ML19224B378

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Health Physics Procedure 1741,Revision 2:operation of Internal Liquid Scintilation Counter
ML19224B378
Person / Time
Site: Crane 
Issue date: 09/07/1976
From:
Metropolitan Edison Co
To: Mullinix W
NRC/IE
References
1741, TM-0125, TM-125, NUDOCS 7906150018
Download: ML19224B378 (12)
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Text

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a TMI DOCUMENTS _

DOCUMENT NO:__

/ ST OF DOCUMENT PROVIDED BY COPY MADE ON METROPOLITAN EDISON COMPANY.

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'"1da R. Hullinix, NRC t

I 7906150018 201 305

MiASTER LUPT 1741 Revision 2

DO NOT REMOVE 09/07/76

) C071 THREE MILE ISLAND NUCLE AR ST ATION o

I' STATION HEALTH PHYSICS PROCEDURE 1741 IfiSiRUMEf1T OPERATI0I4 0F Art INTFRNAL LIQUID SCINTILLATION COUNTER Table of t'ffective Pages Page Date Revision Page Date Revision Pg Date Revision 1.0 02/12/74 0

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THREE MILE ISLAND NUCLEAR STATION HEALTH PHYSICS PROCEDURE 1741 - Instrument Operation of an Internal Liquid Scintillatior. Counter 1.0 PURPOSE The purpose of tnis procedure is to discuss the operation of the Internal Liquid S-

.tillaiton Counter.

2.0 DI SCI. SION 2.1 Radioactivity can be detected by means of a scintillator.

A scintillatcr is a substance which fluoresces when bonbarded with high energy radiation.

This substance could be a solid like NaI (Tl) crystal or a liquid like PPO (2, 5 - diphenyl-oxazole).

In liquid scintillation the scintillator is re-ferred to as the solute. The solute is dissolved in a sol-(!

vent (e.g. toluene,. xylene). The scintillation solution can

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be either external to the sample or internal (mixed with the sample).

When the scintillating solution (sometimes called the cocktail) is external to the sample, the container housing tne scintillator absorbs low energy radiation with high L.E.T.

(linear energy transfer).

Most beta radiation and all alpha particles would be shielded from interacting with the scin-tillator.

Therefore, external liquid scintillation counting is applicable only to samples containing penetrating radiation (gamma rays).

- - TJ 201 30'*

e4 em e-1.0

Revision 2 09/07/76 When the cocktail is mixed with the sample all barriers be-tween the ionizing radiation and the solute (fluor) are removed.

Therefore, thir method is applicable to counting sanples containing soft beta particle (H-3, C-14, Cl-36, etc.).

The energy from the beta particle is absorbed by the scintil-lation cocktail cauaing the solute to become excited.

The excited solute returns to the unexcited state by emitting 1*ght photons. These photons are detected by two bialkaline photo-multiplier tubes, amplified by a series of dynodes and then registered on a scaler.

The two photomultiplier tubes give essentially 4 pi counting geometry.

2.2 The Packard Tri-Carb has three fully independent and simul-taneous channels of pulse-height analysis.

These channels can be set to detect the same energy radiation or detect different energies.

Lower and upper (.iscriminators are also provided with each channel.

3.0 REFERENCES

3.1 Instruction Manual #2080 for Packard Model 3320 Liquid Scin-tillatien Spectrometer System.

3.2 Station Chem Procedure 1951.

I 4.0 EQUIPMENT I

4.1 Packard Model 3320 Liquid Scintillation Spectrometer 4.2 Scintillation Counting Vials 4.3 Tritium standard 4.4 Cesium-137 source 4.5 Packard scintillation cocktail

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Revision 2

l 1741 09/07/76 5.0 OPERATING IllSTRUCTIONS 5.1 The Packard Tri-Carb is left on at all times.

Maintaining a constant high voltage helps to stabilize the photomultiplier t ubes.

5.2 SPECTROMETER

'5.2.l DISPLAY - Displays 6-digit readout of accumulated counts or ' lapsed counting tiue.

Time displayed within one-hundredth of a minute or one-tenth of a second.

5.2.2 DISPLAY Switch - Allows displaying counts or time.

Orr position turns dirplay atf.

5.2.3 STOP - Stops cealers and tin.wr.

In the automatic mode, print-out ind sample changing occur and countir., starts automatica).y.

5.2.4 RESET-dTART - Resets and starts scalers and timer in the manual mcdc.

5.2.5 PRESET TIME - Determines the elapsed tiac when counting stops.

PRESET COUNT switch can cause the same action if it is satisfied first. Time range is 0.1-100 minutes or 1-1000 seconds.

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5.2.6 MINUTES-SECONDS - Allows selection of time base for PRE-SET TIME switch and display.

5.

2.7 BACKGROUND

SUBTRACTION - Digital indicator and thumb-I wheel selector to set background counts for automatic g

i subtraction from gross counts.

Adjustable from 0000 to 9999 counts.

PRESET COUNT switch must be in the BACKGROUND SUBTRACTION position for this selector to ~

operate.

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eVision 09/07/76 5.2.8 PRESET COUNT (THOUSANDS) - Determines the point where counting stops (first scaler to reach this point).

PRE-SET TIME switch can cause the same action if it is satis-fled first. BACKGROUND SUBTRACTION position allows the use of the BACKGRDUND SUBTRACTION selector in conjunction with a preset count of 900,000.

5.2.9 REJECT - Selects the minimum gross counts acceptable during the first minute tr 100 seconds of counting, depending on the setting of the MINUTES-SECONDS switch.

Setting the PRESET TIME switch for the 1 minute or 100 seconds or lesscr times disab3es the low level reject function.

5.2.10 MODE SELECTOP MA!!. - Provides continuous scaling until STOP button is ocpressed.

5.2.10.1 CHECK PRESET - Provides preset scaling and timing of pulses based on power source frequency (50 Hz - 3000 cpm; 60 Hz-k 3600 cpm) to verify proper scaler and timer operation.

5.2.10.2 MANUAL PRESET - Provides preset scaling and timing. Count-ing stops when either the PRESET COUNT switch or PRESET TIME I

switch setting is satisfied or the STOP button is ac-tuated.

s 5.2.10.3 AUTO PRESET - Provides automatic preset scaling, timing, print-out, and cample changing (SAMPLE CHANGEP switch in OPERATE). With the SAMPLE CHANGER switch in REPEAT, a sample is counted repeatedly.

5.2.11

- Concentric switches and variable control permit,

GAIN precise setting of the gain.

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vis on 09/07/76 Inner switch determines range (10% or 100%) of outer switch and the variable control combined.

For exa g le, with both outer switch and variable control set at 5, an inner switch setting of 10% presents a gain of 5.5% '

to the channel.

Placing the inner switch at 100% pre-sents a gain of 55% to the channel.

I 5.?.12 WINDOW SrLECTION Two discriminator potentiometers in combination with window and gain controls adjust a precise counting window in the res ective channel.

5.2.i3 WTNDow 2%, 4%, 8% - Establish a fixed counting windoa between lower limit setting and selected percentage of full scale.

Counting

.ndow widths of 20, 40, or 80 divi-sions are produced (narrow-window mode).

A-B, C-D, E-F - Provide a counting window based on the setting of the respective discriminator controls (wide-I window mode).

A-=, C-=, E-= - Provides a counting window with a pre-b set lower limit and no upper limit (integral mode).

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5.2.14 DISCRIMINATOR POTENTI0 METERS - Left control ( A,C,E) sets lower limit.

Right control (B,D,F) sets upper limit.

Each control has 1000 divisic s adjustable in ten turns and a locking lever.

5.2.15 MAIN POWER 0FF - System de-energized.

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7-ON - System energized except for the high-voltage powe 201 all supply.

7 H.V. - Entire system energized.

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5.0

09/07/76 5.2.16 AUT0ftATIC CONTROL UNIT GROUP REJECT - Stops ccunting end timing and rejects 6.2.16.1 all samples until a green group start insect pases the r

load position.

5.2.16.2 SAMPLE CHANGER STOP - Unloads sample and stops.

OPER/,TE - Unloads, indexes, and loads samples sequentially.

REPEAT - Retair,s sample for repeated countir 3 5.2.16.3 AUTOMATIC STANDARDIZATION AUTO - Sets the system for AES operation.

OUT - Turns off the AES feature.

IN - Manual override that permits extended AES counting.

Lamp lights to indicate when the external standard is in the counting position.

GAIN SETTINGS - The settings for the IIquid scintillation spec-5.3 trometer will be dependent on the radionuclida to be counted.

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The bulk of the samples to be counted will be tritium.

Sug-p gested settings for counting tritium and other radionuclides g

Modi-1 are found it. the Packard Instruction Manual (Ref. 4.1).

fications of these settings may be necessary at T'iI due to the t

I presence of high energy beta emitters in water samples.

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Increasing the gain shifts the beta particle spectrum to the i

S.3.1 A decrease in gain moves the spectrum to the left.

right.

The optimum settings can be obtained as follows:

CONTROL POSITION PRESET MAN a.

MODE SELECTOR b.

PRESET TIME ~-

1

.' MIN'JTEb

._T c.

MINUTES-SECOND3 6.0

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PRESET COUNT 900,000 c.. DISPLAY CilANNEL 1 f.

WINDOW A-B g.

LOWER DISCRIMINATOR (A) 50 UNITS h.

UPPER DISCRIMINATOR (B) 1000 UNITS i.

SAMPLE CHANGER OPERATOR (WINDOWN TROM 50 to 1000)

NOTE:

These settings are to be used only for-obtaining Optimum gain settings.

Their use for counting sampics will result in counting of high energy betas as well as tritium.

5.3.2 The GAIN adjustments for Channel #1 are vo.ied to determine the peak settings.

Turn the inner knob of the coarse GAIN to 100%, the outer knob to 1.

Press the RESET-START button to initiate a one minute count. Record the read-out on the DISPLAY.

5.3.3 Advance the outer control knob for the coarse GAIN to 2.

Again take a one minute count (press RESLT-START button).

Record counts.

5.3.4 Continue the above procedures until one minute counts have been taken at each of the 9 positions on the coarse GAIN control knob.

5.3.5 Determine the gain setting yielding the highest count rate. Turn the coarse adjustment two positions below that setting. Advanse the fine gain adjustment knob to 5.

Take counts as before until the coarse adjustment knob is two positions above the peak setting.

5.3.6 Note the highest count rate and turn GAIN control knobs to that setting.

Further adjustment of the fine GAIN may i..rease the count rate s_1ghtly.

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5.4 The actual optimum settings for the Tri-Carb involve both sample counts and background counts.

The settings which give the highest sample squared to background ratio are considered optimum.

5.4.1 EXAMPLE

Ratio

, Sample)2

(

Sample Bkg Window Cain Counts Counts

Bky, a.

50-1000 5 5'u 3511 79

1. 6 >. 105 b.

50-1000 53t 3300 42 2.6 x 105 The setting of choice is "b" because of the higher sample squared to background ratio.

5.5 Tew samples analyzed at a nuclear power plant will contain only one beta-emitting nuclide.

As a result it is desirable to determine what contributions higher energy beta particles will have on tritium counts, channel #1.

Therefore, channel

  1. 2 is set to detect beta particles of average energy that are likely to be found in liquid releases from a PWR.

Beta particles from a CS-137 source are counted to represent these i

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beta emitters, i

5.5.1 Set the channel #2 window frou C-=.

Set the lower discrimi-nator (C) just above the upper discriminator in channel #1.

(Cain should be set so sufficient counts are registered in channel #2.

Once a GAIN setting has been selected, it is important to maintain that GAIN setting for subseauent sample counting.)

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5.5.2 Set the upper discriminator (B) in channel #1 for maximum re-jection ofCs-137 counts without sacrificing more than 10%

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of the counts for tritium detection.

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09/07/76 5.5.3 Count several Cs-137 samples with activities ranging from 3

10 to 106 dpm.

5.5.4 Plot the total counts per minute observed in channel #3 on the ordinate and the percentage contribution by the Cs-137 sample to channel #1 on the abscissa (see Figure 1741-1).

5.5.5 The above plot can be used to determine the contribution of h'gh energy beta found in most water samples at TMI to the tritium channel.

5.6 The third channel will be used to count the external standard.

The external standard is usei to determine the influence of various quenching agents in the sample on the detection scintillations by the photomultiplier tubes.

Sampics to be counted at TMI in the Tri-Carb will be aqueous.

Variations in quenching should not be a problen.

However, if samples with various degrees of qwnching are to be counted, a quench correction curve can be constructed as follous:

I 5.6.1 Prepare 10 samples with a known amount of tritium in each.

These sampler should be counted to determine f

the exact activity added.

5.6.2 Add varicus amounts of quenching agent to the 10 vials.

The quenching agent should be typical of the quenching g

expected in actual samples.

5.6.3 Recount the samples to detcendne counting efficiency (C.E.).

(C.E. will decrease with increases in quenching),

and count each sample with the external standard in place.

5.6.4 Plot C.E. vs. externa 1 standard cpm.

This curve can be used to determine the C.E. of samples with unknown -

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e activity and various degrees of quenching.

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09/07/76 f,. 7 Background is determined as per HPP 1710.

5.8 Efficiency is determined as per HPP 1711.

5.9 Automatic Operation 5.9.1 Turn SAMPLE CE.sNGER switch to STOP and MODE SELECTOR switch to AUTO PRESET.

5.9.2 set spectrometer controls in accordance sith the nuclides to be counted (see above procedures).

5.9.3 LOAD sa iles into a series of unused sample holders (see HPP 1804 for sample preparatien).

Note the saple hnider numbers.

5.9.4 Place a green group start ahead of the grcup and a red group end after the last vial to be counted.

5.9.5 Allow sufficient time for samples to cool (approximately 15 minutes).

5.9.6 Initiate counting by turning SAMPLE CHANGER t'o OPERATL.

5.9.7 After designated samples have been counted, stop counting operation by waiting until sample is counting and turning the SAMPLE CHANGER switch to the STor position.

C/UTION:

DO NOT TURN SAMPLE CHANGER when sample is NOT being.

counting.

Doing so can result in damage to the changer mechanism.

I 5.5.8 Background and efficiency determinations should be completed on a daily basis. A blank for backgrcund should be made fresh on a weekly basis.

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