Scintillation yield of thin polystyrene films

Journal of Luminescence 31 & 32(1984)567-569 North-Holland, Amsterdam

567

SCINTILLATION YIELD OF THIN POLYSTYRENE FILMS A. M. Boteiho do Rego, M. I. Morals and J. Lopes da Silva Centro de QuThilca Fisica Molecular, Complexo I (INIC) Instituto Superior Tecnico, Lisboa Portugal Specific luminescence of thin dopped polystyrene films are studied in order to analyse the effects of singlet excitons degradation on the scintillation by cc particles. 1. INTRODUCTION The influence of the nature and energy of the ionizing particles on the specific radioluniinescence of organic scintillators, dS/dx, has been the sub— ject of previous studies 1—4 Theoretical curves calculated from the expressions proposed in ref 2 fail to fit the experimental results for low-energy particles,the failure being more evident for results concerning the delayed emission 2 Here, we analyse the effects of quenching on the value of the scintillation (prompt plus delayed component) induced by a heavy ionizing particle of energy E and specific energy loss dE/dx, measuring the specific luminescence dS/dx of polystyrene films dopped with different concentrations of DPA. 2. THEORETICAL In a recent publication we have deduced the following expression for the delayed specific luminescence (mean number of photons emited per unit path length of the incident particle, as delayed signal of the scintillation): {l

=

+

~2~

+

~

-

~ -

-

~

~ (1

-

exp(-(l

-

~

~ BT)) exp(-(l

exp(- ~2~ BT)) exp(- ~2~ BT) ~ (1

-

exp(-(~

1- ~

-

~

~ BT)

+

~ ~

(1)

Quantities (1 - ~l~’~2 and - ~ are the fractions of energy transferred by the incident particle to the track, blobs and spurs respectively. These ones represent the three distinct regions considered in theparameter track model proposedthe 2~ST1~~T ~ 2RTT/WT definsa the quencher concerning before ~. BT = degradation of triplet excitons, where RST and RTT are the critical singlettriplet and triplet-triplet

interactions respectively, and WT is the mean

0022—2313/84/$03.OO© Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

568

AM. Botciho do Rego et al.

/

Scintillation yield of thin polrstsrenc

tunis

energy required to produce a T

1 state, ~ is the quantum efficiency of fluorescence and cc is the number of singlet states S~formed per T1 disappearing during a T1 — T~ interaction. Similarly we deduced in this work the following expression for the specific prompt luminescence: {(1

=

+

~l

-

-

~

~

~

exp(-(l

-

~

~

Bs)

+

~2 ~

exp(- ~2 ~

~

B5)

+

(2)

where W5 is the mean energy required to produce a S1 state and B5 is the quencher parameter defining the degradation of singlet excitons. If we assume that this degradation can take place through interactions such as S +S -~S +S --S +S (a) 1 1 n o 1 o S +T ~T +S -~T +S (b) 1 1 n o 1 o we can write B~= 2R55/W5 + ?RST/WT, Rss being the critical distance for singlet—singlet interactions. Adding (1) and (2) we obtain the total specific

luminescence dS/dx. 3.

RESULTS AND DISCUSSION In fig 1 we plot experimental values of dS/dx obtained by single photoelec-

tron technique with three films of polystyrene containing different concentra—

~1o~ 2

ii

I

I

102

ENERGY (M&~)

FIGURE 1 Specific luminescence as a function of particle energy of polystyrene films containing DPA concentrations of 0 M(O), 3.102 M(cl) and 6.102 M(cc). Calculated curves

AM. Boteiho do Rego et al. / Scintillation yield of thin polystyrene films

569

tions of DPA: 0 N, 3 10-2 N and 6 lO2 M. Theoretical values proportional to dS/dx defined by (WT/ncc)(dS/dx) = A + WB (where A and B are the quantities within bracketts in (1) and (2) respectively and W

=

WT/IWS) were fitted to experimental points, and good agreement was

obtained. Values of B~,B~and WT/WS are indicated in table 1 TABLE 1 Values of parameters BT, B~and WT/WS for different solute concentrations

~DPA]

BT cm MeV~

B~ cm MeV~

WT/WS

N

8.0 1O~

0.1 lO~

0.13

3 io2 M

1.2 lO~

0.4 lO~

0.33

6 l02 M

1.6

1.0 lO~

0.61

0

Similarly to what is observed with BT~B~values are higher for increasing values of DPA concentration. Nevertheless, a more pronounced increasing of B~is remarked here, which means that the mean energy W 5 decreases more swiftly than WT with increasing solute concentration. This can be attributed to a higher efficiency of solute singlet states production by subexcitation electrons. Every fitting realized conduced to B~values lower than BT which is admissible assuming that only the quasi static degradation process (a) takes place. This could be justiffied

by considering that primary singlet and triplet

excitons are produced in separate sites. REFERENCES

1)

J.

Lopes da Silva and R. Voltz, Rev. Phys. Appl. 7 (1972) 127

2)

3. Lopes da Silva, 3. Physique 39 (1978) 561

3)

L. Muga and Diksic, Nucl. Inst. Meth. 122 (1974) 553

4)

A. N. Botelho do Rego and J. Lopes da Silva, Chem. Phys. Lett. 105 (1984)

559 5)

A. M. Botelho do Rego and J. Lopes da Silva, sub. to Tnt. 3. Rad. Phys. Chem.

6)

Fuchs, F. Heisel and R. Voltz, 3. Phys. Chem. 76 (1972) 3867