PHYSICAL REVIEW B VOLUME 21, NUMBER 9

Comments and Addenda

1 MAY 1980

k»~'inc; tipc's of c'on)n))In)c'alic&ns: (I) Cc&n)n)c nts c&n pape'Is p)c'I'ic&I)sit pt)blithe'cf in The Physical Review o) Physical Review Letters.t' ) Acjcjc'nclc) tc& paper» prc ~ic&I)sly' pIII&lishc'cl in The Physical Review I» Physical Review Letters, in n'hic I) thc «cfclitic&nal i)I/i&I'n)c)lie&n

I&roc&/~ an sc nt to aI)thors.

Optical spectroscopy of scintillations occurring in unirradiated glycine

and triglycine sulfate: L-alanine

D. Wayne Cooke and Siraj M. KhanDc'partn)c'nt of Phc'sic's. Men)phis Slate Uni n'rsiti', Men)phis, Tennessee 38/52

Chester Alexander, Jr.Departn)ent of'Phisicsand Astronomy, Unific)siti of Ataban)a, Univ)siti, Alaban)a 35486

(Received 10 A ugust 1979)

Several research groups have reported observations ot' scintillations in well-known ferroelec-tric crystals. Previously we reported similar activity occurring in unirradiated glycine (nonter-roelectric) but did not report the optical spectrum associated with the scintillations. In this paper

we present the optical-spectroscopy results f'or scintillations occurring in single crystals ot'glycine

and L-alanine-doped triglycine sulf'ate (LATGS). Seven lines were identified in glycine and

eight lines in LATGS, all belonging to the second positive system of N2. The spectrum results

f'rom an electrical discharge f'rom the crystal surf'ace, which has a layer of N2 adsorbed onto it,

to ground.

In recent years several research groups have re-ported observations of intense scintillations (electricaldischarges) in ferroelectric crystals' and, in onecase, in a nonferroelectric crystal. ' Yockey and Asel-tine' conducted a thorough study of potassium dihy-

drogen phosphate (KDP) that had been irradiated by

y rays and subjected to a temperature ramp. The op-tical spectrum of the scintillations- occurring in KDPwas recorded, yielding nine lines that were identifiedand assigned to the second positive system of N2. Itwas suggested that the second positive N2 spectrumresulted from electrical discharges from the crystal tothe ambient nitrogen gas that had evolved from theliquid N2 which was being used to cool the sample.By replacing the liquid Nq with liquid Ar, efforts weremade to photograph an Ar spectrum. This experi-ment did not yield an Ar spectrum„ instead, it pro-duced the second positive N2 spectrum. The authorssuggested that the appearance of the N2 spectrumwas due to the fact that the experimental apparatuswas not designed to be evacuated and therefore smallamounts of N2 diffused into the immediate vicinity.Some of the scintillations were partially polarized, in-

dicating that they came from internal breakdown ofthe crystal. A model based upon the strong internalfields that exist in ferroelectrics was proposed to ex-plain the scintillations. Real charge liberated in the

crystal by ionizing radiation was most likely trappedat domain boundaries. These domains anneal andcoalesce over a broad temperature range, therebyreleasing real charge to the surface of the crystal. Itwas noted that the crystal must be in the ferroelectricphase during irradiation and that scintillations arepeculiar to ferroelectrics.

Cooke and Alexander' have observed intense scin-tillations and current pulses in glycine which resultfrom heating or cooling the sample between 77 and300 K. Glycine did not require irradiation for thescintillations to manifest themselves, and further-more, available evidence indicates that glycine is non-ferroelectric. If the scintillations observed in glycineare due to the same physical effect that producesscintillations in ferroelectrics, then the model ofYockey and Aseltine will require modification to ex-plain scintillations adequately. Although Cooke andAlexander' did not propose a model to explain thescintillations in glycine, it was suggested that theyprobably occur as a result of a temperature gradientthat exists across the sample. Current pulses accom-panied the scintillations and the direction of the mea-sured current flow was dependent upon the directionof the temperature gradient. No optical spectrum wasreported for the scintillations occurring in glycine.

In this paper we report optical-spectroscopy results

21 4166 1980 The American Physical Society

21 OPTICAL SPECTROSCOPY OF SCINTILLATIONS OCCURRING. . . 4167

CRYO- TIP

~TRANSFER LINE

CONTROL HEATER CRYO- TIP VACUUM PUMP

Al BLOCK ~

ELECTROOES=—

METAL STRIP

~GLYCINE ORI' ' LATGS CRYSTAL

GAUGES

I «I M0N0 . , + HvlCHROM

tMv )&' I, Itvr P

ELECTRO-

)DRY

(atEMR

[HEarER +

~ ELECTRO-METER

xyRECORDER

QIa sa

STEEL ORNYLON SCREW

DRIVE xyRECORDER

FIG. 1. Sample holder used in detecting scintillations in

glycine and LATGS.

for scintillations occurring in single crystals of glycineand L-alanine-doped triglycine sulfate (LATGS). Byscanning the spectrum slowly with a monochromator,we have identified seven lines in glycine and eightlines in LATGS, all of which belong to the secondpositive system of N2. %'e believe that the N2 spec-trum is due to molecular nitrogen that has been ad-

sorbed onto the crystal surface which is subsequentlyexcited by the electrical discharges. The ubiquitouspresence of N2 on the surface of crystals might ex-plain why other workers have identified the spectra ofscintillations as that belonging to the second positivesystem of N~.

Single crystals of glycine (4 x 4 x 7 mm') orLATGS (4 x 7 x 10 mm3) were placed on analuminum-block sample holder (shown in Fig. 1) andinserted into an evacuated (10 ' Torr) cryostat. Fol-lowing evacuation, the crystal was cooled at a non-linear rate of about 30 K/min to 77 K and subsequentlyheated at a rate of about 10 K/min to 280 K. The tail

section of the cryostat was provided with two quartzwindows so that the optical radiation produced by thescintillations could be simultaneously detected by two

photomultiplier tubes (PMT's). One PMT recordedthe total light emitted regardless of its wavelength,whereas the second PMT was placed at the exit slit ofa 4-m monochromator and detected only that radia-I

tion possessing a particular wavelength. The outputof each PMT was amplified by an electrometer andrecorded on an xy recorder. A block diagram of theexperimental apparatus is shown in Fig. 2.

The scintillation spectrum was obtained by the fol-lowing method: (i) subject the crystal to a tempera-

FIG. 2. Block diagram ot'experimental apparatus used in

detecting scintillations in glycine and LATGS.

ture ramp to produce frequent occurrence (4—5/sec)of scintillations, (ii) detect the occurrence of a scin-tillation by observing a signal on recorder 1, (iii)scan the spectrum slowly (10 nm/min) with themonochromator and observe the signal on recorder 2,(iv) when both recorders produce a signal in coin-cidence, note the wavelength reading of the mono-chromator, (v) after the most intense lines havebeen identified, repeat the experiment several timesby setting the monochromator dial at the identifiedwavelength and observing the coincidence signal ofthe two recorders, (vi) As a final check on the posi-tion of the spectral line, move the monochromatordial 0.3 nm to each side of the line and see if a coin-cident signal is observed; if not, then the spectral linehas been properly identified. An estimate of theresolution of the monochromator was made by

recording a mercury spectrum and determining thefull width at half maximum of the 253.7-nm line(FTHM —0.3 nm) and also noting that the doubletat 313 nm could barely be resolved. Although a rea-sonable estimate of the accuracy of the spectral linesis +0.3 nm, as determined by the monochromatorresolution, the accuracy of the wavelength dial is only+1.0 nm. Therefore, the spectral lines presented in

Table I are only quoted with an accuracy of +1.0 nm.A previous attempt to photograph the spectrum by

passing the radiation through a quartz-lens arrange-ment and focusing it onto the entrance slit of a 1-mmonochromator was unsuccessful. Type-0 plateswere used as the detector but, unfortunately, were ofinsufficient sensitivity to reproduce the spectrum.

4168 COOKE, KHAN, AND ALEXANDER 21

TABLE I. Summary of spectroscopy results (wavelengthsin nanometers).

Glycine TGS:L-alanine Second positive N2

315.4 + 1.0336.8 + 1.0357.3 + 1.0371.0+ 1.0375.2 + 1.0380.0 + 1.0390.0 + 1.0

312.9 + 1.0315.6 + 1.0336.6 + 1.0357.3 + 1.0370.8 + 1.0375.2 + 1.0379.9+ 1.0390.8 + 1.0

313.60 (8)b315.93 (9)337.13 (10)357.69 (10)371.05 (8)375.54 (10)380.49 (10)389.46 (7)

'Taken f'rom R. Pearse and A. Gaydon, The lrkI~(ifieatir»~ o/'

Moleeulal Spectra. 4th ed. (Wiley, New York, 1976).Relative intensities of the spectral lines according to Pearse

and Gaydon.

Prominent spectral lines produced by a scintillationin glycine are shown in Table I. Seven lines are iden-tified and compared with the second positive systemof nitrogen as given by Pearse and Gaydon. 9 Column3 of Table I represents the prominent lines of thetransitions occurring between the C n„and theB 'ng states of N2. The numbers in parentheses arethe relative intensities of each spectral line as deter-mined by Pearse and Gaydon. Also presented inTable I are the spectral lines resulting from scintilla-tions in LATGS. These lines also correspond to thesecond positive system of N2.

The spectroscopy results of Table I exhibit reason-able agreement with the data of Yockey and Aseltine'for y-irradiated KDP. A noteworthy exception is thepresence of a spectral line at 390.0 nm in glycine and390.8 nm in LATGS, which was absent in KDP. Thefirst negative system of N2+ exhibits a very strongline at 391.4 nm, and, due to the uncertainty in ourmeasurements, it is possible that the 390.8+1.0-nmline in LATGS belongs to this system. Robertsonand Baily4 have reported the spectrum associated withscintillations in irradiated and unirradiated triglycinesulfate (TGS) and identified five lines associated withthe second positive system of N2.

Spectroscopy results of the present work, and allprevious work, indicate that the predominant opticalspectrum produced by a scintillation is that belongingto the second positive system of N2. We attributethis to a layer of N2 that has been adsorbed onto thesurface of crystals due to their exposure to the am-bient environment. This layer is probably not re-moved by evacuation at 10 5 Torr. When a dischargeoccurs at the surface of a crystal it will produce a N2spectrum because of this adsorbed layer. Discharge-energy data' " indicate that the energy associatedwith a scintillation is sufficient to ionize molecular

N2. Thus the spectrum of the scintillations providesvery little information concerning the internal chargeproducing mechanism but fortuitously appears as asurface effect. The details of the transition layer(e.g. , TGS-N2-atmosphere) are clearly beyond thescope of this paper; however, this transition layermay explain the polarization exhibited by KDP as re-ported by Yockey and Aseltine. ' The mechanismwhereby a scintillation is produced in a ferroelectricor nonferroelectric crystal is unclear, and we offer nomodel in the present work. We can, however, offersome insight into the problem that should be valu-able in future theoretical developments.

Apparently the scintillation phenomenon cannot bebased upon ferroelectricity. This conclusion is sup-ported by the following experimental facts: (i) scin-tillations are observed in glycine —a nonferroelectriccrystal; (ii) scintillations are observed in pressed pel-lets of glycine, indicating that domain structure isunimportant; (iii) scintillation activity in ferroelectriccrystals does not appear to depend upon the orienta-tion of the ferroelectric axis with respect to the direc-tion of the temperature gradient. We cleaved a singlecrystal of TGS such that the ferroelectric axis wasperpendicular to one of the crystal faces. Scintilla-tions were observed for two orientations of the crys-tal; viz. , ferroelectric axis parallel to the sample hold-er and ferroelectric axis perpendicular to the sampleholder (see Fig. 1). From previous work' it wasknown that the scintillations resulted from a dis-charge from the upper surface of the crystal to thelower surface via the metal strap and screws. Thus,we expected frequent occurrence of scintillationswhen the ferroelectric axis was perpendicular to thesample holder surface but not when it was parallel.Several measurements indicated that the scintillationswere independent of the ferroelectric axis orientation.

In conclusion, we have shown that the emissionspectra of scintillations observed in nonferroelectricand ferroelectric crystals are identical and we attributethis spectrum to an adsorbed layer of molecular N2through which the discharge occurs. In addition, anytheory proposed to explain electric discharges in crys-tals subjected to a temperature gradient should not bebased on ferroelectric phenomena.

ACKNOWLEDGMENTS

The authors acknowledge the assistance of Ms.Joanne Rhodes in obtaining much of the experimen-tal data and to Dr. Michael Garland for use of labora-tory equipment. Fruitful discussions with ProfessorI. Miyagawa were extremely helpful. This work wassupported in part by a grant from Memphis StateUniversity Faculty Research Fund. This supportdoes not necessarily imply endorsement of researchconclusions by the University.

21 OPTICAL SPECTROSCOPY OF SCINTILLATIONS OCCURRING. . . 4169

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