

the 350-750 nm range before and after exposure and after the heat cycle

indicated that the heat cycle had the desired effect. Tests with a

spare, full-sized crystal indicated no thermal damage at this rate and

maximum temperature. Our data do not the possibility of faster,

larger improvement factors.

ization of the cycl ing

exclude

different temperature cycles with the

Lack of time has prevented a systemat

parameters.

same or

ic optim

The basic cycle is generated by the feedback of the thermocouple

readings to a processor in which an a priori determined relation between

temperature and analog signal to the oven power supply has been stored.

The basic equation governing the

temperature (T) of an oven supplied

dQ dT - = C, - = 12R - dt dt

time (t) dependence of the

by a constant current I is given by

dQ -

I + PB (1)

dt loss

C, is the heat capacity of the oven, Q is the quantity of heat supplied,

and R is the resistance of the filament. Pg is the amount of heat

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delivered to the oven per second by the friction of the circulating air.

The heat loss is due to a combination of conduction and convection and is

given by:

(2)

where T is the difference between the temperature and the ambient

temperature and K is the equivalent convection coefficient. In order to

ensure a linear relation between temperature and time in the crystals we

set

T(t) = T(t=O) + rt .

Solving (11, (2) and (31 we have

with

I = JaT + l3 (4)

a q K/R , R = (YCp - PB)/R . (5)

Equation (4) controls the feedback loop from the thermocouple to the

source of current. The numerical values of the constants have been

determined by running the oven at different ramp rates (e.g., Y and -7)

and recording the temperature as a function of current. Equation (4)

implies that, in order to have the crystal temperature increase linearly

with time, the rate of change of the oven current must decrease as the

temperature goes up.

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4. COMPUTER CONTROL OF THE OVEN TEMPERATURE

Three computers were available to control and monitor the heat cycles

in the three ovens, an ECLIPSE 1000 and two MICRO systems assembled from

single MORROW DESIGN boards. The ECLIPSE, operating under the RDOS

operating system, was limited to two programs running concurrently; the

MICRO ran under the single user CP/M operating system.

The interfacing of the computers to the different control circuits is

shown in fig. 12. The ECLIPSE-to-CAMAC interface is a type SEN 2023A +

20238 crate controller, the MICRO-to-CAMAC interface is a TRANSIAC type

6001.

As shown in fig. 12, for each oven there are two basic units in CAMAC:

an AOC to read the temperature and a home-built PSC (Power Supply

Control) unit to control the power to the oven. A DAC unit was used to

send status information to the monitor computer.

4.1. Therm0 Couple Read-Out

Each oven was equipped with five Cu-Constantan thermocouples, one for

monitoring the room temperature and four placed in the oven. With

several thermocouples per oven there is adequate protection against

breakage of a junction generating a run-away condition in the

temperature/power feedback loop.

A parametrization of literature data (accurate to l°C in the range of

interest) gives the following relation between voltage and temperature

for Cu-Constantan:

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T = 23.895 V - 0.2442 V2 .

Each thermocouple signal is amplified in a one stage NIM amplifier

shown in fig. 13. Each thermocouple is connected to a cold junction,

whose temperature is monitored with a LM 3911 (National) chip with a

sensitivity of 10 mV/OC and a output of 0 V at O°C. The six analog

signals are digitized in a type 32 (Joerger Enterprises) AOC. The

correction for the variation in temperature at the cold junction is made

in software.

4.2. Control of the Power in the Ovens

After averaging the temperature read by the thermocouples, and

checking for anomalies due to breakage of a junction, eq. (4) is used to

calculate the power required to maintain a slope of 5OC/hour in each

oven. An analogue signal is then generated at the output of the PSC and

fed to the oven power supplies, either directly (ovens it1 and #2) or via

a home-built NIM voltage-to-current converter (oven 113).

In the PSC (see fig. 14) several other functions have been combined:

it contains a real time counter used to monitor the progress of the heat

cycle; it generates an alarm in case the program stops, and it provides a

battery supported power supply control signal for use in emergencies.

The real time counter counts a 60 Hz cycle which is periodically

latched prior to read-out. The signal is converted to a

second/minute/hour sequence. The "second" signal updates a counter,

which is periodically cleared by the software. In the absence of a clear

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an alarm condition is generated. The emergency back-up provision

consists of two registers in series with a DAC-. In each CAHAC cycle the

desired current is calculated and written into the first register. This

number is then transferred to the second register, and the content of the

first register is replaced by the back-up current. The content of the

second register is converted into an analogue control signal and sent to

the oven power supplies. Upon an alarm or a power-up (after a power

failure) the back-up current which was saved in the first register is

transferred to the second register and converted into an analogue signal

and used to prevent the oven from cooling down.

In the case of oven 83 which was controlled with an SCR supply two

additional circuits were constructed, a voltage-to-current converter and

a power monitor. The latter unit will be described below. The voltage-

to-current converter (see fig. 15) converts a O-5 V signal to a 4-20 mA

signal which is fed to the current loop of the SCR controller for oven

83. The circuit is set up around a 747 dual operational amplifier, the

first amplifier being used for summing, the second for voltage-to-current

conversion. The circuit is housed in a single width NIM module.

4.3. Monitorinq of the Power/Temperature Cycle

In order to monitor the heat cycles each of the three oven control

programs generating the power/temperature cycles also generates five

status words. The status words are transmitted, via 5 DAC (type 232,

Jorway) and ADC (type 32, Joerger Enterprises) channels to a monitor

program which stores and prints the relevant information. In ~addi tion,

in ovens 81 and 82, the voltage over a resistor in series with the

The power monitor for oven 83 is a one-board CAMAC unit. Its function

is to count the number of half-periods the SCR controller is in the "on"

state within a 1.25 second interval. The ratio of this number to the

number of half peridos equivalent to full power (1.25 x 60 x 2 = 150) is

then the fract ion of the (known) full power de1 ivered to the oven . The

"on" state of the SCR controller is detected as a voltage drop of 2.5 V

over a shunt in one of the phases of the AC output. This voltage drop is

sensed by two opto-couplers, switched in antiparallel and wired in "OR"

in order to provide an output signal for each half-period.

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heatinglelements is read into the same AOC. For oven 113 a power monitor

detects the on/off fraction of the SCR supply voltage.

A typical heat cycle, including cooling of the container plus crystal

after completion of the cycle is approximately five days long. The MICRO

in which the monitor program resided was connected to the SLAC MICOM

switch which enabled remote terminals and dial-up lines to be used for

keeping track of the status of the power supplies, the computers and the

temperatures in the ovens.

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' 5. PROTECTION OF THE CRYSTALS AGAINST THERMAL SHOCK

In preparation for the heat cycle the crystals were removed from the

hexagonally shaped vessel (in the dry room, using gloves and facemasks)

and placed in one of a number of reusable Aluminum containers of

22” X 7-114” X 7-l/4” each. A maximum of 12 crystals could be processed

simultaneously in the three ovens.

In each container the crystal was placed at the center and the rest of

the container filled up with aluminum-oxide powder (following the

experience of the Harshaw Chemical Co.) surrounding the crystal with a

layer of at least 1" thickness.

The powder chosen was 38 Alundum, white color 99.5% AXtOo, crystal

form a-Alumina, grain size 32 micron (grit size 3201, effective density

1.8 g/cm3 <true density 3.9 g/cm3). The amount per container was 45 lbs,

the total amount used was 675 lbs. One container was filled instead with

10 lbs 99.98% AR203 (Micropolish 61, crystal form y-Alumina, grain size

0.05 micron, effective density 0.48/cm3. Since the powder is in direct

contact with the NaI, it must contain no chemicals that react with the

NaI, and not form cake-like deposits on the surface of the crystal during

the heat cycle. Deposits can be removed with e.g. Emery paper but this

tends to damage the surface and adversely affect the uniformity of

response of the crystal. Y-Alumina, a mixture of Aluminum sulfate and

Ammonium sulfate, is made by subjecting the mixture to a two-stage

calcination process. In the first stage a cubic lattice structure is

formed (which is also the raw material for ruby- and sapphire crystals),

in the second stage a grain size of 0.05 micron, of low hardness is

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attained, with only a lo-15% admixture of a-Alumina with 0.1 micron grain

size and hexagonal lattice structure.

Although grain size, hardness and chemical purity make micropolish B

the best possible powder in our application we have nevertheless

determined that the 4-l/2 times less expensive a-Alumina of 32 micron

gave adequate results and therefore used it for most crystals. We have

found no signs of chemical activity and no cake-forming in our range of

temperatures (d 300°C). This had the important advantage that no

treatment of the surfaces, other than the one facing the photomultiplier,

was necessary after the heat cycle. This led to a considerable saving of

time.

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6. THE HEAT CYCLE

As a first step in the processing of the crystals the thermocouples

plus amplifiers were calibrated and the oven constants in eq. (4) were

determined for the three ovens.

For about l/3 of all crystals the effect of the heat treatment was

determined by measuring the response to a la7Cs source before and after

the cycle. This will be described below. Several crystals showed

residual radiation damage after one cycle up to 250°C. For the remaining

crystals the cycle was therefore changed to a maximum of 300°C, while for

some other crystals the cycle was repeated. Of the 60 crystals in the

PEP-9 array, one was found to be cracked and was not treated. Four

crystals of the same size as the PEP-9 crystals were obtained from the

Stanford High Energy Physics Laboratory 131, to serve as spares in the

event that PEP-9 crystals would crack as a result of the treatment.

Three of them were cycled.

The cycling data are summarized in table II. The cycling program

started on June 28, 1982. The time development and oven occupancy in the

period June-September 1982 is plotted in fig. 16.

The main risk factors in the processing are power outages, computer

failure and thermocouple failure. This risk was reduced by means of

stand-by power supplies and by software checks on the readings of several

thermocouples per oven. Figure 17 shows the result of a power "glitch"

(before the standby supply was operational) lasting several hours which

resulted in a temperature drop of 10°C in one hour. No cracking resulted

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from this failure. After the installation of the standby supply the

system performed reliably for -10 weeks without failures.

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7. 'VISUAL INSPECTION OF THE CRYSTALS BEFORE/AFTER THE HEAT CYCLE

In preparing for the heat cycle each crystal was lifted out of the

vessel, the aluminized mylar and the white reflective paper (Millipore,

see table I) were removed and the crystal was placed in one of the

containers and surrounded by powder. Visual inspection showed a wide

variety in the intensity of the discoloring that had taken place as the

result of radiation damage. A few crystals looked white but were

surrounded at all sides by brown crystals. In some cases the Millipore

paper had become yellow and removing it left an apparently undamaged or

less damaged crystal. In most brown crystals the discoloring was not

uniformly distributed along the crystal but went through a maximum at

-10 cm (~4 rad length) from the end where the electrons enter, i.e., at

the shower maximum. From this observation it follows that the most

likely cause of the damage is high energy electrons, and not e.g.

synchrotron photons, which would have deposited their energy in the first

few millimeters only.

Several crystals showed mechanical damage of unknown causes. Some

crystals had developed fissures running along the crystal near the

surface in a vein-like pattern. Some others had small cracks,

sufficiently close to the surface to have little effect on the

transmission. A point of concern was the behavior of crystals 15 and #6

which had been reannealed (along with 10 other cyrstals in the North

array) at the Harshaw Chemical Company one year ago. These crystals had

developed a microstructure resulting in a cloudy appearance. As shown in

fig. 4 their long term deterioration was worse than that of any other

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South array crystal, while in the one-shot dose of November 13, 1981 they

were not affected more strongly than other crystals.

After the heat cycle most crystals were visibly clearer than before

they went into the oven. In a number of cases however residual browning

made it desirable to repeat the cycle. There was no correlation between

the extent of the original damage and the residual damage after the first

cycle, except that in the crystals with fissures the residual browning

appeared to be concentrated in the region of the fissures.

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8. ENERGY RESOLUTION BEFORE/AFTER THE HEAT CYCLE

In order to obtain a more quantitative measure for the effect of the

heat treatment, measurements were made with a l 37Cs source on 20 crystals

before and after the heat cycle. The remaining crystals were measured

after the heat cycle only.

The principle effect of radiation damage is a worsening of the energy

resolution through a non-uniformity in response along the crystal axis.

In a crystal that has been damaged, either mechanically, resulting in

cracks, or by radiation, resulting in discoloring, a given amount of

energy deposited far from the photo-multiplier will result in less

photoelectrons than the same amount deposited near to the photo-

mu1 tip1 ier.

The relation between the pulse height observed with a photo-multiplier

and the position of deposition of photon energy was measured by

irradiating the crystal transversely with a collimated 137Cs (662 keV1

source at nine positions along the crystal. The pulse heights blere

measured with an amplifier + pulse height analyzer assembly (see

table I).

The results of these measurements have been summarized in tables III

and IV and in figs. 18, 19 and 20. Figure 18 shows examples of the raw

data, i.e. the pulse height (PHI in arbitrary units corresponding to the

662 KeV peak versus position along the crystal, before and after the heat

cycle, for crystals 81, 87, 837 and #58. In fig. 18 the photo-multiplier

is at the right hand side of the plots, at ~20”. Before the heat

I

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treatment the PH is low at the far end, increases towards a maximum at

*15”, the decreases towards the near end of the crystal. After the heat

cycle the PH first decreases slightly, then increases towards a maximum,

also at ~15~“~ and decreases again towards the interface with the photo-

mu1 tip1 ier.

This dependence can be understood qualitatively by assuming that the

extent of the radiation damage is proportional to the energy deposited

per unit length by a shower initiated by an incident photon or electron.

Me than have an area of reduced transparency centered on the shower

maximum. Characterizing this area by a thickness x2-x1 = d and an

equivalent mean free path X for absorption of NaI scintillation light and

assuming 100% transparency for the nondamaged part of the crystal and

100% reflection at the surfaces we have for the number of photoelectrons

at the photo-multiplier from regions A, B and C:

nIBI x-x1 n(C) d -= cash - , -= cash - . n(A) x n(A) x

PM

The result, for crystal #58, is shown in fig. 18, for X = 38.5 cm. It is

evident that even without the introduction of an x dependence in X (with

an x-dependence following the shower profile) the features of the data

are in agreement with this qualitative picture.

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In table III are summarized the characteristic features of the pulse

height (PHI/position data for all crystals before and after the heat

cycles. The measured PH’s were normalized to the average PH of crystal

855, which had the highest average PH after two heat cycles. The

relative PH is listed for the far end of (~2”) of the crystal, for which

the effect of the heat treatment is expected to be most pronounced: also

listed is the average PH, calculated over the range 2”-15”, excluding the

data at >15” affected by the geometry of the end face. Also listed is

the “uniformity” of the crystals. For the untreated crystals

“uniformity” is defined as the ratio of PH’s at 2” and 15”, the

approximate positions of the minima and maxima, see fig. 18. For the

heat-treated crystals the definition is less obvious since the non-

uniformity is now caused by geometrical effects and residual radiation

damage in comparable proportions. As seen e.g. in crystal 1158, fig. 18,

the PH first decreases, the increases towards the maximum at 15”. This

is possibly due to the fact that the fraction of ‘*direct” light, which

must penetrate through the residual brown zone, versus “reflected” light

(at the far end and the sides) which can “bounce around” the brown zone,

increases as the point of origin of the scintillation light approaches

the photo-multiplier window. For the heat-treated crystals we

operationally define the “uniformity” as the ratio between minimum and

maximum PH in the range 2”-17”.

The data in table III show the following features:

1. The relative PH’s of the 20 crystals for which measurements were

made before the heat cycles are in the ZO-50% range.

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2. The uniformity before cycling is u70-90%. A notable exception is

crystal %25 which had 100% uniformity (and no visible browning)

before cycling.

3. Before cycling the average PH is larger than the PH at the far end

(with the exception of crystal 825) as is expected if radiation

damage dominates the distribution of pulse heights.

4. After the first cycle the PH's are in the 40-90% range and there is

no systematic difference between the average PH and the PH at the

far end.

5. After the first cycle the uniformity is >95% for most crystals. The

lowest value is 86.7% (for crystal #6).

6. For several crystals a second cycle resulted in a increase in PHI

with only a small or no increase in uniformity (e.g. crystal #55).

For crystals 81, #7, 820 and 839 the first cycle was interrupted on

the way down from 25OOC and restarted upwards to a maximum of 300°C

(see table II) without removing them from the oven, hence no

measurements were made after the first cycle.

The data of table III are shown in fig. 19. The plot of relative

average PH versus uniformity indicates that before the heat treatment

there is a clear correlation between average pulse height and uniformity;

on the other hand, after the treatment the uniformity is nearly constant

and larger than -92% while the relative PH's are extended over a large

range, from 45% to 100%.

This large spread in light yield once the crystals have been heat-

treated and regained their transparency can be ascribed to differences in

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reflectivity of the surfaces and in the transparency of the exit windows

among the crystals examined. While in the untreated crystals these

effects are masked by the very much stronger effect of radiation damage,

the cured crystals are expected to be sensitive to the surface conditions

since for scintillation light originating at points 315" more than 90% of

the (isotropically emitted) photons will make at least one reflection at

the surfaces.

The sensitivity to surface conditions is further illustrated in table

IV, where the effects of surface cond tioning have been collected for six

crystals. The data on these crystals indicate that polishing C51 of the

exit window has no significant effect that recompensation (sanding of

the surfaces parallel to the long axis, in order to improve the

uniformity of response) improves the PH by a few percent, without

altering the uniformity, and that packing the crystals in white Millipore

paper results in a 25-50% increase in relative average PH, and a small or

no increase (with the exception of crystal 847) in uniformity.

During the measurements of the PH of the 662 KeV peak at various

points along the crystals, data were also obtained on its width (FWHM).

The data at 2" have been collected in a plot of resolution versus PH, see

fig. 20. Whereas in a perfect crystal PH y the number of photons N * the

energy E and FWHM * fi u AE and thus AE/E +, N-1'2, in a radiation-damaged

crystal additional broadening is expected to occur, provided the

generation of scintillation light is sufficiently non-local to exhibit

the effects of non-uniformity of response.

- 27 -

Figure 20 shows that the average FWHM before and after the heat

treatment. Both sets of data are consistent with the expected square

root relationship, indicating that the energy deposition of the

(collimated) 662 KeV photons is too local to exhibit the effect of the

non-uniformity caused by the radiation damage. No conclusion can

therefore be drawn as to the energy resolution expected at high energies,

where the energy loss is due to showering and spreads over many radiation

1 engths.

- 28 -

9. REASSEMBLY, ENERGY RESOLUTION AT 14.5 GeV

After completion of the cycles and the quality checks the crystals

were wrapped in Millipore paper and aluminized mylar and reinstalled in

the hexagonal vessel shown in fig. 10. Three crystals (#53, which had

cracked when first installed several years ago; #5 and #6 which had been

reannealed at Harshaw one year earlier and had developed microstructure

since then) were not returned to the array but replaced by three of the

four “spares” obtained from the Stanford High Energy Physics Laboratory

c41. The replacement of 85 and 86 was motivated by the fact that 86 had

the lowest uniformity and that both crystals had a milky appearance which

could result in a worsering of the resolution at high energy. The

“spares” were heat-cycled once and had excellent uniformity C96.9, 98.5

and 97.8% for the replacements of #5, #6 and #53 (placed in slot W37, see

ref. L411 respectively].

Afte’r reinstallation and closing of the vessel the light pipes were

put back and another check was made with the 13’Cs source. The loss in

PH due to the window and light pipe relative to the data at 2” in the

test set-up varies between 1 and 20%, with an average loss of 7.4%.

Finally the assembly was reinstalled in the experimental set-up at PEP

and checked with 14.5 GeV Bhabha scattered electrons. The analysis of

the Bhabha scattering data is beyond the scope of this paper. Figure 21

shows the PH spectrum of an “or” of al 1 60 crystals taken before the

occurrence of the radiation damage (June 19811, after the radiation

damage (February 1982) (see also fig. 91 and after the heat treatment

(November 1982) of the South array. For comparison, the undamaged and

- 29 -

untreated North array is also shown. The difference between the November

1982 South array measurement and the earlier source tests performed in

the dry room is that the energy deposited is now a factor 2 x lob higher

and that each crystal is equipped with a different photo-multiplier.

Comparison of the resolution in the North and South arrays at the three

stages in fig. 21 indicates that the heat treatment has resulted in an

improved energy resolution at high energies as well.

ACKNOWLEDGEMENTS

The project described has benefited from the collaboration of many

people. We would like to thank L.P. Keller, H.A. Weidner, F.T. Halbo,

E.K. Johnson and H.M. Boatner of the SLAC Experimental Facilities

Department for their active participation in setting up the oven

facility. We are very much indebted to Kelly M. Foley (1982 summer

student), to G.J. Warren and to Jean Francis, E. Garwin and T. Roder of

the Applied Physics Group for help in the preliminary tests with small

crystals. We thank 0. Pellett for making available the U.C. Davis Micro

and ECLIPSE Computers for the process control. We thank Professors R.

Hofstadter and W.X. Vernon for very useful discussions at the early

stages of the project. We are indebted to Dr. David Berley (National

Science Foundation) for permission to use four additional crystals as

back-ups for the project.

- 30 -

REFERENCES

Cl1 F.R. Charvat et al., Metallopraphic Specima Preparation teds.

T.L. McCall and W.M. Mueller, Plenum Publishing Corporation, New

York).

lIZI Results obtained with small crystals will be reported elsewhere.

E3l These crystals were used earlier in the Crystal Ball experiment at

SLAC.

141 Crystal #53 was replaced by crystal #37. Crystal t37 was replaced

by HEPL/UCSD W6. Crystal #5 was replaced by HEPLIUCSD P3, crystal

86 by HEPLIUCSD #2. Another rearrangement was the exchange of

crystals 819 and 820 in the array (this provided a better fit).

E5l A recipe for polishing NaI crystals is described in: Jean Francis,

SLAC-TN-82-2, September 1982.

- 31 -

TABLE I

Commercial Data on Instruments and Materials

1. WHITE REFLECTIVE PAPER

Millipore Corp., Bedford, MA 01730. Cat. No. S52 JO 62E2

2. HYGROMETER

Panametrics, Inc., Waltham, MA 02154. Model 2000.

3. OVENS

Blue M Electric Co., Blue Island, IL 60406.

Oven #l: Model TA-792B-1 208 V / 1 Phase / 60 Hz Maximum Temperature 316OC 25 X 20 X 20 inch

Oven #2: Model POM-256B-1 208 V / 1 Phase / 60 Hz Maximum Temperature 343OC 25 X 20 X 20 inch

Oven 43: Model CW-8812 440 V f 3 Phase / 60 Hz Maximum Temperature 649OC 36 X 16 X 24 inch

4. OVEN POWER SUPPLIES

Oven 81: Hewlett-Packard, 100 Locust Avenue, Berkeley Heights, NJ 07922. DC Power Supply, SCR-10 Series, Model 6475C.

Oven t2: Same as Oven #l.

Oven #3: Research Inc., c/o Johnson Associates, 1280 Scott Blvd., Santa Clara, CA 95050. Model 662C-00-71-11, 440-48OV/3 phase/70 A, Power Controller.

5. NaI CRYSTALS

Harshaw Chemical Co., 6801 Cochran Road, Solon, OH 44139.

6. ALUNDUM POWDER

Abrasives Unlimited, P.O.Box 2058, San Leandro, CA 94577. AR203 powder, 99.5%, grain size 32 micron, 2.66 $/lbs.

Buehler Ltd., 2120 Greenwood Sheet, Evanston, IL 60204. Micropolish B, 0.05 micro Y-AR203, No. 40-6301-080, 210 8/5 lbs.

- 32 -

7. ELkTRONICS

- CAMAC Interface Type SEN 2023A, 20238 SEN Electronique, S.A. Ave Ernest Pictet 31 1211 Geneva 13 Switzerland

- CAMAC Interface Type Transiac 6001 Transiac Corporation 560 S. Antonio Road, Suite 101 Palo Alto, CA 94306

- Northern Pulse Height Analyzer Type TN1705

- Type 32 ADC, Joerger Enterprises

- Type 232 DAC, Jorway

- 33 -

TABLE II

Heat Treatment of NaI Crystals

Pass/fail rate for crystals in cycles tl, #2 and 113.

Temperature is maximum temperature for the cycle.

Ramp rate = 5°C/hour, up and down, for all cycles.

Nos in brackets: crystals with response measured before and after cycle.

62 Crystals

--'/ 250°C 250o-150°L3000 '1 45(4) 4(4)

3oooc 13(12)

l(l)

'1 Downward ramp interrupted at 100°C, then reversed up to 3OO*C.

- 34 -

TABLE III

Light Yield and Uniformity of Response Before/After Heat Cycles

!

BEFORE FIRST HEAT CYCLE AFTER FIRST HEAT CYCLE AFTER SECOND HEAT CYCLE

tYSTAL Rel Pulse Height Uni- Rel Pulse Height Uni- Rei Pusle Height Uni- Formity Formity Formity

At Far Average At Far Average At Far Average End End End

% % % % % % % % %

(I) (21 (3) (4) (5) (6) (7) (8) (9) (IO)

1 18.6 21.8 74.5 - 57.9 57.2 96.9 2 - 58.1 57.5 97.3 - 3 - 60.2 59.2 97.4 - 4 - 67.5 65.9 97.3 - 5 - 48.5 50.1 92.6 - 6 - 53.0 55.5 86.7 - 7 30.9 34.3 83.6 - 58.1 57.9 98.7 8 17.9 21.3 73.2 62.3 60.7 96.3 - 9 - 57.9 58.5 96.1 -

IO - 74.8 74.1 98.3 - 11 - 62.8 61.8 98.3 - 12 - 83.6 81.7 96.6 - 13 - 14 - 66.7 66.1 98.1 - 15 - 64.4 63.5 96.8 - 16 - 60.0 59.2 97.4 - 17 - 61.8 61.1 96.7 - 18 - 57.4 57.2 95.6 - 19 33.2 35.8 85.4 54.8 53.6 96.7 - 20 29.1 32.1 81.2 .- 49.1 49.7 94.4 21 27.2 32.5 71.8 55.3 54.7 96.7 - 22 - 50.9 50.1 98.0 - 23 - 62.0 59.8 95.8 - 24 - 61.5 60.6 97.0 - 25 54.5 53.8 100.3 69.8 67.6 95.2 - 26 - 57.6 56.2 96.4 - 27 - 56.8 55.5 96.8 - 28 - 61.8 59.6 95.4 - 29 - 62.6 60.9 96.3 - 30 - 60.2 60.3 96.2 - 31 - 75.3 73.6 96.9 - 32 - 73.2 71.9 96.5 - 33 - 89.3 87.4 97.1 - 34 31.1 33.7 85.4 65.9 65.2 98.0 - 35 24.9 29.1 74.5 59.4 59.0 98.3 -

- 35 -

I

(II (2; (31 (4) (5) (6) (7) (8) (9) (IO)

36 24.9 28.8 76.5 49.8 49.7 94.0 75.5 75.4 92.8 37 39.4 41.0 91.4 67.0 66.7 97.3 - 38 35.9 39.2 84.5 58.4 58.5 98.2 - 39 33.9 36.8 85.3 - 54.8 55.1 95.4 40 23.3 27.9 75.3 57.1 56.0 96.8 - 41 24.9 29.5 74.9 53.2 52.9 97.1 - 42 - 50.9 50.5 94.0 - 43 - 54.5 54.2 96.3 - 44 - 65.7 64.6 96.8 - 45 - 37.4 38.0 92.2 45.7 45.9 95.0 46 - 50.6 50.6 96.0 - 47 - 53.7 55.6 90.4 81.0 80.4 97.8 48 - 53.5 54.0 94.4 76.6 76.6 98.3 49 - 64.1 63.5 98.0 - 50 - 42.6 42.6 96.4 48.0 47.4 96.2 51 - 46.2 46.7 94.6 45.2 45.2 96.1 52 - 54.5 54.6 95.4 - 53 - 54 - 69.3 68.0 96.6 - 55 - 49.6 49.7 94.9 103.6 100.0 95.0 56 37.1 38.2 92.9 93.4 93.7 98.1 - 57 22.9 26.3 75.9 47.5 47.5 94.0 - 58 23.3 27.5 72.8 43.1 43.4 89.0 76.0 75.6 94.7 59 53.4 55.2 92.3 61.3 62.0 96.7 - 60 55.9 57.7 93.8 66.5 66.7 97.7 -

1. "Rel Pulse Height" is defined as the ratio of the pulse height corresponding to the 662 KeV peak of the 13'Cs spectrum to that of the average pulse height of crystal it55 after two heat cycles, i.e. the maximum average pulse height in the table.

2. "At the Far End" refers to a position -2" away from the end of the crystal. The photo-multiplier is at the opposite end.

3. "Average" is defined as the pulse height averaged over position in the range 2"-17". The point closest to the photo-multiplier has been omitted from the average as the light collection at >17" is affected by photons escaping through the end face on which the photo-multiplier is mounted.

4. "Uniformity" is defined, for untreated crystals as the ratio of pulse heights (662 KeV) at -2" to the pulse height at -15". For the entries after one or two - heat cycles "Uniformity" is defined as the ratio between minimum and maximum pulse height in the range 2"-17".

- 36 -

TABLE IV

Effect of Surface Treatment and Packing Material on Light Yield and Uniformity of Response

Crystal Treatment PULSE HEIGHT

At Far End Average Uniformity % %

24

After First Cycle No Treatment

Window Polished

61.3 60.5 96.2

61.5 60.6 97.0

30 After First Cycle

Window Polished + Recompensation

60.2 60.3 96.2

66.7 64.8 95.3

After First Cycle 63.9 62.2 95.5 33

+ Recompensation 65.9 64.3 96.5 + New Millipore Paper 89.3 87.4 97.1

36 After First Cycle

+ Recompensation + New Millipore Paper

54.0 54.0 94.4

75.5 75.4 92.8

47

After First Cycle + New Millipore Paper

After Second Cycle + Recompensation + New Millipore Paper

40.5 41.5 90.6 53.7 55.6 90.4

42.6 43.3 92.0 81.0 80.4 97.8

58 After Second Cycle

f Recompensation f New Millipore Paper

44.4 44.7 91.7

76.0 75.6 94.7

- 37 -

FIGURE CAPTIONS

I. Side view of one of two identical spectrometer arms used in the

PEP-9 experiment at the PEP e+e' storage ring at the Stanford Linear

Accelerator Center (SLAC).

2. On-view of the NaI array. The PEP beam pipe passes through the hole

3.

4.

5.

6.

7.

8.

9.

IO.

11.

12.

13.

14.

15.

in the center.

Ratio of pulse heights in the 60 crystals of the North (N) array

exposed to 6oCo before and after a four week period of PEP operation

within the period August-November 11, 1981.

Same as fig. 3 for the South (S) array.

Same as fig. 3 for data taken on November 11 and 18, 1981.

Same as fig. 5 for the South array.

The combined effect over the period August-November 18, 1981, for

the North array.

Same as fig. 7, for the South array.

The energy resolution of 14.5 GeV Bhabha-scattered electrons and

positrons before (June 1981) and after (February 1982) the exposure

of the South array.

Exploded view of the NaI array.

Lay-out of the power and control circuitry for the ovens.

Electronics linking the oven to the MICRO and ECLIPSE computers.

Amplifier for the read-out of thermocouples.

Power supply control unit, for transmitting instructions to the oven

power supplies.

The voltage to current converter used with the SCR controller of

Oven W3.

- 38 -

16. Time development and oven occupancy.

17. Temperature variation during a power outage.

18. Pulse height as a function of position along the crystal for the 662

KeV line of a collimated 13’Cs source. The photo-multiplier is at

the right hand side, at ~20”.

19. Uniformity versus relative average pulse height, as defined in the

text, before and after the cycling.

20. Resolution versus pulse height of the 662 KeV 13’Cs peak.

21. Bhabha scattered 14.5 GeV electrons before the radiation damage

(June 19811, after the radiation damage (February 19821, and after

the heat treatment (November 1982) of the South array. For

comparison the undamaged and untreated North array is also shown.

f

Muon Drift Chamber -Shower Counter Lead Shield

7 I \ Na (open position)

\ Septum Magnet \ DC3

Air Cored Quad DC2

r Prop. Tubes

I II II I II I! Ii/ TWO PHOTON

Iron Absorber

I I I I I I I I I

9 8 7 6 5 4 3 2 I .,llCl

4.11 meters

Fig. 1

41 42 43 44 45 41 42 43 44 45

40 22 23 24 40 22 23 24 25 25 46 46

39 21 39 21 9 9 10 11 10 11 26 47 26 47

38 20 38 20

28 49 28 49

60 36 18 60 36 18

59 35 17 16 15 30 51 59 35 17 16 15 30 51

58 34 58 34 33 33 32 32 31 ’ 52 31 ’ 52

57 56 55 54 53 57 56 55 54 53

11-82 NUMBERING OF THE CRYSTALS 4456A2

Fig. 2

4

3

2

I

0

Ii-82

I I I I I

N Source Ratio Aug/Nov I I

-

10 20 30 40 50 60 CRYSTAL NUMBER 4456A3

Fig. 3

12

IO

8

6

4

2

0

11-82

S Source Ratio Aug /Nov I I

-

- -

L 4 k -

I

-

IO 20 30 40 50 6.0 CRYSTAL NUMBER 4456A4

Fig. 4

2.0

1.5

I .o

0.5

0

11-82

I I I

‘1 - s

-

IO

N Source Ratio Nov WNov I8

20 40 50

7,

.

60

CRYSTAL NUMBER 4456A5

Fig. 5

6

4

2

0

11-82 CRYSTAL NUMBER 4456A6

1

S Source Ratio Nov I I/Nov I8

i

20 30

1

60

Fig. 6

-n

-. c(1 . v

0 -P

03

n, 0

NU

MBE

R

OF

CR

YSTA

LS

z5

s g

$ I

I I

I I

I I

I I I

I2

IO

8

6

4

2

0 0

11-82

I I I I I I I I I

-

-

-

-

S Source Ra tio Aug/Nov I8

-r b

l-l J I I I I I I I

4 8 I2 I6 PULSE HEIGHT RATIO

4 20

.4456A8

Fig. 8

1.2

0.8

0.4

0

1.2

0.8

0.4

0

/ I ’ I I , I I I

- North Array I (a) - _ June 1981

0.6 GeV-+ -

1.2

0.8

0.4

0 4 8 12 16 20 0 4 8 12 16 20 I I-82 BHABHA ENERGY 44M”Y

Fig. 9

60 SODIUM IODIDE CRYSTALS

> CENTER HEXAGON

PLATE FRONT ’

11-82 PEP-9 NaI CRYSTALS AND HOUSING 4456AiO

Fig. 10

POWER SUPPLY OVENS #I & #2

3 3 Phase

0

3 Phase 060 Hz /

11-82

Model - 6475C -+-W-l

t I 6 tl

Current Read out

Signal From Comp Interface Overtemp

0-6.2V/O-230V Protection

Blower Motor

POWER SUPPLY OVEN #3

0 /

I I

SCR Power

/

I /

I 00-71-I I

4

Blower Motor

Overtemp Protection

+ Alarm

Controi Sign01 From Comp Interface

4-20 mA/O-100% Of Full Power 4456All

Fig. 11

MICRO 0

C

ei A C

ADC

5x - DAC +

5x 1 DAC

5x DAC t

o-6V I I

ROOM TEMP

6x lcl- ADC

Fig. 12

-I MICRO

-0 ECLIPSE

El ECLIPSE

4456Al2

I

r-- 1 r--- l I I 1 I

Input I I

_-_--_ 1 I

\1 1 I Output

I Channel

Constantan’ v 1; I5747~yg+- I I

I I

I 1 I I I I I I

I I

I I

I I

I I I I I I

&p

f---- I -

G p-j--&

I I I I

I -12v , 1 I

I ” I I 41 I

I 47p 2ov I

4 I I J ” I

I

4

IN3064 470

I I

I XT I t-------- Identical Circuit

‘--------- -

72 31

l-1 !i tsink 7 LM

391 I --- 8

I 1 Hea L-

II -82 4456A23

I 4 5K 0

0

Cold Junction Tempout

3K9 8K2

-12v +12v

THERMOCOUPLE READOUT Five channel non-inverting amplifier with cold-junction temperature readout for copper -constantan

Fig. 13

Alarm

ro PSC

To CAMAC Read

/ / >

1 I

Signal Second Minute Hour 0 n

>ConyF;ing+ Counter --+ Counter ---+ Counter 60Hz i

f- <

From CAMAC Write

CAMAC Decoding

Register + Battery > I Backup

‘

DAC -Analog Out

c POWER SUPPLY CONTROL UNIT (PSC)

Power supply control with batter back-up Real time counter Alarm signal generator

11-82 4456A73

Fig. 14

Input >

IK P2

24V

tw 1 l2V

CURRENT LOOP CONVERTER Input 0 to 5 Volts Output 4 to 20 mA

CURRENT Adjustment - Connect mA meter over output - Apply input voltage = OV - Adjust PI for 4mAoutput current - Apply input voltage=5V - Adjust P2 for 20mA output current

II-82 4456A14

Fig. 15

7 -. a G

P=

HEA

TCYC

LE

CO

MPL

ETED

86

3 UI

N N

UM

BER

OF

CR

YSTA

LS

IN O

VEN

C

RYS

TALS

(%

I G

-

-

0 s 3

-

- - -

ll-82

TIME (h) 6 7 8 9 IO I I 12

I

6h 2lmin t

-7

Slope

.7-L -6 “C/h

0 a----

Slope -4.9 “C/h

Slope - 16 “C/h

‘. +2.7 “C/h\ : 0 0

40 0

-/ .O

0 0 Problem 0 0 Temperature

0 0 During Anneal ing l Crystals #I5 and #30

z 7/20/82 q/e

0..

3600 3700 3800 3900 4000 4100

ELAPSED TIME (min) 4456816

F ig. 17

HtAl I REATMENT

360 /

320

300

150

360

340

240

220

200

After

I I

Crystal 1

I I 1

Before

I I I

Before

I I I

t

Crystal 7 340

i

320 0 _

300 I I I

:,:/Al

I 60 I I I

425 - Crystal 58 -

400 -

375

175

125

I I I

0 5 IO I5 20

<I ‘1.7 POSITION ALONG CRYSTAL (inch) 44’,1,11 I I

Fig. 18

RELATIVE AVERAGE PULSE HEIGHT AND UNIFORMITY 0 q 20 Crystals Before

l 0 58 Crystals After Heat Treatment

zz

; 80 z 3

70

OocoO 0

QB O 0

60 I I I I I I I I 20 30 40 50 60 70 80

RELATIVE AVERAGE PULSE HEIGHT

20 40 60 80 100 12-82 RELATIVE AVERAGE PULSE HEIGHT

-

90

70 80 90 100 UNIFORM ITY 4456A21

Fig. 19

3 60 0 I I I I I

s 0 Resolution vs Pulse Height

s 66’2 keV Photons,

k 50 - 00 20 Crystals Q

8 Heat

Z Treatment: 0 F

8 Q9 0 Before

3 40 - OO

% l After

b? *%

08

30 4 ? l

lg - a l o IT 0 Y a 6 20 I I I I

100 200 300 500 700

?I-82 AVERAGE PULSE HEIGHT (ARB. UNITS) 4456A24

Fig. 20

I .2 I I I I I I I I I

- North Array _ June 1981

-

I I I L I I

0.8

0.4

0

I.2

0.8

0.4

0

0.8

0.4

0

400

200

I.06 1.2

0.8

0.4

0

1.6

1.2

0.8

0.4

cl

I I

_ South Array _ June 1981

0.4 GeV- -

I I I I I I I

- North Array _ Feb 1982

L- ’ I

I

0.5 GeV

-il- !

0 4 8 12 16 20 l-83 BHABHA ENERGY 445681

Fig. 21